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eHam Forums => Amplifiers => Topic started by: VE7RF on December 31, 2011, 04:44:27 AM



Title: Length of coax from xcvr to amp affecting input swr, why ?
Post by: VE7RF on December 31, 2011, 04:44:27 AM
With short length's of coax, [no more than aprx 3'], the input swr between
either my FT-1000-D  or my FT-1000-MK-V... and the input to my Drake L4B
is dead flat.   Once the coax is lengthened  to aprx 6', the input swr rises,
and can't be brought down to 1:1  No amount of slug tuning  will bring it back
down flat.  It will bring it down a tiny bit though.  What is  going on here ??
 
On my HB amp,  I use a variable tuned input, consisting of a pair of broadcast caps
+  tapped coil,  [pi- net].  The coax can be 25' long, and it can always be tweaked dead flat
from 160-10m.  The hb amp resides in the workshop next door.
 
Having to resort to changing all the C1 + C2 values in the drake amp is a pita. Then
even if I do that, and then change the coax length yet again, I will  have to go through
the same C1 + C2 rigmarole again.  I have 4 x L4B's..and this is getting to be a real pita
with this input swr.  Does anybody else notice this effect ??   Is their  some  'magic length'
of coax that will solve my problem ?   I don't understand the mechanism why the swr changes
with coax length.   I mean, coax length  to a dummy load has no effect on swr,  so why should
it, when feeding the tuned input of a GG triode amp ?
 
tnx.......... Jim   VE7RF 


Title: RE: Length of coax from xcvr to amp affecting input swr, why ?
Post by: KH6AQ on December 31, 2011, 05:32:01 AM
Why do you want a 1:1 VSWR? Is the VSWR low enough for the transceiver to operate without folding back?


Title: RE: Length of coax from xcvr to amp affecting input swr, why ?
Post by: W1QJ on December 31, 2011, 05:36:45 AM
Really Jim? 


Title: RE: Length of coax from xcvr to amp affecting input swr, why ?
Post by: W8JI on December 31, 2011, 06:17:47 AM
It is all explained here:

http://www.w8ji.com/tuned_input_circuit.htm

Anyone who understands transmission lines and SWR also understands SWR is constant with line length, so long as the SWR detector null and line have the same impedance and losses are low.

Any SWR change with line length obviously has to be a spurious or harmonic in the system.

Class AB grounded grid amplfiers have considerable harmonic energy at the cathode.

Use enough Q in a low pass at the cathode, and the problem will go away unless the meter or coax is bad.



Title: RE: Length of coax from xcvr to amp affecting input swr, why ?
Post by: VE7RF on December 31, 2011, 05:16:19 PM
Why do you want a 1:1 VSWR? Is the VSWR low enough for the transceiver to operate without folding back?

##  Both my MK-V's  start folding back  with a 1.5:1  swr., but still have plenty of po to drive the amps.

Jim


Title: RE: Length of coax from xcvr to amp affecting input swr, why ?
Post by: VE7RF on December 31, 2011, 05:44:16 PM
It is all explained here:

http://www.w8ji.com/tuned_input_circuit.htm

Anyone who understands transmission lines and SWR also understands SWR is constant with line length, so long as the SWR detector null and line have the same impedance and losses are low.

Any SWR change with line length obviously has to be a spurious or harmonic in the system.

Class AB grounded grid amplfiers have considerable harmonic energy at the cathode.

Use enough Q in a low pass at the cathode, and the problem will go away unless the meter or coax is bad.



##  yes, I realize that.  I also tried  inserting my coaxial dynamics wattmeter, with the low power slug in rvs..same results. [ cd meter installed on the output of the yaesu].   I also tried my rack mount bird 43..and also my array solutions  powermaster.  All the external meters  + the yaesu meter all  jive with each other dead on.

##  The C1 + C2 values on all the drake L4B tuned inputs  are almost double the values  used in the heath SB-220, so the Q  should be plenty high enough.  The only thing changed was the 6 x grid pins are now directly bonded to the chassis, and the 6 x grid caps + 2 x chokes have been removed.  But I have 4 x L4B's..and one of them still has the floating grids, and grid caps/chokes etc...and that amp too, has high  input swr with longer cable lengths.

##  The input swr on 10m, however, on the 3  x L4B's  with grids directly chassis grnded is now sky high, real high, and that's with short length of coax to xcvr.  Dunno why the 10m input swr is affected on 10m, but that's another issue itself.

##  On the L4B tuned input, there is NO coax  from the C2 cap to the cathode !  The double wafer small bandswitch is located right next to the cathodes..and all they use is a .01uf @ 2 kv disc ceramic, wired directly from tuned input bandswitch to cathode.

##  Bypass swr is also dead flat on the L4B's..and doesn't deviate, until u get up to aprx 50-60 mhz. [mfj-259b on input, and 50 ohm plug on output]

##  I'll try some new coax with new connectors.   I have 2 x MK-V's...and 2 x FT-1000D's...and  4 x L4B's, and any combo of any xcvr + any amp will produce identical [ high swr] results  with longer cable to xcvr.  So it's not a one off case of a screwball swr meter in one of the 4 x yaeus's....esp when I also installed external bird/cd/powermaster meters..and get the same results.

## It's baffling to say the least.  BTW, ur tuned input write up on ur site was very informative. I didn't realize the T type tuned input [ using 2 x coils and one cap] was a lo pass filter,..except it won't attenuate harmonics.  We tried that config on a 6m GG amp, and no amount of tweaking the variable cap or tweaking either/both coils would result in an swr below 1.5:1  We dumped it..and went to a PI net, with 2 x small adjustable caps and a fixed coil.  problem solved, flat input swr, loads of grid current/drive to the cathode, and eff was way up.

##  I can always use the built in auto tuner in the yaesu's if required, in conjunction with the L4B's. In the mean time, I will make up a 1/2 doz new jumper's, of varying length's, and run some more tests. perhaps a mess of large diam ferrite beads slid over some of the jumpers will do something. I have both type 77 and type 43 beads.

Later... Jim  VE7RF


Title: RE: Length of coax from xcvr to amp affecting input swr, why ?
Post by: VK4TUX on January 01, 2012, 03:38:49 AM
It can be a weird thing. I replaced my patch leads a couple of weeks ago;
Between Txcr and amp I used a 6ft length (amp=sb-220) Input swr good on all bands bar 15m which was real high.
I then made a 9ft length and now input swr perfect on all bands.
I note 9 ft is 3 times what you have in 3 feet for which you have good results.
If you multiply an antenna length x 3 a similar matching effect occurs;
i.e; 1/4 wl to 3/4 wl is a 3x factor, Maybe a coincidence, however that worked for me.
I'm guessing as the termination impedances are not perfectly matched the coax acts similar to a tuning stub or matching transformer.


Adrian ... vk4tux


Title: RE: Length of coax from xcvr to amp affecting input swr, why ?
Post by: W8JI on January 01, 2012, 04:06:17 AM
##  The C1 + C2 values on all the drake L4B tuned inputs  are almost double the values  used in the heath SB-220, so the Q  should be plenty high enough.  

The L4B, reverse engineering the network, looks like an effective Q of 2.1 on the input. Assuming a good layout that might be 15 dB or so attenuation on the second harmonic.

I'm not sure how the SWR meters respond to a second harmonic in the reverse direction, or how much power is in the 2nd harmonic at the cathode in the L4B, but Qeff of 2.1 might not be enough, or there could be layout problems limiting the attenuation through the network.  

If the coaxial cable matches the SWR meter null exactly, and loss in the cable is low, cable length can't affect SWR. Any length sensitivity has to be cable problems, instability in the radio, or spurious from the amp.

I've had to go up to Q~5 on some amps.


Title: RE: Length of coax from xcvr to amp affecting input swr, why ?
Post by: K6AER on January 01, 2012, 08:32:38 PM
Are you sure the transceiver output impedance is 50 ohms?


Title: RE: Length of coax from xcvr to amp affecting input swr, why ?
Post by: M0HCN on January 01, 2012, 09:10:07 PM
Transceiver output impedance will make NO difference to the SWR measured on the line.....

If you have drive power to spare, a pad in series with the line will significantly reduce the reported VSWR, 6dB or so may help enough.

Regards, Dan.


Title: RE: Length of coax from xcvr to amp affecting input swr, why ?
Post by: VE7RF on January 01, 2012, 09:21:26 PM
Transceiver output impedance will make NO difference to the SWR measured on the line.....

If you have drive power to spare, a pad in series with the line will significantly reduce the reported VSWR, 6dB or so may help enough.

Regards, Dan.


### They only do 200w  out...and the L4B  requires 110w  to drive it to 800 ma.  Even if a 3 db pad is used, then the xcvr is running at 200w.... so it's imd [xcvr] will be worse.  Plus the 3db will have to handle 100w.

Jim


Title: RE: Length of coax from xcvr to amp affecting input swr, why ?
Post by: VK4TUX on January 01, 2012, 11:37:10 PM
Transceiver output impedance will make NO difference to the SWR measured on the line.....

If you have drive power to spare, a pad in series with the line will significantly reduce the reported VSWR, 6dB or so may help enough.

Regards, Dan.


The important measured swr here is that determined by the transceiver swr detection circuitry.
It's all about keeping that happy for best power output.

Adrian ... vk4tux


Title: RE: Length of coax from xcvr to amp affecting input swr, why ?
Post by: VK4TUX on January 01, 2012, 11:44:51 PM
  Does anybody else notice this effect ??   Is their  some  'magic length'
of coax that will solve my problem ?   I don't understand the mechanism why the swr changes
with coax length.   I mean, coax length  to a dummy load has no effect on swr,  so why should
it, when feeding the tuned input of a GG triode amp ?
 
tnx.......... Jim   VE7RF 
[/quote]

The amp input circuit is not purely resistive like a non inductive dummy load.

Try 9' ?

Adrian ... vk4tux


Title: RE: Length of coax from xcvr to amp affecting input swr, why ?
Post by: VE7RF on January 02, 2012, 12:25:22 AM
##  The C1 + C2 values on all the drake L4B tuned inputs  are almost double the values  used in the heath SB-220, so the Q  should be plenty high enough.  

The L4B, reverse engineering the network, looks like an effective Q of 2.1 on the input. Assuming a good layout that might be 15 dB or so attenuation on the second harmonic.

I'm not sure how the SWR meters respond to a second harmonic in the reverse direction, or how much power is in the 2nd harmonic at the cathode in the L4B, but Qeff of 2.1 might not be enough, or there could be layout problems limiting the attenuation through the network.  

If the coaxial cable matches the SWR meter null exactly, and loss in the cable is low, cable length can't affect SWR. Any length sensitivity has to be cable problems, instability in the radio, or spurious from the amp.

I've had to go up to Q~5 on some amps.

## Point well taken.  On a similar note, on my hb tuned input for the GG 3CX-3000/6000A7 tubes, I did not want to build 9 x bandswitched tuned inputs, so built just an adjustable pi net tuned input, with 2 x broadcast caps [padded on 160m only] and a tapped coil. I can make the Q anything I want, so tried a Q of 5-6 for an experiment.   To test the tuned input, I installed watt meters on both sides of it..then into a dummy load.  Low bands is ok, but on the high bands, with 200w going into it, I only got 160w out of it.  I increased the coil by 1/4 turn, and reduced the 2 x caps, and re-tuned for flat swr, then the Po rose to 195 watts.  The coil was 6 ga solid cu wire, and I could get it warm with 200watts. That equates to 2A  from the xcvr. Then with a Q of 5-6, it becomes 9-12 A of circulating current in the coil alone.  BW is less, but no big deal since it's fully adjustable anyway, via front panel. .  Once hooked to the cathode though, the C1 + C2 values are off by quite a bit, but a 1:1 swr could always be obtained.

##  Now imagine what happens with real small ga wires typ used on tuned inputs, and high Q. Inserting the watt meter between the tuned input and a dummy load , and varying the Q vs a real eye opener.

##  Other's have told me that when they went from 3'  to 6' of coax o their TL-922, that the swr rose as well. Some are suggesting that the added 3' of coax is like adding another 30 pf to the input of the amp.  I find this hard to believe, since the same 6' would have zero effect on a  dummy load.

##  I have also tried using arco trimmer caps   [padded on the low bands] and adjustable slugs or fixed coils.  The values used are dead on with the spread sheet, into a dummy load, but again, when hooked to the cathode, the caps are off by a bit, but always result in a  flat swr.

## OK, what happens if 6-20' of coax is used between xcvr and the input of an AL-1500/AL-1200 /alpha, dentron, sb-220, henry, etc, etc??  

## some have also suggested to use an electrical 1/2 wave of coax [cut for 3500 khz]..which then becomes 1 wave on 40m..and 2 waves  on 20m...and  3 x waves on 15M...and 4 x waves on 10m.

## some have suggested installing a T connector at the input to the amp, then inserting small amounts of C, to tune out the swr.

## Perhaps the answer is to replace the silver mica's used for C1..with arco's ?

Jim  VE7RF


Title: RE: Length of coax from xcvr to amp affecting input swr, why ?
Post by: VK4TUX on January 02, 2012, 03:42:42 AM

## Perhaps the answer is to replace the silver mica's used for C1..with arco's ?

Jim  VE7RF
[/quote]

Jim, I guess you still have both the 3' & 6' patch coax pieces. In your massive amp resources kit can you find a 259 double female joiner to connect them together to make a 9' length?
If you could try that I would be interested to see the result if that's not too much trouble?

Adrian ... vk4tux


Title: RE: Length of coax from xcvr to amp affecting input swr, why ?
Post by: W1QJ on January 02, 2012, 08:07:24 AM
Amp Supply amps all have arco trimmers for C1 and C2 with their ATI-6 boards. Their coils are fixed.  Just the opposite of most other amps.  Keep in mind that most of these older amps we have been discussing  L4B, SB-220, TL-922 etc. were designed to operate around their own transmitters/transceivers like T4X,TR-4, SB 401 HW-101, TS-520, TS-820.  All these had tube finals with adjustable pi nets.  None of these radios had built in SWR meters so we really didn't know what the swr was to the transmitter.  They had enough drive regardless to push the amp pretty hard as they would not cut back in power.  The intersting thing is when the swr meter reads high, is the radio actually folding back power?  SOmetimes you may notice an swr but not a foldback in power.  If that s the case it could be the harmonic power reflection.


Title: RE: Length of coax from xcvr to amp affecting input swr, why ?
Post by: W8JI on January 02, 2012, 03:54:45 PM
Hi Lou,

Actuaklly the direction coupler sees any power from the outside as full reflected power. I have a 30 dB high power (2.5 kW) pad on my test bench. If I run 1000 watts backwards into that pad, so there is 1 watt backfeeding my IC751A while it is transmitting, the 751A SWR meter moves up to ~1.2:1 and the radio just barely starts to fold back on some bands.

If I run 2 kilowatts, so it backfeeds the 100 watt rig with just 2 watts, the ICOM noticably shuts down and shows a 1.3 SWR. 2 watts of backwards power is the trigger point for my 751A's detector.

The cathode of a tube in a higher power grounded grid amp can have well over a dozen watts of harmonic energy, and if the tuned input doesn't knock that energy all out, the exciter sees it just as if it were normal reflected power.

73 Tom

PS...
Some of the other answers fly in the face of how standing waves work. Although Dan covered this, I'll repeat it. We all should know, if we understand SWR, that line length does not change SWR if the SWR detector and line are matched (as long as the line has low loss). We also should know exciter source impedance does not affect SWR on the line.

This leaves only a few things:

1.) The interconnect cable is bad

2.) The SWR detector in the radio is misadjusted

3.) The radio is oscillating or making a spurious that changes level with line length

4.) The amplifier is putting harmonics or spurious back into the radio

None of this mattered, as Lou already said, when we had old tube rigs. But we should still know how SWR works on transmission lines. The source impedance cannot change SWR downstream.


Title: RE: Length of coax from xcvr to amp affecting input swr, why ?
Post by: K6AER on January 02, 2012, 06:54:55 PM
If you have activated the antenna tuner in a transceiver and the output impedance is say 100 ohms the cable length will be critical when coupling into the amplifier. It will depend where the VSWR nodes combine or cancel to produce different impedances along the line. A VSWR meter will only read the line to the terminating load. The radio is still seeing VSWR if it is tuned to something other than 50 ohms out from the antenna tuner and is set to an impedance other than 50 ohms.

When driving the amplifier make sure your antenna tuner is turned off.


Title: RE: Length of coax from xcvr to amp affecting input swr, why ?
Post by: VE7RF on January 02, 2012, 11:58:55 PM
If you have activated the antenna tuner in a transceiver and the output impedance is say 100 ohms the cable length will be critical when coupling into the amplifier. It will depend where the VSWR nodes combine or cancel to produce different impedance's along the line. A VSWR meter will only read the line to the terminating load. The radio is still seeing VSWR if it is tuned to something other than 50 ohms out from the antenna tuner and is set to an impedance other than 50 ohms.

When driving the amplifier make sure your antenna tuner is turned off.

##  I disagree. As long as the amp has an existing tuned input.... using the built in auto tuner is not an issue.   I shut off the auto tuner.  Then tune the amp.  THEN switch the auto tuner on..then tweak the auto tuner for flat swr. Xcvr is happy.  Amp is easily driven.

Jim  VE7RF


Title: RE: Length of coax from xcvr to amp affecting input swr, why ?
Post by: VK4TUX on January 03, 2012, 01:09:26 AM
##  I disagree. As long as the amp has an existing tuned input.... using the built in auto tuner is not an issue.   I shut off the auto tuner.  Then tune the amp.  THEN switch the auto tuner on..then tweak the auto tuner for flat swr. Xcvr is happy.  Amp is easily driven.

Jim  VE7RF
[/quote]


Jim is probably not interested in responding to any of my comments, however for the rest here I had a situation where I made up a new 6ft coax length (properly soldered plug body > braid & 1000v megger tested, with 20 amp conductivity test also; I am an electrical tradesman) and tried it out, working well, apart from 15m K3 ind swr which was unusually very high. Amp is a sb-220.

I grabbed my Yaesu tuner and placed it inline with its own extra 3' patch lead. I thought; "Ok I will just use the tuner on 15m and put it in bypass for the other bands".
I did that because I use the amp 15m position for 17m, which had a good input swr as per above setup, but previously both 15 & 17 were ok, and neither perfect as you would expect on the one band input.

Well when I connected all that up I had it in bypass and went through the bands and they were all 1:1 swr on the K3.

15m had gone from very bad to very good, with the tuner not being utilised and just in bypass .

Then I think ok 6 & 3, lets try that, and did with the same good result.

I went to each band and tested tx into the cantenna load and all good 1:1 on the K3 every band once amplifier was tuned correctly.

The amplifier input network impedance match effects the linearity of the result signal if not good, and I wondered whether using a tuner cleans up a signal effected for that reason or the signal stays bad despite good power transfer?

I have always avoided using a tuner in the input line if possible and found a way here.
The input coax length appears to become part of the amp input network impedance match,
despite not agreeing with reported theory.

I can only put it down to the method the transceiver swr detection circuit uses to get it's result. I think good found lengths may vary for different amps and setups, I cannot see the problem with experimenting.

Adrian ... vk4tux



Title: RE: Length of coax from xcvr to amp affecting input swr, why ?
Post by: VE7RF on January 03, 2012, 03:03:44 AM

## Perhaps the answer is to replace the silver mica's used for C1..with arco's ?

Jim  VE7RF

Jim, I guess you still have both the 3' & 6' patch coax pieces. In your massive amp resources kit can you find a 259 double female joiner to connect them together to make a 9' length?
If you could try that I would be interested to see the result if that's not too much trouble?

Adrian ... vk4tux
[/quote]

##  I'm gonna try that..if I can find a good one.  And no, the coax is not bad.   The swr is dead flat when amp in bypass mode too.   The external bird/cd/as meter's all jive with the yaesu meter.  It's the yaesu meter that's the one that will determine if the po of the xcvr folds back, not the external meters.

##This will all change..again, when the new upper shelf is finished on desk #2.  I tried this b4 and it worked good.  I string the L4B's, nose to tail, all in series..and tune em up on different bands.  They will all easily handle 2 kw on bypass. A rotary switch ensures that only one amp's key line is on at any one time. IE: all the amps are switched to operate mode.   Key line from xcvr goes into the rotary switch box...and multiple outputs, one to each L4B.  I can also tune more than one amp up on the same band as well.

##  I find this hard to believe that nobody else has this problem with rising/varying swr with different lengths!  For folks who have adjustable arco's for each band,I'll bet even they have the same issue with different lengths.  Except they can re-tweak the input swr down flat.  How many of these newer amps use arco's ??

Later.. Jim  VE7RF


Title: RE: Length of coax from xcvr to amp affecting input swr, why ?
Post by: VK4TUX on January 03, 2012, 05:48:16 AM

## Perhaps the answer is to replace the silver mica's used for C1..with arco's ?

Jim  VE7RF

Jim, I guess you still have both the 3' & 6' patch coax pieces. In your massive amp resources kit can you find a 259 double female joiner to connect them together to make a 9' length?
If you could try that I would be interested to see the result if that's not too much trouble?

Adrian ... vk4tux

##  I'm gonna try that..if I can find a good one.  And no, the coax is not bad.   The swr is dead flat when amp in bypass mode too.   The external bird/cd/as meter's all jive with the yaesu meter.  It's the yaesu meter* that's the one that will determine if the po of the xcvr folds back, not the external meters.

Yes I mentioned exactly that * in an earlier post here. Putting an out of rig swr meter in the line to amp is a waste of time.
I never mentioned bad coax, must have been another poster, yes swr great here too with amp on stdby when using 6', same deal.

At the end of the day as long as the feedpoint impedance (an amp or antenna input) is a good match, and the transceiver swr detection is happy then all is good. It's a shame Rich got banned here, he is very knowledgable on the subject.

##This will all change..again, when the new upper shelf is finished on desk #2.  I tried this b4 and it worked good.  I string the L4B's, nose to tail, all in series..and tune em up on different bands.  They will all easily handle 2 kw on bypass. A rotary switch ensures that only one amp's key line is on at any one time. IE: all the amps are switched to operate mode.   Key line from xcvr goes into the rotary switch box...and multiple outputs, one to each L4B.  I can also tune more than one amp up on the same band as well.

##  I find this hard to believe that nobody else has this problem with rising/varying swr with different lengths!  

Try google and you will see abundant comment on the subject. Apart from that many use antenna tuners to deal with it, but the issue with that is you may not detect a change in the amp input , relay failure, tube filament or supply failure that would show up as bad rig swr all of a sudden esp with auto-tuners.

On the sb-220 band positions 10m is used for 12m & 10m
                                           15m is used for 17m & 15m
                                           20m is used for 30m & 20m
40=40 and 80 =80 and these two can be optimised for the label band, however the others are a compromise which brings up the situation in the first place. However the right coax length can be found that keeps things happy on all bands incl warc.

Tom is right about dedicated hi-Q separate inputs for every band you use, but with a sb-220 things are different unless you want to leave the unlabelled bands out and optimise and raise Q on perfectly tuned input networks for what is on the dial.

Adrian ... vk4tux



Title: RE: Length of coax from xcvr to amp affecting input swr, why ?
Post by: VK4TUX on January 03, 2012, 06:11:27 AM

##  I find this hard to believe that nobody else has this problem with rising/varying swr with different lengths!  

Try google and you will see abundant comment on the subject.

An example here;

"
>From time to time, people have mentioned how Collins used to recommend a
certain length of coax between the KWM2 and 30L1 amplifier.  As the
following anecdote may illustrate, this isn't just ancient history.  I hope
this story helps someone else troubleshoot a similar problem.

Recently, I redid the T/R relay system in my SB-220 to add QSK, and was
perplexed to discover, after the modification, that the amplifier suddenly
showed a high input SWR on 40m (only), as well as abnormally high gain,
grid current and plate current on that band only.  Visions of an HF
oscillation, or some other abnormality, had me digging for answers. 

Thanks to Tom, W8JI, who asked if I had changed anything else at the same
time.  Eureka!  I had "improved" the jumper from exciter to amplifier by
shortening it about 8 feet.  Just now, I put the old one back in, and
everything is back to normal.  I don't pretend to understand WHY this
happens, or why it happens only on one band, but there you have it.



73, Pete Smith N4ZR
"


Adrian ... vk4tux


Title: RE: Length of coax from xcvr to amp affecting input swr, why ?
Post by: G3RZP on January 03, 2012, 07:03:45 AM
I have been told by some users that the 'magic length' of cable for the 30L1 provides an impedance which keeps it more stable on 15 and 10. I cannot say that I'm impressed by the design: a 40 microhenry plate choke gets a lot of current through it on 80m, and the lack of neutralisation begs for problems at the top end of the frequency range. Add to that the low values of grid bypassing capacitors - presumably to give negative feedback - and there are troubles stored up. The mechanical mounting of the PSU equalising/bleeder resistors is a real bodge job.

Of course, the low values of grid bypass cap do give negative feedback in a perfect world with no stray inductance or capacitance and thus zero size tubes to have zero lead length.


Title: RE: Length of coax from xcvr to amp affecting input swr, why ?
Post by: W8JI on January 03, 2012, 07:09:24 AM
Jim,

The problem you have is not unusual, and manifested itself in the 1980's when solid state rigs appeared. You aren't the first one to notice it. Ameritron ended up with a Drake TR5 from Dentron that had some weird big finals subbed in for the OEM parts, and no active SWR foldback. I suspect that was to make it  work with the poor or no tuned inputs. The Q and arrangement of ATI6 boards, when they used them, was all over the place. On ten meters, some just had one trimmer and a wire jumper to replace the coil!

No one needed to pay attention to back-fed harmonics when exciters had no SWR detection and foldback.

Of course the TR5 had other issues, it would oscillate out-of-band with some loads, making me wonder how anyone used it to adjust amplifiers.

73 Tom


Title: RE: Length of coax from xcvr to amp affecting input swr, why ?
Post by: W8JI on January 03, 2012, 07:17:16 AM
I have been told by some users that the 'magic length' of cable for the 30L1 provides an impedance which keeps it more stable on 15 and 10. I cannot say that I'm impressed by the design: a 40 microhenry plate choke gets a lot of current through it on 80m, and the lack of neutralisation begs for problems at the top end of the frequency range.

The 30L1 makes a great oscillator on 15 and 10 meters. Try running some stability tests as you mesh the load cap. Collins fought for years because they insisted on wapping feedback around two stages in transmitters, and those floating grids. The floating grids were a good idea in the 30S1, but that amp had no grid current and had a tetrode with a screen to shield the input from output.

Quote
Add to that the low values of grid bypassing capacitors - presumably to give negative feedback - and there are troubles stored up. The mechanical mounting of the PSU equalising/bleeder resistors is a real bodge job.

Of course, the low values of grid bypass cap do give negative feedback in a perfect world with no stray inductance or capacitance and thus zero size tubes to have zero lead length.

Not even then. The problem is grid current.  Calculate the phase and level of feedback as the grid cycles through its current range.

The grid caps are the lower part of the divider, and the grid-cathode time-varying and frequency-varying impedance is the upper half of the divider. It has to be the silliest circuit in the world for a mult-band amp with grid current.



Title: RE: Length of coax from xcvr to amp affecting input swr, why ?
Post by: G3UUR on January 03, 2012, 07:45:31 AM
Some of the other answers fly in the face of how standing waves work. Although Dan covered this, I'll repeat it. We all should know, if we understand SWR, that line length does not change SWR if the SWR detector and line are matched (as long as the line has low loss). We also should know exciter source impedance does not affect SWR on the line.

None of this mattered, as Lou already said, when we had old tube rigs. But we should still know how SWR works on transmission lines. The source impedance cannot change SWR downstream.

There's no new physics here, just some more you don't fully understand. A pair of GG 3-500Z tubes present a load to the driver that is a non-linear function of the drive voltage, and as such it is quite different from the constant impedance loads used in the elementary transmission line theory you're considering, Tom.

The source impedance does affect the distortion caused to the drive signal voltage and the reflected harmonic energy seen on the transmission line. What is needed right at the cathode/filament is a voltage source with very little source impedance to drive the linear without too much distortion being caused by non-linear loading. That's very dependent on the phase delays of all the networks and cables between the driver and tube cathode/filament.

Bruene VSWR sensors are fairly flat with frequency, whereas pick-up line types are more sensitive to harmonics unless some form of compensation is used, but they'll all indicate harmonic reflections pretty well unless the input matching network attenuates them well.

The L4B input matching has, as Tom said, an output Q of 2.1. It also has an input Q of around 1.5 and overall the circulating current is 2.3 times the output current at a load of 80 ohms. This is apparently what the network was designed to match, but the actual impedance is half this at the peak of the combined grid and cathode current and is infinite over part of the cycle when the tubes are cut off. Obviously, a circulating current of 2.3 times at the average impedance is not sufficient for any length of connecting line and a higher Q is required for solid-state drivers to smooth out the load variations. A Q of 10 would definitely be enough, but many handbooks say more than 5 should be used - this lower figure is close to the Q (6.28) that provides energy storage in the network equal to that passing through.  

The real question is why just 3ft of coax between the transceiver and the linear works so well and other longer lengths don't. Perhaps, if you'd read Warren Bruene's article more thoroughly, Tom, and understood it, rather than dismissing it because he disagrees with your views on the HFTPA conjugate match question, you'd have been able to model it and tell us!

Without sufficient Q and a good flywheel effect to smooth out the non-linear load at the cathode/filament, the low-pass filter and the connecting cable, along with the input matching network phase delays will determine the source impedance seen right at the tube and affect the extra distortion created there. Rigs withe different low-pass filter designs will require different "magic lengths" of patch lead for the same type of amplifier and the same band, and different magic lengths will be required on different bands. You might find a length that works on most bands, but it might take quite some effort to find it. The simplest solution is to just re-design the input networks, Jim. Trying to understand exactly what's going on may take quite a lengthy study and not be worth to time required.  

Happy New Year de Dave, G3UUR.
  


Title: RE: Length of coax from xcvr to amp affecting input swr, why ?
Post by: AF6LJ on January 03, 2012, 12:01:04 PM
Hi Lou,

Actuaklly the direction coupler sees any power from the outside as full reflected power. I have a 30 dB high power (2.5 kW) pad on my test bench. If I run 1000 watts backwards into that pad, so there is 1 watt backfeeding my IC751A while it is transmitting, the 751A SWR meter moves up to ~1.2:1 and the radio just barely starts to fold back on some bands.

If I run 2 kilowatts, so it backfeeds the 100 watt rig with just 2 watts, the ICOM noticably shuts down and shows a 1.3 SWR. 2 watts of backwards power is the trigger point for my 751A's detector.

The cathode of a tube in a higher power grounded grid amp can have well over a dozen watts of harmonic energy, and if the tuned input doesn't knock that energy all out, the exciter sees it just as if it were normal reflected power.

73 Tom

PS...
Some of the other answers fly in the face of how standing waves work. Although Dan covered this, I'll repeat it. We all should know, if we understand SWR, that line length does not change SWR if the SWR detector and line are matched (as long as the line has low loss). We also should know exciter source impedance does not affect SWR on the line.

This leaves only a few things:

1.) The interconnect cable is bad

2.) The SWR detector in the radio is misadjusted

3.) The radio is oscillating or making a spurious that changes level with line length

4.) The amplifier is putting harmonics or spurious back into the radio

None of this mattered, as Lou already said, when we had old tube rigs. But we should still know how SWR works on transmission lines. The source impedance cannot change SWR downstream.

5. the amplifier input is not presenting a 50 ohm impedance to the radio cable and bridge.



Title: RE: Length of coax from xcvr to amp affecting input swr, why ?
Post by: G3UUR on January 03, 2012, 12:22:03 PM
5. the amplifier input is not presenting a 50 ohm impedance to the radio cable and bridge.

Not only is it something other than 50 ohms, it's not constant over the RF cycle either!


Title: RE: Length of coax from xcvr to amp affecting input swr, why ?
Post by: AF6LJ on January 03, 2012, 03:36:44 PM
5. the amplifier input is not presenting a 50 ohm impedance to the radio cable and bridge.

Not only is it something other than 50 ohms, it's not constant over the RF cycle either!

That is very true.
The older radios were better suited to handle this situation.


Title: RE: Length of coax from xcvr to amp affecting input swr, why ?
Post by: VE7RF on January 03, 2012, 10:28:24 PM
5. the amplifier input is not presenting a 50 ohm impedance to the radio cable and bridge.

Not only is it something other than 50 ohms, it's not constant over the RF cycle either!

That is very true.
The older radios were better suited to handle this situation.

##  Sue.... take something simple..like a GG SB-220.  With it's oem  5 V zener for bias, the tubes have a 210 deg conduction angle.  That means they don't conduct for the other 150 degs !
A PI net tuned input is used to provide some flywheel action at the cathode. Just put a scope across the cathode on any GG amp, with and without the tuned input in place, and you can see the waveform distortion instantly.   Those peak RF pulses from the tube go around in a big loop..and have to end back up at the cathode.  Those pulses are aprx 3 x the dc plate current for class AB...and aprx  4 x the dc plate current..for Class C.  The tubes can't source anything. They can only sink current.  The return path for those pulses is via the chassis, then up through the C2 cap of any tuned input !  With out the tuned input installed, the path is then via the braid of the 3' of coax to xcvr, then through the C2 load cap of the tube xcvr pi net....then back down the center conductor of the same 3' coax....then back to the cathode. [ that C2 cap in either case has to handle one helluva lot more current than you think. ] 

##  These days, we use SS xcvr's..and each band has it's own high power LP filter.  I firmly believe the Xcvr's  LP filter + coax + the 210 deg conduction angle of the tubes + loaded Q of the amps tuned input all interact.  Change bands, and presto, a different xcvr's LP filter is now being used.  This has nothing to do with defective coax from xcvr to amp, nor defective swr meter inside the yaesu. [ which jives 100% with all my external meters, like coaxial dynamics, bird, array solutions, etc].  This is not transmission line theory either.  On BYPASS, the swr is dead flat into either the ant, or a dummy load. The difference being, we are then driving a 50 ohm, non reactive load...and NOT a the cathodes of pair of tubes..that only conduct 210 degs of each cycle.

## As long as the tuned input in the amp has adjustable C1+C2  caps, they can always be tweaked for a flat swr back to the SS xcvr.  [ and no need to run a sky high loaded tuned input Q]

##  as noted before, I tried loaded Q's of 5-6 on my HB 3CX-3000A7 amp's  tuned input, and with 200w of drive.. I can easily warm up solid 6ga Cu wire used for the 4uh tuned input coil. That tuned input consists of a 4 uh coil, aprx 17 turns of 6 ga wire, wound on a 1.5" ID. The coil is suspended in mid air, between 2 x broadcast caps. each broadcast cap is a 4 x section type..with all 4 x sections strapped in parallel.  Each section is 18-540 pf.  Each broadcast cap is padded with 4 x 500 pf HT-50 type doorknobs on 160M only.  The 4 uh coil is then tapped for each ham band.

##  Using a loaded Q of 5-6 works fine on the low bands, but is flaky on the upper bands.  I tested the assy, with wattmeters on both sides of the HB  tuned input assy....then 50 ohm coax into a dummy load. [the dummy load is not the cathode obviously, but the 3x3 tube is very close to 50 ohms].  With 200w applied, I was only getting 160w into the dummy load on the upper bands. I had to increase the uh on the coil by 1/4 turn on 15m..and retweak both caps..then the PO  shot up to 195 watts.  Now this is with 6 ga solid CU wire used for a tapped 4 uh coil.  Now try the same stunt with the more typ puny 18-20  ga wire, and higher loaded Q on any tuned input, and you will be in for a rude awakening.  And when finally hooked to the cathode of the 3x3 tube, the c1 + c2 values are now off a bit, and in some cases are way off  from the spreadsheets, depending on coax length.

##  My conclusion is,  SS xcvr's with their LP filters for each band, plus  old style bandswitched tuned inputs consisting of 2 x fixed caps  + adjustable slug tuned coil just won't cut it these days.  The short term 'fix' is just to use the auto tuner in the xcvr, if required. The yaesu's will do well over 200w po, and the L4B only requires 110 watts of drive.

## It's a good thing I didn't try and design  9x  bandswitched tuned inputs for the hb 3CX-3000A7 amp. [ that will handle 200-400w each. ] What a pita that would have been.  Loaded Q's of 3-6 is fine, provided you can adjust from the front panel. Otherwise you can't compensate for the narrower swr BW..and cable lengths.  I used a pair of jackson bro's 4" ball drives, with 6:1 vernier's for the C1 + C2 caps on the hb tuned input. they are marked 0-100 across the 180 deg arc. 4" diam = 12" circumference = 6" across the 180 deg.  Then it's a snap to just dial up various pre-sets for each band, and usually several per band.  We used the exact same hb tuned input for the larger YU-148 tubes.  Then coax length is a non issue, and the swr is always 1:1  from 1.8-30 mhz continuously.  As a real test, we stuffed a 800w cxr though it on each band for 10 mins, with no heating issues.

Later... Jim  VE7RF 


Title: RE: Length of coax from xcvr to amp affecting input swr, why ?
Post by: G3UUR on January 04, 2012, 07:42:05 AM
##  Sue.... take something simple..like a GG SB-220.  With it's oem  5 V zener for bias, the tubes have a 210 deg conduction angle.  That means they don't conduct for the other 150 degs !
A PI net tuned input is used to provide some flywheel action at the cathode. Just put a scope across the cathode on any GG amp, with and without the tuned input in place, and you can see the waveform distortion instantly.   Those peak RF pulses from the tube go around in a big loop..and have to end back up at the cathode.  Those pulses are aprx 3 x the dc plate current for class AB...and aprx  4 x the dc plate current..for Class C.  The tubes can't source anything. They can only sink current.  The return path for those pulses is via the chassis, then up through the C2 cap of any tuned input !  With out the tuned input installed, the path is then via the braid of the 3' of coax to xcvr, then through the C2 load cap of the tube xcvr pi net....then back down the center conductor of the same 3' coax....then back to the cathode. [ that C2 cap in either case has to handle one helluva lot more current than you think. ]

Jim, the cathode current includes a sizeable grid current, which in the case of a pair of GG 3-500Z tubes means a grid contribution at the peak of up to 1.3A. This makes the peak cathode current 1.5 times the peak plate current.

Your claims of a Q of 5-6 for the tuneable input filter on your HB amplifier don't seem to stack up with your C1 and C2 capacitor values either. With a limit of 2160pF on 75/80m, the circulating current in the input matching network will be around 3 times the output current (Q=3). However, this combined with the increased output capacitance (up to 26pF/meter rather than 14pF/meter in the L4B) would seem to be enough to get a reasonably stable input impedance for any solid state rig to work into.

You'll be able to get Q values of 5-6 easily on the higher bands, of course, because the reactance of your variables will be adequate to achieve that up there. I don't know why you were so surprised that you could get so much loss in the input network at the higher frequencies. Switched inductors are not the best way to achieve high Q values and a few simple calculations of the very low intermediate impedance in the pi-network matching input circuit will soon tell you how high the circulating current will be. XL/Q will give you the loss in the coil and you know the current, so you should be able to estimate the loss prior to building the network. The intermediate impedances can be very low and the currents very high in GG input matching circuits when you're starting from 50 ohms. None of this ought to be a surprise.

Dave.


Title: RE: Length of coax from xcvr to amp affecting input swr, why ?
Post by: W8JI on January 04, 2012, 05:03:07 PM
5. the amplifier input is not presenting a 50 ohm impedance to the radio cable and bridge.



Not true.

If line loss was low and the impedance was a stable impedance, and a linear load, SWR would not vary with cable length even if the SWR was not unity. 

SWR would be the same if the line were 3 feet long or six feet long, even if the SWR was 3:1.

73 Tom


Title: RE: Length of coax from xcvr to amp affecting input swr, why ?
Post by: KF7CG on January 05, 2012, 10:51:55 AM
What makes the input of an amplifier different from any other load at the end of a coax?

If the coax impedance doesn't match the impedance of the load you will get standing waves in the coax and the attendant impedance transformations that go with feedline transformers.

The well know cases used to be reducing the 200 ohm impedance of a folded dipole driven yagi by using a quarter wave of 75 ohm coax as a transformer.

This may be an extreme in the length case but it shows the concept. 4 to 1 if measured from radio to folded dipole, 1 to 1 if measure at end of coax transformer.

Different frequencies and coax electrical length have different effects on presented impedance and on SWR. Look at the books on transmission line effects and substitute resistive (complex if you want to fight the math) loads/sources at the ends of the line.


David
KF7CG


Title: RE: Length of coax from xcvr to amp affecting input swr, why ?
Post by: VK4TUX on January 05, 2012, 11:21:24 AM
What makes the input of an amplifier different from any other load at the end of a coax?
David
KF7CG

The conduction angle of the tube .

Adrian ... vk4tux


Title: RE: Length of coax from xcvr to amp affecting input swr, why ?
Post by: G3UUR on January 05, 2012, 11:47:35 AM
What makes the input of an amplifier different from any other load at the end of a coax?

If the coax impedance doesn't match the impedance of the load you will get standing waves in the coax and the attendant impedance transformations that go with feedline transformers.

What makes the input of a grounded grid amplifier different is that the input current is a non-linear function of the drive voltage and that makes the impedance vary over the part of the cycle the tube is conducting. Also, as Adrian has pointed out, the tube is off part of the time and for that time 100% of the drive signal will be reflected back. For the rest of the time when the tube is conducting the reflected power will vary and at a couple of points in the cycle it might be zero fleetingly. This makes a very non-linear load!

One way round this problem is to use an input network with some energy storage to even out the impedance seen by the driver. If the circulating current in the input stage is high enough the flywheel effect will provide a stable load impedance for the driver even though the load on the input matching circuit is varying considerably. The matching circuit only has to have a high enough Q and match to the average value of the varying load. This average value can be found by considering that the power going into the matching network must equal the power going out.

Dave. 


Title: RE: Length of coax from xcvr to amp affecting input swr, why ?
Post by: VE7RF on January 06, 2012, 05:35:19 AM
##  Sue.... take something simple..like a GG SB-220.  With it's oem  5 V zener for bias, the tubes have a 210 deg conduction angle.  That means they don't conduct for the other 150 degs !
A PI net tuned input is used to provide some flywheel action at the cathode. Just put a scope across the cathode on any GG amp, with and without the tuned input in place, and you can see the waveform distortion instantly.   Those peak RF pulses from the tube go around in a big loop..and have to end back up at the cathode.  Those pulses are aprx 3 x the dc plate current for class AB...and aprx  4 x the dc plate current..for Class C.  The tubes can't source anything. They can only sink current.  The return path for those pulses is via the chassis, then up through the C2 cap of any tuned input !  With out the tuned input installed, the path is then via the braid of the 3' of coax to xcvr, then through the C2 load cap of the tube xcvr pi net....then back down the center conductor of the same 3' coax....then back to the cathode. [ that C2 cap in either case has to handle one helluva lot more current than you think. ]

Jim, the cathode current includes a sizeable grid current, which in the case of a pair of GG 3-500Z tubes means a grid contribution at the peak of up to 1.3A. This makes the peak cathode current 1.5 times the peak plate current.


###  Right you are Gunga Din !  ur dead on abt the added grid I.

Your claims of a Q of 5-6 for the tuneable input filter on your HB amplifier don't seem to stack up with your C1 and C2 capacitor values either. With a limit of 2160pF on 75/80m, the circulating current in the input matching network will be around 3 times the output current (Q=3). However, this combined with the increased output capacitance (up to 26pF/meter rather than 14pF/meter in the L4B) would seem to be enough to get a reasonably stable input impedance for any solid state rig to work into.


##  when I did that experiment, I used the 160m padder caps on 80m for the extra C.  That gave me an extra 2000pf on each air cap. Now this was for 80m only. So 4160pf max, per cap. 

You'll be able to get Q values of 5-6 easily on the higher bands, of course, because the reactance of your variables will be adequate to achieve that up there. I don't know why you were so surprised that you could get so much loss in the input network at the higher frequencies. Switched inductors are not the best way to achieve high Q values and a few simple calculations of the very low intermediate impedance in the pi-network matching input circuit will soon tell you how high the circulating current will be. XL/Q will give you the loss in the coil and you know the current, so you should be able to estimate the loss prior to building the network. The intermediate impedances can be very low and the currents very high in GG input matching circuits when you're starting from 50 ohms. None of this ought to be a surprise.

##  I never said it was a surprise, just an eye opener, that's all.  On 80m, with high Q, the 6 ga wire is ok. On 40m, it semi ok.  On the upper bands, if u want to run  a  q of 6-10, u better use some 3/8" tubing for the coil.  The L4B tank coil is wound with 1/4" tubing,.and runs HOT on 15M, and real hot on 10M.  that's with 800ma of plate current..and Q's > 10.  That's only 10-12 a of current  through a 1/4" tubing coil.

## Now with 200w  from an xcvr..and a Q of 10 for a tuned input.. = 20A current.  And 20A  will cook  just abt anything on the upper bands.  Anyway, the point being..as long as both C1 and C2  on a pi net for a tuned input is adjustable, then flat swr can always be obtained..with any length of coax. I can calculate current flow..but the question is... what will be the temp rise over ambient?   I hear so much bs.. I thought I  would just run some tests.

Later... Jim  VE7RF

Dave.


Title: RE: Length of coax from xcvr to amp affecting input swr, why ?
Post by: VE7RF on January 06, 2012, 06:11:21 AM
What makes the input of an amplifier different from any other load at the end of a coax?

If the coax impedance doesn't match the impedance of the load you will get standing waves in the coax and the attendant impedance transformations that go with feedline transformers.

What makes the input of a grounded grid amplifier different is that the input current is a non-linear function of the drive voltage and that makes the impedance vary over the part of the cycle the tube is conducting. Also, as Adrian has pointed out, the tube is off part of the time and for that time 100% of the drive signal will be reflected back. For the rest of the time when the tube is conducting the reflected power will vary and at a couple of points in the cycle it might be zero fleetingly. This makes a very non-linear load!

One way round this problem is to use an input network with some energy storage to even out the impedance seen by the driver. If the circulating current in the input stage is high enough the flywheel effect will provide a stable load impedance for the driver even though the load on the input matching circuit is varying considerably. The matching circuit only has to have a high enough Q and match to the average value of the varying load. This average value can be found by considering that the power going into the matching network must equal the power going out.

Dave. 

## How do we calculate the average value of the  varying load ??  It's 50 ohms when driven..and sky high during periods of non conduction. [ this is on a 3CX-3000A7].  Ur right, a loaded Q of 3 is more than ample..in some cases. ..and will work, as long as both C1 and C2 caps are adjustable.   In my old  ssb, theory and practise book... it mentions the 20.5' cable that collins used on the 30L1.    " Unless the Z match between xcvr +  output and pa input is reasonably close, the Z variations will be reflected into the driver plate circuit and appear as phase modulation of the SS signal.   The length of the cable  is selected so that when it is added to the lumped constants of the pi net input... the effective length is equal, electrically to  1/2 wave  length..or multiple on each amateur band" .

## same book, but re: to the 30S1.  "to match the two, an ingenious method is used which is a  combo of cable + bandswitched pi nets.  The 20.5'cable  and tuned input are such that  even multiples of 180 deg phase shifts will be provide between driver plate and  pa grid.  Even multiples are needed, since  modulation components  change the amps cathode Z, and this  change is translated into a shift in  reactive  Z  at the driver plate. This shift  phase modulates the driver..and in turn increases overall distortion``

// OK... how do u propose to make high Q ..low esr, coils for the tuned inputs. I`ll be damned if I`m gonna build 9 x pi nets.. and mount it, so all 18 x arco`s can be tweaked.  Just to recap, how high does the loaded Q of the pi net have to be..to match the average load. How do i calculate the average load.   what`s the min loaded Q I can get away with . 

later... Jim  VE7RF


Title: RE: Length of coax from xcvr to amp affecting input swr, why ?
Post by: W8JI on January 06, 2012, 08:57:57 AM
Jim,

You certainly have the problem of fractional-cycle loading, anyone knows that.

The cable length affecting SWR reading is a result of the harmonics created by the tube for a fraction of a cycle.

There is considerable harmonic energy at the tube cathode, and regardless of Q, if the harmonics are not bypassed and prevented from reaching the exciter, they will show as reflected power on a directional coupler.

There are ten ways to nit-pick the working of this, and argue how many angels can standing on the head of a pin, but the root cause of the directional coupler seeing an SWR that isn't really there is harmonic content in the waveform.

If you had a Q of 10 in a high-pass to the cathode, you could still have the harmonic energy issue in a grounded grid stage conducting less than 360 degrees.

This is why the network has to be near the cathode, and be a good low pass or band pass, to be reliable. 

73 Tom


Title: RE: Length of coax from xcvr to amp affecting input swr, why ?
Post by: G3UUR on January 06, 2012, 12:17:46 PM
## How do we calculate the average value of the  varying load ??  It's 50 ohms when driven..and sky high during periods of non conduction. [ this is on a 3CX-3000A7].  Ur right, a loaded Q of 3 is more than ample..in some cases. ..and will work, as long as both C1 and C2 caps are adjustable.   In my old  ssb, theory and practise book... it mentions the 20.5' cable that collins used on the 30L1.    " Unless the Z match between xcvr +  output and pa input is reasonably close, the Z variations will be reflected into the driver plate circuit and appear as phase modulation of the SS signal.   The length of the cable  is selected so that when it is added to the lumped constants of the pi net input... the effective length is equal, electrically to  1/2 wave  length..or multiple on each amateur band" .

## same book, but re: to the 30S1.  "to match the two, an ingenious method is used which is a  combo of cable + bandswitched pi nets.  The 20.5'cable  and tuned input are such that  even multiples of 180 deg phase shifts will be provide between driver plate and  pa grid.  Even multiples are needed, since  modulation components  change the amps cathode Z, and this  change is translated into a shift in  reactive  Z  at the driver plate. This shift  phase modulates the driver..and in turn increases overall distortion``

// OK... how do u propose to make high Q ..low esr, coils for the tuned inputs. I`ll be damned if I`m gonna build 9 x pi nets.. and mount it, so all 18 x arco`s can be tweaked.  Just to recap, how high does the loaded Q of the pi net have to be..to match the average load. How do i calculate the average load.   what`s the min loaded Q I can get away with . 

Jim,

The input impedance of a pair of 3-500Z tubes in GG, if I remember it correctly from years ago when I calculated it, runs from about 30 ohms at the peak down to about 90 ohms just before they turn off. You can calculate an effective value of input resistance by splitting up the cycle into 10 degree segments and considering the power consumed by each of the various values of load presented to the matching circuit and doing a weighted average of that. The effective value is then the value that would consume the same power if applied continuously. I've done that and it comes out very close to a much easier way, which entails working out the cathode current waveform for an ideal voltage source driver (zero output impedance) and doing a Fourier transform to find out the relationship between the level of the fundamental waveform and the peak current. Both methods give practically the same value, which is around 2.2 times the value at the peak for a pair of 3-500Z tubes, so it's about 66 ohms, theoretically. Bear in mind this is for the ideal situation of a voltage source driving the cathode and in real amplifiers with some source impedance the value is slightly higher, probably 70 to 80 ohms.

When we're considering driving old GG amplifier designs with solid-state rigs there's no real alternative to upping the Q of the input matching network if we want to get away from using magic lengths of inter-connecting cable. The consequence, as you know, is a more limited bandwidth and the need for adjustment to cover the entire amateur band in some cases. Not much can be done about that, other than automating the tuning system, which in an old amplifier wouldn't be worthwhile.

A bit more imagination can be used in the design of matching networks to ease the need for such large values of capacitance, though. Some experimenting with how close to the wind we can sail on Q and pF per meter on the output capacitance will be required, but that should be no problem for some. I don't have the time at the moment because I'm doing up my house to sell. My wife and I are already involved in charity work in the Caribbean and need to downsize so we have somewhere smaller in the UK to use as a base, and hope to be abroad much of the time for the next few years. Much of my gear is packed away or in the process of being packed away.  You have several L4B linears with input SWR problems, so would seem to be in a very good position to conduct a few tests yourself, Jim. Tom should also be in a similar position to do these tests easily. I would think there are many L4B and SB220 owners who could do with some help on this score. I suppose there could be similar problems with some of the Jap GG linears as well. 

Dave.


Title: RE: Length of coax from xcvr to amp affecting input swr, why ?
Post by: VE7RF on January 08, 2012, 04:12:13 AM
Jim,

You certainly have the problem of fractional-cycle loading, anyone knows that.

The cable length affecting SWR reading is a result of the harmonics created by the tube for a fraction of a cycle.

There is considerable harmonic energy at the tube cathode, and regardless of Q, if the harmonics are not bypassed and prevented from reaching the exciter, they will show as reflected power on a directional coupler.

There are ten ways to nit-pick the working of this, and argue how many angels can standing on the head of a pin, but the root cause of the directional coupler seeing an SWR that isn't really there is harmonic content in the waveform.

If you had a Q of 10 in a high-pass to the cathode, you could still have the harmonic energy issue in a grounded grid stage conducting less than 360 degrees.

This is why the network has to be near the cathode, and be a good low pass or band pass, to be reliable. 

73 Tom

###  I just looked at the schematic of the AL-1500... and it depicts pi nets for each band, consisting of fixed caps, and a slug tuned adjustable coil.   The C1 + C2 values  for each band of the AL-1500  are no where near as high as the L4B tuned input values [80-10m, the L4B has no 160m].

##  OK, I just looked at the tuned inputs on the AL-82..which uses 2 x 3-500Z... and it too, has lower values  than the L4B.  In both the AL-1500 and also AL-82... C5 + C6 are the tuned input caps..with C5 closest to the cathode.  The AL-82 also uses fixed caps and a slug tuned coil.

##  I will have to do some tweaking and experimenting on one of the 4 x L4B's.  It's a bit awkward, since the caps and slugs are below the chassis..and with bottom lid off, the air is lost to the tubes [blower + chimneys].

Later... Jim  VE7RF


Title: RE: Length of coax from xcvr to amp affecting input swr, why ?
Post by: VK4TUX on January 08, 2012, 04:31:50 AM
This guy (W2AJI)

http://home.earthlink.net/~cherokeehillfarm/id2.html (http://home.earthlink.net/~cherokeehillfarm/id2.html)

has an interesting take on the subject;

"Why does this work as it does? 

In cases involving RF signals, some time will pass during the 'round trip of the reflected energy and the phase of the reflection will also depend upon this length of time. Imagine that a resistor in a black box is at the end of a length of cable. From the outside world this length of cable will give the reflection from the resistor a phase shift since the signal must make a round trip through the length. If a 100 ohm resistor has an SWR of 2, a cable long enough to invert the signal after the round trip will make it look like a 25 ohm resistor, also with an SWR of 2 but with inversion (a cable with a multiple of 1/4 wavelength would do the trick). Since the impedance looking into this black box is a function of the SWR and the cable length, it can be seen that intentionally mismatched lines can be used to transform one impedance into another. Notice that the 1/4 wave cable inverts the impedance and preserves the SWR. This impedance inversion may be used to match two impedances at a particular frequency by connecting them with a 1/4 wave cable with an impedance equal to the geometric mean of the two impedances. (The geometric mean is the square-root of their product.) A 50 ohm, 1/4 wave cable will match a 25 ohm source to a 100 ohm load : sqrt(25 x 100) = 50. Of course, it is not always easy to find the desired impedance cable!

Multiples of 1/2 wavelength will give enough delay that the reflection is not inverted and the impedance will be the same as the load. Such cables may be used to transfer the load impedance to a remote location without changing its value (at one frequency).

Other cable lengths will transform an impedance which differs from the cable's impedance with a reactive component. If the load is a lower impedance than the cable, a length below 1/4 wave will have an inductive component and above 1/4 (but below 1/2) wave a capacitive component. If the load is a higher impedance than the cable, the reverse is true. Above 1/2 wavelength, the reactance will alternately look capacitive and inductive in 1/4 wave multiples. This reactance will combine with the load's reactance and offers the possibility of resonating the reactive component of the load. Therefore, a cable with the "right" length and impedance can match a source and load with different resistance and reactance values. Obviously, these calculations can become quite involved and most engineers resort to a Smith chart, a computer program or perhaps the most common method, trial and error with a SWR meter or return loss bridge!

Our trial and error experiments with an external or internal transceiver SWR meter, suggest that the solution length is usually between 20 and 22 feet.  Therefore, considering for example that on 20 meters where this length is greater than a 1/4 wavelength, your input SWR is too high and your amplifier is presenting a load impedance of less than 50 ohms, this solution length will have a capacitive component. This would be true for 15 meters as well.

It is also possible that the longer length of coax acts as a "stub" and strips shield currents from the interconnecting cable which are fooling the internal SWR bridge, and therefore it eliminates power foldback.  In this case, a line isolator installed between the driver and the amplifier may also be of major benefit in eliminating the power foldback.

This solution is simple to achieve, and inexpensive as well.  Just a little patience is required when trimming the cable.  In the event that your transceiver does not directly read SWR, you may install a SWR bridge coupler onto the output connector of your transceiver to obtain the solution length of cable.

If your linear amplifier input circuits are tunable, you can finish up with adjusting the input coils for the absolute lowest input SWR on each band.  You will be pleased to see that after following this solution, your transceiver will be able to operate at an even lower level of drive to produce full amplifier output. "


Title: RE: Length of coax from xcvr to amp affecting input swr, why ?
Post by: G3UUR on January 09, 2012, 03:20:41 AM
Jim,

You certainly have the problem of fractional-cycle loading, anyone knows that.

Apparently not everyone!


Title: RE: Length of coax from xcvr to amp affecting input swr, why ?
Post by: G3UUR on January 09, 2012, 04:18:49 AM
There is considerable harmonic energy at the tube cathode, and regardless of Q, if the harmonics are not bypassed and prevented from reaching the exciter, they will show as reflected power on a directional coupler.

There is but you seem to only half understand the problem. Considerable fundamental reflected power occurs until the Q is high enough for the flywheel effect to stabilize the load to the driver. A high enough Q in this context is still very low, perhaps somewhere between 1 and 3. Nobody really knows yet because the designers of 1980's and onwards GG linear amplifiers didn't understand enough to ask the question and did things by trial and error rather than measuring what input network Q was required to just be able to get over the problem.

Once you've decided on a configuration to use for the matching network, the attenuation of the harmonics and Q are inextricably linked. If you increase the capacitor values of a CLC pi-network input matching circuit to increase the harmonic attenuation, you inevitably increase the Q. There's no way round that and if you know your theory you just accept that both are being increased if you're talking about increasing the Q of the network. The real question, which you "experts" still haven't answered is how little Q can we get away with and how does that differ for the various input matching circuits?

Quote
If you had a Q of 10 in a high-pass to the cathode, you could still have the harmonic energy issue in a grounded grid stage conducting less than 360 degrees.

Trust you to think of an absurd comparison, Tom!  No one would use this configuration, so why raise the issue. The rest of us are not talking about using a Q as high as 10 in any configuration, though we know it would provide the necessary stabilization of the load for the driver, because the operational bandwidth would be too narrow. In case you hadn't noticed, we're discussing where between 2 and 5 we'd find the best compormise for use with a solid-state driver. Above a Q of 2 the bandwidth between the 1.5:1 SWR frequencies no longer allows operation over the whole band on 75/80m, but there is still harmonic contamination in the reflected signal. How little can we get away with above this?

The second harmonic is probably only in the region of 7dB down at the cathode in the ideal case of the driver looking like a perfect voltage source. A CLC pi-network input circuit with a Q2 of 2 might give you 17dB attenuation in the ideal case, but would probably be lower by a couple of dB because the driver is not conjugately matched. You'd see a 1.2:1 SWR in this perfect case and much worse in practice.

A simple fix would be to use yet another low-pass filter bank between the amplifier input and the driver transceiver, but that would not address the problem of IMD impairment if that's happening as a result of increased non-linearity at the cathode/filament due to a higher than desirable source impedance. However, these filters would only need to be third order but their phase delay would need to be included in the number of 180 degree phase delays between the solid-state PA in the driver and the tube cathode to optimise the source impedance.

This should all have been addressed thirty years ago and shouldn't have been left until now. Someone has been derelict in their engineering duty and we know where to point the finger, Tom. It's all very well you criticizing other designers like Warren Bruene but he had very good theoretical knowledge for his day, unlike some of you modern designers. He also had to make do with limited test equipment compared with todays designers.

Jim, the figure given for the input impedance of a pair of 3-500Z tubes by Bill Orr is around 60 ohms, so it's not as high as the optimum output inpedance of the L4B matching networks would suggest. You've also got to be very careful comparing the input matching circuits of the different amplifier designs because of the reactance of the filament chokes and other circuitry around the input. Very often this reactance is a bit low at the low frequency end of the range and has to be tuned out by some of the output capacitance of the input network. So, unless you know the exact impedance looking into the cathode/filament on the various bands with the tubes off, you have to rely on the input values and the inductor values to give you a hint of what the output capacitance should be solely for matching to the tube input impedance.

Dave.


Title: RE: Length of coax from xcvr to amp affecting input swr, why ?
Post by: G3RZP on January 09, 2012, 04:25:44 AM
But the Q needed to have the harmonic current low enough for one transceiver not to object may well not be enough for another. So to cover all transceivers now and in the future may not be possible if a relaively wide band (eg 75/80) is to be covered.


Title: RE: Length of coax from xcvr to amp affecting input swr, why ?
Post by: G3UUR on January 09, 2012, 11:23:44 AM
But the Q needed to have the harmonic current low enough for one transceiver not to object may well not be enough for another. So to cover all transceivers now and in the future may not be possible if a relaively wide band (eg 75/80) is to be covered.

Yes, what's just over the threshold for one may well be under the threshold for another transceiver as far as SWR protection is concerned. Add to that the different phase delays of the different low-pass filter designs in use and you've got the possibility that the total phase delay with one length of cable will be wrong for another make of transceiver. When you're on the limit because you want the widest possible bandwidth, and are sailing close to the wind on Q and harmonic attenuation, a slight change in low-pass design could make the source impedance at the cathode just a little too high so it generates a second harmonic level that pushes the VSWR up over the top. Obviously, this will be more of a problem at the band edges where the VSWR is rising anyway because of the input matching network.   


Title: RE: Length of coax from xcvr to amp affecting input swr, why ?
Post by: VE7RF on January 10, 2012, 02:20:52 AM
There is considerable harmonic energy at the tube cathode, and regardless of Q, if the harmonics are not bypassed and prevented from reaching the exciter, they will show as reflected power on a directional coupler.

There is but you seem to only half understand the problem. Considerable fundamental reflected power occurs until the Q is high enough for the flywheel effect to stabilize the load to the driver. A high enough Q in this context is still very low, perhaps somewhere between 1 and 3. Nobody really knows yet because the designers of 1980's and onwards GG linear amplifiers didn't understand enough to ask the question and did things by trial and error rather than measuring what input network Q was required to just be able to get over the problem.

Once you've decided on a configuration to use for the matching network, the attenuation of the harmonics and Q are inextricably linked. If you increase the capacitor values of a CLC pi-network input matching circuit to increase the harmonic attenuation, you inevitably increase the Q. There's no way round that and if you know your theory you just accept that both are being increased if you're talking about increasing the Q of the network. The real question, which you "experts" still haven't answered is how little Q can we get away with and how does that differ for the various input matching circuits?

Quote
If you had a Q of 10 in a high-pass to the cathode, you could still have the harmonic energy issue in a grounded grid stage conducting less than 360 degrees.

Trust you to think of an absurd comparison, Tom!  No one would use this configuration, so why raise the issue. The rest of us are not talking about using a Q as high as 10 in any configuration, though we know it would provide the necessary stabilization of the load for the driver, because the operational bandwidth would be too narrow.


#####  any HP won't work anyway. And any LP config with a Q=10  would result in meltdown of the coils.


 In case you hadn't noticed, we're discussing where between 2 and 5 we'd find the best compromise for use with a solid-state driver. Above a Q of 2 the bandwidth between the 1.5:1 SWR frequencies no longer allows operation over the whole band on 75/80m, but there is still harmonic contamination in the reflected signal. How little can we get away with above this?

The second harmonic is probably only in the region of 7dB down at the cathode in the ideal case of the driver looking like a perfect voltage source. A CLC pi-network input circuit with a Q2 of 2 might give you 17dB attenuation in the ideal case, but would probably be lower by a couple of dB because the driver is not conjugately matched. You'd see a 1.2:1 SWR in this perfect case and much worse in practice.

A simple fix would be to use yet another low-pass filter bank between the amplifier input and the driver transceiver, but that would not address the problem of IMD impairment if that's happening as a result of increased non-linearity at the cathode/filament due to a higher than desirable source impedance. However, these filters would only need to be third order but their phase delay would need to be included in the number of 180 degree phase delays between the solid-state PA in the driver and the tube cathode to optimise the source impedance.

This should all have been addressed thirty years ago and shouldn't have been left until now. Someone has been derelict in their engineering duty and we know where to point the finger, Tom. It's all very well you criticizing other designers like Warren Bruene but he had very good theoretical knowledge for his day, unlike some of you modern designers. He also had to make do with limited test equipment compared with todays designers.

Jim, the figure given for the input impedance of a pair of 3-500Z tubes by Bill Orr is around 60 ohms, so it's not as high as the optimum output impedance of the L4B matching networks would suggest. You've also got to be very careful comparing the input matching circuits of the different amplifier designs because of the reactance of the filament chokes and other circuitry around the input. Very often this reactance is a bit low at the low frequency end of the range and has to be tuned out by some of the output capacitance of the input network. So, unless you know the exact impedance looking into the cathode/filament on the various bands with the tubes off, you have to rely on the input values and the inductor values to give you a hint of what the output capacitance should be solely for matching to the tube input impedance.

Dave.

## OK Dave. Here's what I don't get. Are folks who use say an AL-82 [2 x3-500Z's]  with various cable lengths also getting swr problems..when using say a MK-V  or a 1000-D  ??? Yes, I realize the fil chokes xl, etc can affect the input Z somewhat, but it baffles me why I'm the only one.."with the problem".  I'll bet anything if folks used 6-10' of coax from xcvr to amp, they would be screaming blue murder.

## I see that Ameritron makes a standalone set of switchable low pass filters [400w CCS cxr/600w cw/1 kw pep ssb]  called an "ARF-1000".  It has 6 x cut off freqs.  IF you installed this unit right at the INPUT of the L4B, I wonder if that would alleviate the problem?  At least, located there, their would be virtually next to no cable at all, or at the very most, maybe 1-2'.[between standalone LPF's and the input of the amp..and perhaps 6' between xcvr and input of stand alone LPF's. ]. Ameritron list NO attenuation or other specs for the arf-1000. On the front panel, it's called a "ALS-600FB".


###  I tried both versions of a T network [ 2 x caps + coil..and also 2 x coils + a cap] and both were a dead loss for a tuned input. The only thing that works is the PI [CLC]. This is on my hb amps.  I also noticed that some folks are using a PI-L  for a tuned input on monoband 10m amps, but that's another story.

##  If we cut to the chase, Q's between 2-5 will work...and a Q of 3 works great on the hb amps... PROVIDED  BOTH caps are fully adjustable.  Since I can adjust both caps  from the front panel on  the hb amps, BW is NOT an issue....and neither is cable length.  The 2 x broadcast caps + tapped 4 uh coil has to be the simplest, fool proof tuned input I have cooked up so far.  It solved all my problems in one fell swoop.  Sure, using a mess of arco's and T-50's will also work..up to a point.  I tried that route b4, then had problems with the various relay contacts, + the tiny coils over heating on the upper bands etc.  The broadcast caps + tapped 6 ga, 4 uh coil was attempted after a friend had tried it on a 2 x 4-1000 amp [80-15m], with great success. My buddy got the idea from the very last edition of Orr's book.  Orr had used the exact same scheme, but it was for a 160m monoband amp..and also used in another orr project, for a 80m monoband amp.  Orr advised NOT  making it into a tapped coil, multiband config. He claimed it would turn the amp into an osc, if tuned wrong, like to the 2nd harmonic etc.  Such is not the case these days, esp with spread sheets + the mfj-259B + 50 ohm plug test set up.  The pair of broadcast caps + tapped coil config is 100% reliable + repeatable.  It's now been used on everything from 3CX-3000A7's  to YU-148's to  pairs of 4-1000's, GS35B's and a lot of other bigger triodes like 3x10's and 3x15's, etc.   The drawback of course is it can't be retrofitted into 99% of existing GG amps, due to the space it eats up.

##  The standalone ameritron arf-1000 lp filter MIGHT work.  Plan B might be to use a 23' piece of 213-U..and try that on all bands..then shorten it a few inches at a time, then test on all bands.

##  Id still like to see somebody who has a yaesu mk-V or 1000-D, used in conjunction with older amps..or even any of the AL-82/1200/1500 amps [ which all use slug tuned pi nets]..and with random cable lengths tell us they have no input swr problems on any bands.

Later..Jim  VE7RF


Title: RE: Length of coax from xcvr to amp affecting input swr, why ?
Post by: G3UUR on January 10, 2012, 04:06:46 AM
##  The standalone ameritron arf-1000 lp filter MIGHT work.  Plan B might be to use a 23' piece of 213-U..and try that on all bands..then shorten it a few inches at a time, then test on all bands.

##  Id still like to see somebody who has a yaesu mk-V or 1000-D, used in conjunction with older amps..or even any of the AL-82/1200/1500 amps [ which all use slug tuned pi nets]..and with random cable lengths tell us they have no input swr problems on any bands.

Jim,

I think starting off with a length of cable longer than you need and gradually cutting it down is your best option. If you can hit the multiple of 180 degrees on most bands with one length that would be a lot more convenient and cheaper than any of the other options.

I agree, it's odd that more people haven't noticed this problem but, possibly, most amateurs are using very short lengths of connecting cable and have fortuitously found a small multiple of 180 degrees, or close, in their total filter and cable phase delays by chance.

If you ask around on the air, you might eventually find someone with one of the Ameritron GG amps who can tell you how they perform with longer lengths of connecting cable. There must be somebody out there who has had the necessity to use longer than 3' of cable between the transceiver and amp.

I asked one guy I know who uses an SB220 with a K3. Unfortunately, he has a band-pass filter between the K3 and amp because he's a contester and works several bands simultaneously with other transceivers and amps, so he couldn't throw any light on the problem.

I know you've reported problems with heating in the input filter because of high circulating currents at modest values of Q when using your HB amp. I think you said that uses a 3CX3000, which probably has an effective input impedance at or below 50 ohms. A Q of 10 would certainly give you circulating current problems at that impedance, but there no reason why you shouldn't use a broadband transformer right at the cathode to step the input impedance up to around 200 ohms. That would reduce the circulating current at that Q and give you more convenient values of C1 and C2. A tightly coupled bifilar transformer should minimise the leakage inductance presented at the output and fairly faithfully transform the non-linear current waveforms demanded by the cathode. I'm not suggesting you change to this configuration, but it would be one way of operating at a Q of 10 without incurring too much loss in the input matching network, if you ever wanted to. It would require pretty heavy gauge wire though.

73, Dave.       


Title: RE: Length of coax from xcvr to amp affecting input swr, why ?
Post by: W8JI on January 10, 2012, 03:56:48 PM
I just ran an AL811, and AL80B, and an AL1200 with the 3CX1200Z7 tubes with my stepped length 50 ohm line I use for stability testing, and there is no significant SWR change on my radio's SWR meter and an external meter at the radio as I step through various line lengths from ~5 electrical feet to 67 feet in one foot steps on ten or twenty meters.

This was a 3 foot jumper (foam RG58 style), and the test line (small heliax) that adds electrically 1 foot to 64 feet in one foot lengths by adding lines of 1,2,4,8, 16, and 32 feet through a relay switch.

I've never once in my life found an amplifier with adequate input filtering that has a SWR problem with line length.

The AL811 mule I tested is not neutralized, and I removed the 200 ohm input swamping resistor.

On ten meters a bridging tap showed harmonics on the 811 mule were worse of the lot, at -24 dB fundamental signal for the second. It also had the largest wiggle in reflected power as line length was changed, but it was under 1.2:1. The 1200Z7 was -34 dBc 2nd harmonic, and most stable with line length.

The radio alone was in the -60dBc harmonic range, and the amp made the second harmonic increase 36 dB.

If you make sure the radio has an adequate low pass or band pass, and if the SWR meter and line have the same impedance, the problems will go away. At about -15dBc harmonics, line length becomes a real issue.

added text in bold

You can fix a 30L1 by neutralizing it, and increasing Q of the input filters, and grounding the grids properly. I'm not sure about the SB220 because I have not looked at one for this problem. When the SB220 was around, and similar amps, most transmitters were tube types. They had no sensitivity to harmonics from the PA stage back-feeding the exciter.

By the way, if you add the phase shift in the 30L1 network to line length, you will find it is not the value claimed by Collins. It is all over the place from band-to-band. If you neutralize the 30L1 and make it stable, and use adequate input Q and harmonic filtering, the line length sensitivity on the input vanishes.

73 Tom
  


Title: RE: Length of coax from xcvr to amp affecting input swr, why ?
Post by: W5LZ on January 12, 2012, 04:15:06 PM
The very simple answer to the original question is that the output impedance of the radio is not the same as the input impedance of the amplifier.  Or, if makes you feel better, the input impedance of the amplifier is not the same as the output impedance of the radio.
How you go about correcting that has a lot of answers specific to the devices in use.
 - Paul


Title: RE: Length of coax from xcvr to amp affecting input swr, why ?
Post by: VE7RF on January 13, 2012, 03:58:01 AM
The very simple answer to the original question is that the output impedance of the radio is not the same as the input impedance of the amplifier.  Or, if makes you feel better, the input impedance of the amplifier is not the same as the output impedance of the radio.
How you go about correcting that has a lot of answers specific to the devices in use.
 - Paul


##  You just missed the entire point.  The amp is 50 ohms... but only for 210 deg of each 360 deg cycle.  During the other 150 degs the tube is NOT conducting , the Z is sky high.   The problem, imo is not that the  Q of the tuned input isn't high enough.  The real problem is you only have one thing to tweak..and that's the tunable slug !   That design is flawed.  You need adjustable C1+C2 caps......then just tweak em for flat swr..problem solved.  In the old days, none of the american built radios even had swr meter's built into them. And if you did own a wattmeter, you installed it on the OUTPUT of your amp...and not between xcvr + input of amp.

##  Just for a laugh, I'm going to try installing the Ameritron ARF-1000  switchable LP filter's...right at the input coax on the L4B and see what happens.  They appear to be the 5 x LP filters  from the ameritron AL-600.  They put them into a box, with a bandswitch  on the front of the box..and a pair of so-239's on the rear panel.  They rate em for 1 kw pep on ssb...600w on cw..and 400 w cxr/fm/rtty.  $160.00   for  5 x filters in one box is a pretty good price.  I couldn't build it for that money.   On Bypass mode, the most it will ever see is the full 200 watts from the xcvr.  Whether the L4B is in bypass or not...the ameritron switchable LP filters will always be in line..rx and tx. It has cut-offs of 2.3, 4.4, 8.0, 14.5, 21.6, and 30 MHz.  So that will easily handle the entire spectrum from 0-30 mhz.

## I'll let folks know if this  works or not.  I will also measure the insertion loss of these filters.

Later... Jim  VE7RF