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Author Topic: Length of coax from xcvr to amp affecting input swr, why ?  (Read 37372 times)

Posts: 119

« Reply #45 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.   

Posts: 212

« Reply #46 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?

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.


## 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  Huh 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

Posts: 119

« Reply #47 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.


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.       

Posts: 9748


« Reply #48 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
« Last Edit: January 11, 2012, 04:37:53 AM by W8JI » Logged

Posts: 477

« Reply #49 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

Posts: 212

« Reply #50 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
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