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Author Topic: External antenna tuners and ZS6BKW and G5RV antennas  (Read 6892 times)
W8JI
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« Reply #15 on: September 17, 2012, 05:40:53 AM »

He said his mast was 45 feet high. The G5RV feeder is about 30 feet.

How lossy can 15-30 feet of coax required to reach a tuner be?
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W5DXP
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« Reply #16 on: September 17, 2012, 06:36:57 AM »

He said his mast was 45 feet high. The G5RV feeder is about 30 feet. How lossy can 15-30 feet of coax required to reach a tuner be?

I must be confused. He said, "Id need right at 100 feet of ladderline to feed down the 45ft mast and then to the corner of the house the equipment is in." So I assumed he would need ~70 ft. of coax if he uses a standard G5RV. I assumed an "external antenna tuner" means not built into the transceiver but still located in the shack. I guess I need a line diagram of the actual system being discussed. Of course, an autotuner located at the twinlead/coax junction of a G5RV would solve the problem of high SWR on the coax.
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
AE5JU
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« Reply #17 on: September 17, 2012, 10:55:07 AM »


Or put up a resonant fan dipole with coax and skip the tuner altogether. Works the same either way except that even the finest tuners introduce loss. There's a reason that people like resonant multiband antennas...

My first antenna, actually modified from my SWL antenna, was a multiband dipole, with elements for 75, 40, and 20 meter bands.  And I can work those bands without a tuner.

BUT, I have to disagree about skipping the tuner.  With a tuner I can also work 17, 15, and 10 meters. 

I have both manual and automatic tuners.  I can't say that manual tuners will tune better than the LDG AT-1000 and IT-100 automatic tuners I have.  Basically, if the LDGs won't tune, you can't get a match with the manual tuners, either.

BTW, I'm still using the multiband dipole.

73,
Paul - AE5JU
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AK4YA
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« Reply #18 on: September 17, 2012, 12:12:11 PM »

<SNIP>
From reading what you say and ask, it seems like you may (like many or most Hams) misunderstand SWR and what it does.

73 Tom

OK, lets discuss this:

From what I understand, a tuner's VSWR reading is for the link between the tuner and amp.  Is the reflected power figure a measurement of reflected power from the antenna to the tuner?  Or is it what the tuner is reflecting back to the amp?

These questions are why I was asking about balanced meters, because Im interested in data from BOTH sides of the tuner.  As to your statement about ladder line being operated with terrible SWR but little loss..  I never understand that concept until just now after making up an excel spreadsheet and seeing for myself.  Wow.  So now Im off so search for VSWR data for all bands on the 102' and 135' all band doublets.

Last ?:  For a coax-fed antenna that is mistuned for TX operation to the figure of 90:1 at the antenna using coax with 1.6dB of line loss, how bad will it be for receiving signals?  I understand Ill see at least 1.6bB of loss, but am wondering about mismatching in regards to RX operation.
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N4CR
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« Reply #19 on: September 17, 2012, 01:17:14 PM »

Last ?:  For a coax-fed antenna that is mistuned for TX operation to the figure of 90:1 at the antenna using coax with 1.6dB of line loss, how bad will it be for receiving signals?  I understand Ill see at least 1.6bB of loss, but am wondering about mismatching in regards to RX operation.

1.6 db of loss at 1:1 SWR? At what frequency?

You want this: http://vk1od.net/calc/tl/tllc.php

Let's take a stab at it though.

I found Belden 9914 at 344 feet had 1.6 db loss at 1:1 match at 14.175Mhz.

At 90:1 it had 12.59 db loss which means 5.5% of your transmitted power made it to the antenna feed point. (Load end VSWR - bad case Zload)

The rest turned into heat in the coax.

It doesn't matter if you are receiving or transmitting if the load on both ends is the same. If the feed point of the antenna is assumed to be 50 ohms for a perfect match and the impedance of the receiver input connector is assumed to be 50 ohms for a perfect match, the loss is the same in both directions.

Even the loss to receive is turned into heat in the coax. It's just so small it's imperceptible.
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73 de N4CR, Phil

We are Coulomb of Borg. Resistance is futile. Voltage, on the other hand, has potential.
WB6BYU
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« Reply #20 on: September 17, 2012, 01:25:50 PM »

Quote from: AK4YA

From what I understand, a tuner's VSWR reading is for the link between the tuner and amp.  Is the reflected power figure a measurement of reflected power from the antenna to the tuner?  Or is it what the tuner is reflecting back to the amp?



SWR meters in tuners are designed to read the SWR between the radio and the tuner,
because that is where you are trying to minimize it.

If you want to see the SWR on the feedline itself, you can install the tuner backwards,
put an external meter after the tuner, or just switch the tuner to BYPASS and feed the
antenna directly.

The SWR at the end of the feedline is useful to know when setting up an antenna, but
not necessarily something to check every time you readjust the tuner.

Meter accuracy is going to limit the resolution to which you can read high SWR:  there may
be some that can measure a 10 : 1 SWR accurately, but it gets much more difficult
beyond that.

Quote

As to your statement about ladder line being operated with terrible SWR but little loss..  I never understand that concept until just now after making up an excel spreadsheet and seeing for myself.  Wow. 



The VK1OD transmission line loss calculator does this for you.


Quote

So now Im off so search for VSWR data for all bands on the 102' and 135' all band doublets.



You really want impedance data, not just SWR.

Something like this:  http://vk1od.net/antenna/G5RV/


Quote

Last ?:  For a coax-fed antenna that is mistuned for TX operation to the figure of 90:1 at the antenna using coax with 1.6dB of line loss, how bad will it be for receiving signals?  I understand Ill see at least 1.6bB of loss, but am wondering about mismatching in regards to RX operation.



In practice losses are generally similar for RX and TX, but the mechanisms are somewhat
different.  On receive we can provide a 50 ohm receiver input so the SWR is 1 : 1 on the
coax.  This holds the coax losses to 1.6dB in your example.  But let us assume that the
antenna is 5000 ohms (SWR = 100 : 1) and, for the sake of illustration, there is a very short
length of 5000 ohm feedline between the antenna and the coax connection.  On this piece
of imaginary feedline the SWR is again 100 : 1 because it sees a 50 ohm load.  The power
from the antenna flows though this 5000 ohm feedline and hits the 50 ohm of the coax.  
What happens?  Most of the power is reflected back up to the antenna.  Since the antenna
matches our 5000 ohm feedline, that reflected power is all dissipated in the load (re-radiated
from the antenna, actually.)  We can let the length of our 5000 ohm transmission line go
to zero, and assign no loss to it, and have the same result in the real world.

[Brief pause for math:  return loss in dB is given by -20 * log ( (VSWR - 1) / (VSWR + 1 ) ).  
For an SWR of 100 : 1, the return loss is 0.17dB.  That means that the reflected wave is 0.17dB
weaker than the forward wave.  The amount power delivered to the 50 ohm load is the reciprocal
of that, or about 5.75dB down from the incident wave.]  So the loss due to antenna mismatch
is about 6dB.]

You then add the coax loss (1.6dB in your example) and find that on receive signals are
over 7dB weaker due to the antenna mismatch.  That compares with ~13dB loss on
transmit:  not the same, but higher than one might expect if you only looked at the feedline
losses when the receiver input stage perfectly matched the transmission line impedance.
Assuming, of course, that you are not using a tuner in the shack, as that would change
the SWR on the coax in receive mode.



Let me add a disclaimer:  this is the first time I've run such a calculation, and while it seems
like it is based on solid transmission line theory, it wouldn't surprise me at all if someone pokes
a hole in it.  But it is an excellent question, and I've never seen a good explanation of it.


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VE3FMC
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« Reply #21 on: September 17, 2012, 03:11:25 PM »

One word of advice I did not see in any posts. (Or I missed it)

If you use ladder line you can not attach the ladder line to the mast. You must have it standing off the metal mast by at least 1 foot, more if possible. I stand it off from my tower by close to 30 inches.

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AK4YA
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« Reply #22 on: September 17, 2012, 04:17:44 PM »

OK everyone, I think you all have got me squared away here.  Im making a .xls version of the vk1od tool.
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AK4YA
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« Reply #23 on: September 17, 2012, 06:01:18 PM »

OK everyone, I think you all have got me squared away here.  Im making a .xls version of the vk1od tool.

well, squared away except for the part about reverse power meters on tuners and if they measure reverse power from the antenna to the tuner or from the tuner to the amp?

As far as a true balanced tuner vs a pi network with a balun, besides the johnson, what else is there?  I see the johnson advertises 15dB of harmonic attenuation, which is a big + for me.  is this inherent to true balanced tuners or just something extra built into the johnson?  I ask because my amp does not include a LPF and this is something else I'm looking around for.  I see MFJ has a switchable one but if I can get a tuner with one, the more the better.
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W5DXP
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« Reply #24 on: September 17, 2012, 06:12:01 PM »

If you use ladder line you can not attach the ladder line to the mast. You must have it standing off the metal mast by at least 1 foot, more if possible.

One foot is good but 3-4 times the distance between the parallel wires is closer to the minimum spacing. I use 6" electric fence standoffs.
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
N4CR
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« Reply #25 on: September 17, 2012, 06:31:54 PM »

I ask because my amp does not include a LPF and this is something else I'm looking around for.  I see MFJ has a switchable one but if I can get a tuner with one, the more the better.

Why are you looking for a low pass filter?
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73 de N4CR, Phil

We are Coulomb of Borg. Resistance is futile. Voltage, on the other hand, has potential.
AK4YA
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Posts: 106




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« Reply #26 on: September 17, 2012, 06:44:08 PM »

I ask because my amp does not include a LPF and this is something else I'm looking around for.  I see MFJ has a switchable one but if I can get a tuner with one, the more the better.

Why are you looking for a low pass filter?

My solid state amp has no LPF of LPF switching circuitry, so harmonic suppression is something I have to address.  From what I understand, if a T network tuner is designed as LCL then it should offer the suppression I need.  Now Im still not 100% sure of my abilities to read schematics, but the MFJ balanced tuner appears to be CLC based.  http://www.mfjenterprises.com/pdffiles/MFJ-974HB.pdf  Correct me if Im wrong.  I also believe that pi based tuners do offer this suppression, so if this is the case, perhaps a pi tuner with a balun is the way I need to go?
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WB6BYU
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« Reply #27 on: September 17, 2012, 06:46:17 PM »

Quote from: AK4YA

...I ask because my amp does not include a LPF...



If your amp is properly designed and built it SHOULD have sufficient attenuation of
harmonics that you don't need a separate low pass filter.  Typically a low pass filter
has a cutoff frequency around 35 to 40 MHz, enough so that harmonics won't
interfere with TV channels 2 - 6.  But that doesn't help reduce harmonics of, say,
3.75 MHz, which will appear at 7.5, 11.25, 15, 18.75 MHz, etc., since all of these
frequencies are still below the cutoff.  Your amp really needs to have switched filters
for the different bands to provide sufficient rejection of spurious outputs (whether
generated by the amp or the driver.)

Then, as long as those networks provide sufficient attenuation out of band, you
don't need a separate low pass filter.

A CLC "T" tuner is generally considered a high-pass design, not a low-pass. 
A CLC "pi network" tuner is low-pass, but much more difficult to implement in a practical
design because it requires larger capacitors to match low impedances.


Besides, it isn't good practice to depend on an antenna tuner to reduce out of band
signals.  Sometimes you might feed it into a matched antenna that doesn't need a
tune.  Also, filters require a proper termination impedance to work properly, and
you can't count that the impedance of the antenna at the harmonic is such that
the circuit gives the expected attenuation when it is matched at the fundamental.
So make sure that your transmitter + amp is clean and meets the standards for
amateur equipment, then any additional harmonic attenuation you get from a
tuner is a bonus.

You really don't want to put an amp on the air without proper filtering - that's not
a good design, even if everything else about it is state of the art.


Quote

...reverse power meters on tuners and if they measure reverse power from the antenna to the tuner...


I thought I had explained this:  they measure the SWR (reflected power) between the tuner
and the rig.  Unless, of course, the tuner is connected backwards, or switched to bypass mode,
in which case they measure the SWR on the feedline between the tuner and the antenna.
« Last Edit: September 17, 2012, 06:48:59 PM by WB6BYU » Logged
AK4YA
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« Reply #28 on: September 17, 2012, 07:59:35 PM »

SNIP

understood.  looks like an ameritron ARF-1000 switchable filter.  And a MFJ-974HB if I cant talk myself into getting an LDG auto with the appropriate balanced setup
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N4CR
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« Reply #29 on: September 18, 2012, 02:02:41 AM »

SNIP

understood.  looks like an ameritron ARF-1000 switchable filter.  And a MFJ-974HB if I cant talk myself into getting an LDG auto with the appropriate balanced setup

With 300 watts, you'll burn that tuner up.
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73 de N4CR, Phil

We are Coulomb of Borg. Resistance is futile. Voltage, on the other hand, has potential.
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