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Author Topic: My ignorance is showing, antenna tuner  (Read 2631 times)
W9KDX
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Posts: 770




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« on: March 10, 2013, 11:37:57 AM »

Sorry, probably a stupid question but numerous posters point out that an antenna tuner doesn't change the SWR at the antenna, just in the shack. So....

When I use a tuner, my reception improves.  Will I also have an improvement in the signal I transmit?  My guess is that because the transmitter has a better impedance match, it will deliver more to the antenna, even though the antenna does not have any real difference than it did before the tuner was used.

If you need the info, the antenna already has SWR on all bands used lower than 2:1.

Thanks
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Sam
W9KDX
AA4PB
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Posts: 12779




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« Reply #1 on: March 10, 2013, 11:51:21 AM »

The tuner can only change the SWR on the piece of coax between the tuner and the radio. It can't change the SWR on the coax between the tuner and the antenna.

If your feed line has an SWR of less than 2:1 without the tuner then you shouldn't notice any significant difference on receive or transmit signals with or without a properly adjusted tuner. Of course if the tuner is mistuned then it will reduce the received signal so you can hear a significant change by adjusting the tuner. You shouldn't hear a big change when switching a properly adjusted tuner in and out of the line however - if the SWR on the feedline is really under 2:1.
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AC5UP
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« Reply #2 on: March 10, 2013, 12:01:39 PM »

A perfectly matched system where the radio antenna jack, transmission line, and antenna impedance are all equal will provide the lowest loss and best overall transmission efficiency (minus the inevitable transmission line Ohmic loss due to imperfect conductors). It should be noted that perfection is an unnatural state rarely obtained and difficult to maintain.

But, it's a two way street... The transmitter delivers maximum power to the antenna, the antenna delivers maximum signal to the receiver.  Given a 2:1 measured SWR I'm surprised you can hear a difference through the tuner. A mismatch at 2:1 is low enough to be considered insignificant. If your RX is improved it's likely your TX is improved as well since both signals travel the same path.

As a general rule, adding complexity to a system will have a point of diminishing returns. The matching network has some loss and does present another set of knobs to adjust. But, if it works better, it's better. I have a hunch your SWR might be a little greater at some point than what you measured.   Wink
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KD0REQ
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Posts: 896




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« Reply #3 on: March 10, 2013, 12:10:15 PM »

fun physics question:  if you use a tuner to provide a 1.01/1 match to a transciever, and the antenna remains a 5:1 bad match, where do the extra numbers go that aren't on the meter any more?  you can't create or destroy energy, after all, you can just transform it with some losses in the translation.

fun physics answer:  the rest warms the shack.  the antenna doesn't get any better, the tuner just allows the lumped load to accept your transmit power.  more power into a bad antenna?  nope, the antenna is still a 5:1 behind the curtain.  all that additional power is dissipated in heat within the tuner.

which is why people kvetch about tuners arcing and blowing up when "I only have 100 watts, and the tuner is rated at 300."

you receive better because you have a better match, and the microvolts come to you... but not as many as if you had a perfect dipole on tune for that frequency.  you transmit better, albeit a couple dB down, because your watts are not burning up in the feedline and in the final stage of the transceiver.  you limit burning up watts at the tuner, but you can load it better, so there is sum total more amps in the antenna.

there is still no Special Freakin' Magic here.  you reduce the losses and move most of the waste into the tuner.
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N6GND
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Posts: 354




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« Reply #4 on: March 10, 2013, 12:29:58 PM »

I usually adjust my tuner for stronger receiver signals and/or background noise as a first step in my tuning procedure for transmit. Then when I transmit, I have only a very minor tweak for optimum transmit tuning.

That said, I usually switch my tuner to bypass when I am just listening. The signal levels are a bit below optimum like this; if not in bypass, reception through the untuned tuner is significantly poorer than in bypass.

The bottom line, I think, is that for reception the most significant factor is signal/noise so that using the tuner for best strongest signals and/or noise is mostly not necessary. The best use of the tuner for reception remains for pretuning before transmit.
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NO2A
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Posts: 768




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« Reply #5 on: March 10, 2013, 12:34:37 PM »

Sorry, probably a stupid question but numerous posters point out that an antenna tuner doesn't change the SWR at the antenna, just in the shack. So....

When I use a tuner, my reception improves.  Will I also have an improvement in the signal I transmit?  My guess is that because the transmitter has a better impedance match, it will deliver more to the antenna, even though the antenna does not have any real difference than it did before the tuner was used.

If you need the info, the antenna already has SWR on all bands used lower than 2:1.

Thanks
Sam,While the 2:1 match isn`t bad at the antenna end,the problem is with modern rigs seeing any swr that cuts back your power output. For that reason,using the tuner will increase your signal heard because it will enable your rig to put out 100 watts or close to it. It also means your rig will run cooler,very important with today`s rigs.
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AA4PB
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Posts: 12779




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« Reply #6 on: March 10, 2013, 12:42:09 PM »

The tuner does not simply convert extra watts into heat. It provides a proper impedance match between the 50 Ohms transmitter output and whatever impedance appears at the antenna feedline input. That permits the transmitter to couple maximum power into the antenna feedline. The feedline still has the same SWR as it did before and it still has the same loss in terms of dB as it did before. That loss is determined by the matched loss of the coax cable and the SWR.

The loss in a good quality tuner operated within its specified impedance range will have a reasonably small amount of loss. Most of the system loss will occur in the feedline between the tuner and the antenna as determined by the mismatch between the feedline and the antenna.
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KG6AF
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Posts: 345




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« Reply #7 on: March 10, 2013, 01:10:52 PM »

fun physics question:  if you use a tuner to provide a 1.01/1 match to a transciever, and the antenna remains a 5:1 bad match, where do the extra numbers go that aren't on the meter any more?  you can't create or destroy energy, after all, you can just transform it with some losses in the translation.

fun physics answer:  the rest warms the shack.  the antenna doesn't get any better, the tuner just allows the lumped load to accept your transmit power.  more power into a bad antenna?  nope, the antenna is still a 5:1 behind the curtain.  all that additional power is dissipated in heat within the tuner.

That might be true if you have a lousy tuner, but in general it's not correct.  If you've got a 5:1 SWR with a long run of lossy coax, you'll dissipate some power in the coax.  If you run open-line feeder, you'll probably dissipate very little power in the feedline, and most of the power will be radiated.  As far as the tuner is concerned, if it's designed using components with low ohmic losses, it should dissipate relatively little power.

Tuners arc or heat up because (a) the means they use to perform impedance transformation can result in very high voltages and currents and (b) some tuners are marginally designed, and can't deal with those voltages and currents.

I'm not sure where the "tuners are just power wasters" meme came from, but it sure is persistent.
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M0HCN
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Posts: 473




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« Reply #8 on: March 10, 2013, 01:41:27 PM »

I was tempted to sell a load of 6dB high power attenuators I had as 'instant tune auto tuners' down at the local rally, they would guarantee at least 12dB return loss on all bands into any aerial.......

I have never quite understood rating tuners in terms of power, they are really rated in terms of voltage and current, and depending on the magnitude and phase of the load this can have a non obvious relationship to power.
Much better to rate a tuner as 500V/10A or whatever, which in a pure resistive load would be 5KW, but will be a fraction of that into say 30+40j. Even rating the things in VA rather then watts would be an improvement.

73 Dan.
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WB6BYU
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Posts: 13143




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« Reply #9 on: March 10, 2013, 01:52:51 PM »

Quote from: KG6AF

I'm not sure where the "tuners are just power wasters" meme came from, but it sure is persistent.




They may be under the mistaken impression that it dissipates the reflected power so it
doesn't show on the SWR meter.

Typical tuners can be inefficient in some cases, particularly with low impedance lads on 80m and
especially 160m where the capacitors are smaller than optimum.  But generally the efficiency is
pretty good, especially when adjusted to use maximum capacitance.  There is a simple test for
the amount of power dissipated in your tuner:  have an extended QSO at full power, open the
case, and use a digital temperature sensor (e.g. your finger) to see how warm the coil
is.  With 100W, you should be able to detect significant warming even at 90% efficiency.  I have
seen coils get so hot that the connections unsoldered themselves, but usually the warming
is barely detectable, meaning tuner losses must be small.  Compare the temperature of the
coil to, say, a 40W light bulb:  if the coil isn't as hot as the bulb, you aren't dissipating 40W in
the tuner.



Back to the original question:  If your rig already puts out full power into the 2 : 1 SWR, then
adding a tuner to reduce it won't make any significant improvement.  If your rig reduces output
power due to high SWR at that point, then the tuner will allow you to increase the power out
of the radio.
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W5DXP
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Posts: 3553


WWW

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« Reply #10 on: March 10, 2013, 02:53:51 PM »

When I use a tuner, my reception improves.  Will I also have an improvement in the signal I transmit?

It depends on how efficient your transmitter is in the presence of a slight mismatch vs how efficient your tuner is into that same mismatch. This is one of the questions that can be answered by putting a dual needle SWR meter on the output of the tuner and trying both cases with (1) a properly adjusted tuner vs (2) the tuner in bypass mode. Although the SWR will be the same in both cases, the forward power will most likely be greater in one case than in the other. When maximum power is being delivered to the antenna, the forward power reading will be maximum .
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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.
W9KDX
Member

Posts: 770




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« Reply #11 on: March 11, 2013, 09:33:43 AM »

When I use a tuner, my reception improves.  Will I also have an improvement in the signal I transmit?

It depends on how efficient your transmitter is in the presence of a slight mismatch vs how efficient your tuner is into that same mismatch. This is one of the questions that can be answered by putting a dual needle SWR meter on the output of the tuner and trying both cases with (1) a properly adjusted tuner vs (2) the tuner in bypass mode. Although the SWR will be the same in both cases, the forward power will most likely be greater in one case than in the other. When maximum power is being delivered to the antenna, the forward power reading will be maximum .

While I love the theoretical answers, I love even more the practical.  Something so simple, and I missed it.

I got the tuner because even at 2:1 the Ameritron 811H I have had a terrible time tuning up on 40 meters with my OCF dipole.  After the tuner, the problem was gone.  Everything was adjusted on the 811H, it just doesn't like SWR approaching 2:1 and this was confirmed  last week when I tried to tune up on 40M and one of the Antenna knob was 1 number off.  Sure enough, the amp was acting oddly.

I was just curious as to whether there was any point to tuning up on my fan antenna where all the SWR readings are already virtually 1:1.  Now I can just look with and without the tuner and measure the output.

Thanks guys
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Sam
W9KDX
WB6BYU
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Posts: 13143




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« Reply #12 on: March 11, 2013, 12:58:36 PM »

Quote from: W9KDX

I got the tuner because even at 2:1 the Ameritron 811H I have had a terrible time tuning up on 40 meters with my OCF dipole... 



That's interesting:  usually a tube amp like that has enough range in the
output tuning controls (essentially a built-in antenna tuner) to accommodate
a 2 : 1 SWR.  It might be that it is running a bit low on output capacitance,
especially if the load impedance happens to be on the low side (25 ohms). 
If so, then adding a quarter wave of coax (20 to 25 feet) should shift the load
impedance to 100 ohms, which should be much easier for the amp to match. 
And will be much cheaper than a tuner.

(Of course, with an OCFD the feedpoint impedance may shift when you change
the coax length due to the effect of common mode currents.)
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W9KDX
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Posts: 770




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« Reply #13 on: March 11, 2013, 03:43:55 PM »

Quote from: W9KDX

I got the tuner because even at 2:1 the Ameritron 811H I have had a terrible time tuning up on 40 meters with my OCF dipole... 



That's interesting:  usually a tube amp like that has enough range in the
output tuning controls (essentially a built-in antenna tuner) to accommodate
a 2 : 1 SWR.  It might be that it is running a bit low on output capacitance,
especially if the load impedance happens to be on the low side (25 ohms). 
If so, then adding a quarter wave of coax (20 to 25 feet) should shift the load
impedance to 100 ohms, which should be much easier for the amp to match. 
And will be much cheaper than a tuner.

(Of course, with an OCFD the feedpoint impedance may shift when you change
the coax length due to the effect of common mode currents.)

Well, I am dropping the feed line directly at 90 degrees so hopefully I am avoiding as much common mode current as possible.  I'll dig out the analyzer and see what the impedance is at the end of the coax and double check the SWR.  The coax length is close to 25 feet between the line outside and inside.  If I find a particular impedance  other than 50 ohms can cutting or lengthening improve the 40M mismatch without changing 10 and 20 meters?  If so, which way would I go?
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Sam
W9KDX
WB6BYU
Member

Posts: 13143




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« Reply #14 on: March 11, 2013, 04:04:55 PM »

Quote from: W9KDX

Well, I am dropping the feed line directly at 90 degrees so hopefully I am avoiding as much common mode current as possible. 



Not sure that is going to help:  the problem is that the load is unbalanced, so
the two sides of the coax see different impedances.  It may be possible to
reduce common mode current with one or more effective baluns/chokes,
but many of the common solutions actually encourage common mode current
in order to get a better match on 15m.



Quote

I'll dig out the analyzer and see what the impedance is at the end of the coax and double check the SWR.  The coax length is close to 25 feet between the line outside and inside.  If I find a particular impedance  other than 50 ohms can cutting or lengthening improve the 40M mismatch without changing 10 and 20 meters?  If so, which way would I go?


If you add 1/4 wave of coax on 40m it should rotate your impedance half a
turn around the Smith Chart on that band while still leaving the other bands
pretty close to where they are now (as the coax would be a multiple of 1/2
wavelength on those bands.)

The added coax doesn't change the SWR, but rotates the impedance around
the SWR circle.  A low impedance requires more capacitance to match:  if this
is the problem then you would be tuning the amp with the output loading
capacitor fully meshed:  adding some capacitance across the output cap
would also work.  But if the capacitor isn't close to fully meshed then something
else is wrong.

Assuming the SWR is 2 : 1, you should measure between 25 and 100 ohms
resistive, along with some reactance (unless the resistance is near to one
of those extremes.)  The actual reading will vary with coax length, even though
the SWR remains constant.  IF the problem is that the impedance is at the
low end of the range and the amp is running out of load capacitance, then
rotating it to the high impedance part of the circle is a simple solution.
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