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Author Topic: Antenna analyzer hookup question?  (Read 1462 times)
W3END
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Posts: 17




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« on: April 06, 2017, 10:30:00 AM »

What readings would be the most accurate using the antenna analyzer? If you hook it up to the antenna outside or if you hook it up in the shack to the incoming coax? I've asked this question to several hams and have gotten many different answers and explanations. Your thoughts???
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KE6EE
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« Reply #1 on: April 06, 2017, 10:51:04 AM »

Most accurate for what purpose?

If you measure at the antenna feedpoint you are observing the SWR, resonance (X=0) and other characterisics of the antenna proper. This is helpful for tuning your antenna.

At the shack end of your feed line you are observing the SWR, etc., that your rig and tuner have to deal with, which is probably more critical.

The difference in measurements has to do with the SWR excursions along the feedline as a result of any mismatch of the feedline and antenna at the feedpoint.
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W9IQ
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« Reply #2 on: April 06, 2017, 10:53:27 AM »

The antennalyzer will always give an accurate reading, within the limits of the instrument, for the point at which it is inserted.

When you place the antennalyzer at the antenna, it will measure the impedance of the antenna and the SWR of the antenna relative to a 50 ohm (or other) feedline.

If you insert the antennalyzer further down the feedline, it will report the impedance and SWR at that point in the feedline. But it is important to recognize that if the antenna impedance does not match that of the feedline, the reported impedance along the feedline will be the transformed value as determined by the length of the feedline. The SWR will be reported at that point as well but it will be reduced by the losses of the feedline.

Where many people get confused is by trying to adjust an antenna by monitoring its impedance along the length of the feedline. Unless you have a clear understanding of how the feedline transforms (changes) the impedance of the antenna, you can chase your tail for a long and frustrating period. You can correct for the effect of the feedline transformation but most antennalyzers do not do this so it requires a separate process to make the transformations. Thus it is preferable for most hams to place the antennalyzer at the antenna feedpoint if the goal is to adjust the impedance of the antenna. If you want to know the impedance and the SWR that the exciter or tuner will see, then place the antennalyzer at that point in the feedline and it will give you the necessary information.

- Glenn W9IQ
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- Glenn W9IQ

I never make a mistake. I thought I did once but I was wrong.
WB6BYU
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Posts: 16566




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« Reply #3 on: April 06, 2017, 11:27:14 AM »

Generally I agree:  if you want to know the antenna feedpoint impedance (including finding resonance by
searching for the frequency where X = 0 ) then you must either put the analyzer right at the antenna
feedpoint, or correct the measurement for the length of the feedline.

If all you are interested in is the SWR, and/or the frequency of minimum SWR (which often is all you need
for tuning antennas) then you can take the measurement through any length of coax, because the SWR
doesn't change along the coax (except for a reduction due to cable losses.)


Actually it's even a bit more complicated than that...

If you are using something other than 50 ohm feedline (such as 75 ohm TV coax, ladder line, etc.) then
you are better off measuring the SWR in the shack, which will account for any impedance transformations
caused by the assortment of feedline impedances.  In such as case, the purpose (generally) is to provide
a reasonable impedance to the rig, and measuring at the rig tells you how well you achieved that.

If you have common mode currents, then none of the measurements may reflect the load that your rig
actually sees when you transmit, even if the analyzer is perfectly accurate.  That's because the outside
of the coax is part of the antenna (that's what common mode currents are), and when you unplug the
coax from the analyzer and plug it into the back of a transmitter that isn't totally isolated from ground,
it changes the effective length of that part of the antenna, so the SWR may change.  That's not
the fault of the analyzer - you are actually measuring a different antenna when using it than when using
a transmitter that is grounded back to the AC line, for example.  One way to check this is to measure
the antenna with the analyzer, then ground the shield of the antenna coax to the transmitter chassis and
see if the analyzer reading changes.
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WA3SKN
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« Reply #4 on: April 07, 2017, 08:35:37 AM »

The readings will have the same "accuracy", but the readings will probably be different.  How different is what is important.

-Mike.
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NR4C
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Posts: 452




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« Reply #5 on: April 07, 2017, 08:46:30 AM »

Kinda like having a thermometer and asking what is the temperature?  the accuracy is determined by the construction of the instrument.  But do you want inside temp, or outside, or in the shade, sun, or under the apple tree?

...bc nr4c
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AA5MT
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Posts: 106




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« Reply #6 on: April 07, 2017, 10:25:33 AM »

That's the difference between an antenna and an antenna SYSTEM.  If you want to pretune the system, say, a tuner, from the radio point, you tune everything else too, including the antenna. 

CB'ers tune their antenna by changing the coax length! 

Tom
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KF7CG
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Posts: 1182




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« Reply #7 on: April 07, 2017, 10:43:46 AM »

There is only one drawback that I can see to trying to measure antenna characteristics at the feed point. Interactions between the antenna and surroundings not normally present during operation. That is the body of the person doing the measurement, the measuring instrument, and finally any changes in feed point elevation to ease the task of measurement. Remember anything in the near field of an antenna will have some effect on its impedance.

KF7CG
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W9IQ
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Posts: 1355




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« Reply #8 on: April 07, 2017, 10:45:05 AM »

Quote
If you want to pretune the system, say, a tuner, from the radio point, you tune everything else too, including the antenna

I know this is a common believe but it simply is not true.

The problem starts with the name "antenna tuner". It doesn't tune the antenna nor the feedline. The antenna tuner would better be described as an impedance matching device. It matches the impedance present on one side of the tuner to the desired impedance on the other side of the tuner. That is all it does. The effect is that the exciter is able to output full power since it sees no reflections and the reflected power from the antenna is returned to the antenna.

The SWR remains whatever it was on the antenna side of tuner and the complex impedance of the antenna is unchanged - thereby dispelling the myth that the antenna has been tuned. The closest you could come to actually tuning the antenna is to place the tuner directly at the antenna feedpoint. This would be similar to matching devices placed directly at the antenna such as a hairpin match, delta match, etc.

- Glenn W9IQ
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- Glenn W9IQ

I never make a mistake. I thought I did once but I was wrong.
W9IQ
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Posts: 1355




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« Reply #9 on: April 07, 2017, 11:06:33 AM »

Quote
There is only one drawback that I can see to trying to measure antenna characteristics at the feed point. Interactions between the antenna and surroundings not normally present during operation.

That is a very good point. The newer analyzers that can "back out" the impedance transformation caused by the feedline go a long way to addressing this issue. But as BYU pointed out, then there is common mode currents that can raise havoc. With the proliferation of end fed and other common mode current inducing antennas, I suspect more users will be confused with the results of their tuning activities in either case.

- Glenn W9IQ
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- Glenn W9IQ

I never make a mistake. I thought I did once but I was wrong.
AA5MT
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Posts: 106




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« Reply #10 on: April 14, 2017, 10:20:59 AM »

It was called a transmatch in the old days, which may actually be more accurate, as the power meters are set for 50 ohms now.

Tom
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W9IQ
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Posts: 1355




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« Reply #11 on: April 14, 2017, 10:50:56 AM »

There is no difference between them.

- Glenn W9IQ
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- Glenn W9IQ

I never make a mistake. I thought I did once but I was wrong.
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