[TowerTalk] Change in Frequency As Antenna Height Rises

[Jeff DePolo]

> Perhaps this debate is really a measurement problem ---

>Perhaps this debate is really a measurement problem ---

Part of the problem is that some people consider "resonance" and "matched to
the transmission line" (i.e. 1:1 VSWR) as being one in the same when they
are talking about antennas, when they most decidedly are not.

> What is needed is an analyzer that can be calibrated with the 
> measurement plane at the far end of the coax.  A VNWA, 
> AIM4170, SARK110, 
> etc. can be calibrated with S/O/L on the end of the cable so the 
> measurement plane is at the antenna.  Then when jX = 0 you have 
> resonance.  However, even then real R might not be 50 ohms, not 1:1 
> SWR.  Otherwise some complex Z probably transforms to 1:1 SWR 
> on an SWR 
> bridge at some other frequency than Fres depending on the 
> length of coax.
> 
> Grant KZ1W

Maybe I'm not understanding what you were saying, but there's no way a 50
ohm cable can transform a load Z that's not 50+j0 to yield a 1:1 VSWR (ref
50 ohms).  Please clarify.

I agree that the most reliable way of making feedpoint Z measurements is to
move the calibration plane to the end of the transmission line.  But lacking
that, you should be able to get fairly accurate in situ feedpoint Z data by
starting with an accurate measurement of electrical length of the line
(using TDR) and then rotating the Z measured at the source end of the line
accordingly.  Of course you have to factor in line loss as well, which is
best determined via a return loss measurement at the frequency of interest
with the line open/shorted (take the average of RL with the line open and
shorted, and then divide by 2 to get loss), but lacking that, manufacturer's
data is the next best thing.

				--- Jeff WN3A



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