[TowerTalk] Change in Frequency As Antenna Height Rises

[Jim Brown]

Exactly right, Grant.

I regularly measure Z in the shack with my DG8SAQ-designed VNWA, follow 
that with a TDR measurement of the line at VHF, then use Dan's ZPlots 
with short and open measurements of a shorter length of the coax to 
determine Vf as a function of frequency, and then use SimSmith to 
subtract out the transmission line to show the Z at the antenna (and to 
design stub matching networks, when needed). It's important to realize 
that Vf varies a percent or so with frequency, so to get Z at the 
antenna we must use Vf at the frequency of the antenna. The error is not 
great with electrically short lines, but can be a significant source of 
error with longer lines.

73, Jim K9YC

  On Tue,5/31/2016 8:03 AM, Grant Saviers wrote:
> Perhaps this debate is really a measurement problem ---
>
> I think the important thing to remember is that a SWR bridge is 
> probably NOT going to measure resonance when attached to some random 
> length of coax hanging in the air to an antenna.
>
> 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.