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[Towertalk] EXACT resonant freq of a dipole --> AEA CIA HF analyzer?

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Subject: [Towertalk] EXACT resonant freq of a dipole --> AEA CIA HF analyzer?
From: (Tom Rauch)
Date: Mon, 8 Jul 2002 17:37:21 -0400
> Before I buy one, can you please advise whether I will be able to do
> this with an AEA CIA HF analyzer? It sure looks good on the
> manufacturers specifications. I had an Autek RF-1 analyst before and I
> was quite dissapointed with its accuracy. What else do you use for
> tasks like this? Should I get the CIA or the VIA model?
> (Why do I want to measure so exactly? I want to determine the velocity
> factor of the wire used. The velocity factor is introduced by the
> insulation covering the wire. Therefore the wire elements are always a
> bit too long when cutting them to the dimensions taken from a computer
> simulation. This is not a problem with dipoles but with wire yagis and
> quads, where the parasitic elements must have exactly the proper
> resonant frequency.)

The length of feedline from the antenna to the measurement device 
will affect the measured resonant frequency, so you also must be sure 
the feedline is an exact multiple of 1/4 wl to determine EXACT 
resonant frequency.

If it is not an exact multiple of 1/4wl, the line will shift the 
reactance off center or even create a "false' indication of antenna 
zero reactance. That's because the line will have some SWR, and the 
SWR will cause reactance shift in the line. An odd- 1/4 wl line will 
invert the sign of the reactance, but will not alter it's absolute 
value assuming the line is lossless.

The next problem you might have is line loss. Feedlines do not have 
evenly distributed losses between conductor series resistance and 
dielectric losses. The conductor loss dominates the system. The end 
result of using a lossy feedline is it adds reactance to the system, 
skewing the results.

You are trying to measure one of the most difficult things to measure 
with near-perfect accuracy. You will probably learn more about how 
difficult measurements like this actually are than you will about the 
insulation. Be careful the measurement method you use and equipment 
errors don't determine the results!

  73, Tom W8JI 

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