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[TowerTalk] 160m shunt feed followup

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Subject: [TowerTalk] 160m shunt feed followup
From: (Tom Rauch)
Date: Mon, 1 Oct 2001 20:26:48 -0400
> You can back out the value of X using the SWR and value of Z.

> The formula is in the AUTEK manual.  The sign of it is easy to
> determine.

Not true!

> First find the resonant freq.  If your freq of interest is higher then
> the sign of Z is + (inductive) and vice versa.  To get practice, try
> this first on a simple antenna with well known characteristics.  Take
> about 5 points either side of resonance and plot the R and X values.
> It will be pretty obvious when X changes signs.

Unfortunately this is what some analyzers assume. It is reliable 
with lumped components.

It is unreliable when you are measuring a system with transmission 
lines. It is possible to have increasing Z or X with decreasing or 
increasing frequency and the sign to be either capacitive or 
inductive randomly.

Since antennas have transmission lines integrated in them, even 
measuring a simple antenna with that method can prove misleading.

The only reliable way is to measure phase angle, or add a 
reactance of a known sign and see how impedance changes.
73, Tom W8JI 

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