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[Towertalk] Antenna analysers

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Subject: [Towertalk] Antenna analysers
From: (Tom Rauch)
Date: Sat, 20 Jul 2002 22:20:16 -0400
> I do not know for sure but I presume all of todays antenna analyzers
> measure SWR and absolute impedance value (mag Z). From these numbers
> they calculate the vector data (real and imaginary part of Z)

The MFJ analyzer measures real and imaginary parts "directly", and 
also measures SWR directly. It does use SWR and Z to calculate 
Impedance at very low SWR values. The two different values of R and X 
are factored together with a certain "weight" or priority. 

As SWR increases above 2:1 the MFJ relies more and more on the 
"direct" R and X reading, and less on SWR because of the slope of 
accuracy when using each one at various SWR readings.

As you know, SWR is less and less accurate as the SWR is higher. By 
the same token, direct R and X is not easy to calculate when X is 
nearly zero and R is centered on 50 ohms (half voltage in the 

The sign of reactance is not given, because there is no phase angle 
detection. Some other analyzers give the sign but only "guess". What 
they do is perturb the frequency and watch the change in absolute 
value of reactance. The problem with that is an antenna and 
transmission line can decrease reactance as the frequency is raised 
when the sign is inductive, or can increase reactance with increasing 
frequency when the sign is capacitive. Just the opposite of a lumped 
component! Of course they can also behave like a lumped component.

Since that happens randomly, the reactance sign (+ or -) is really 
just a guess using that common method.

I'd always check the sign by using a lumped component in series with 
the output port to confirm it is correct if I was doing anything that 
was "+ or - sign critical", unless I had an analyzer that directly 
measured phase.

The reason sign is not measured is the cost to do it correctly and 
accurately.73, Tom W8JI 

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