> You can back out the value of X using the SWR and value of Z.
True.
> 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
W8JI@contesting.com
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