09 February 2016

Calculate VSWR for varying Reactance Values X

Great question from Richard N4PBQ as follows:

"So as I'm analyzing my various antennas I understand that my antenna is at resonance when my analyzer reports X=0 for a particular frequency.  But that never seems to be the case outside of my dummy load and even there the reactance is reported as X=7 for 28 MHz.

So is there a rule of thumb for acceptable amount of reactance or some calculation that I can use to determine the loss in DB for a given frequency and the reactance?"

I got to thinking about this. The complex math becomes messy quickly with many possible combinations and permutations if both R and X are allowed to vary for the load impedance ZL. However if one keeps R = 50ohms constant and assume that the source impedance is 50 ohms then the analysis simplifies.

The referenced table shows a range of X values up to 20ohms for a VSWR not exceeding 1.5:1

Simplified math derivation

Calculate VSWR for varying Reactance Values X

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