Exploiting your antenna analyser #28

Resolving the sign of reactance – a method

Many analysers do not measure the sign of reactance, and display the magnitude of reactance, and likewise for magnitude of phase and magnitude of impedance… though they are often incorrectly and misleadingly labelled otherwise.

The article The sign of reactance explains the problem and dismisses common recipes for resolving the sign of reactance as not general and not reliable.

This article gives an example of one method that may be useful for resolving the sign of reactance.

My correspondent has measured VSWR=1.68 and |Z|=66 and needs to know R and X. From those values we can calculate R=60.3 and |X|=26.9.


The method involves adding a short series section of known line, short enough to provide a measurement difference in R, and that R would be different for the case of =ve and -ve X, all of these measured at the same frequency.

There is a risk when the measurement is of an antenna system that significant common mode current may alter the measurement, so lack of consistency with expectation flags a potential problem that needs to be investigated.


An additional 8′ section of RG-8U was inserted and the impedance looking into that section was measured at 14.17MHz.


Now let us predict the input Z to an 8′ section of RG8 with loads of 60.3+j26.9 and  60.2-j26.9  using a good transmission line calculator.

For the +ve X case, the R component of Zin is predicted to be 57Ω, for the negative case it is predicted to be 32Ω. These are sufficiently different for the test to be conclusive.


In the event, the measured R was close to the 32Ω predicted for the case of X being -ve.


On the basis of Rin being close to prediction for the -ve X case, it can be reliably concluded that at the first point Zin=60.3-j26.9Ω.

The importance of this was that the known value allowed calculation of the feed point impedance of the antenna which was at the end of 6′ of RG8X to be 79.2+j14.6Ω. That informs the design of a matching scheme.

Watch the blog for continuing postings in the series Exploiting your antenna analyser. See also Exploiting your antenna analyser – contents.