The sign of reactance – SM6WHY’s take

As the popularity of low cost, low end antenna analysers increases, client software appears to enhance the capability of the analyser.

The SARC-100 is one of these low end analysers, it and its many close derivatives are marketed under various model names.

The sign of reactance discusses a major weakness of these and many other low end instruments in that they do not ‘measure’ the sign of reactance, displaying the magnitude of reactance and leaving it to the user to solve the sign problem.

SM6WHY is one of the many who have produced software for the SARC-100 that purports to solve the sign of reactance problem. He gives this graphic on his website to demonstrate the capability of his software used with a SARC-100 (which does not sense the sign of reactance).

Above is part of the graphic he offers. Though the image is poor quality, the VSWR plot appears smooth and quite typical of that which might be obtained by measuring an antenna system near its VSWR minimum.

However the accompanying Smith chart plot which has points plotted with both negative and positive reactance is inconsistent with the VSWR plot and appears flawed. 

Remembering that VSWR along the Smith chart curve is simply the distance to the chart centre, so as you ‘travel’ along the Smith chart curve, the distance of each point to the centre should follow the VSWR plot

Smith chart curves of real system resemble spirals where short sections of the spiral resemble circular arcs about some centre (not necessarily the chart centre though), and those arcs are clockwise with increasing frequency.

So, the fact that the two ends of the curve here end in clockwise arcs is a warning that something is very wrong.

The graphic above is the same graphic with the contrast improved to see the SWR curve more clearly, and my annotation of some Smith chart points for discussion.

If you make the assumption that the plotted R value is approximately correct, and that the magnitude of X is approximately correct, you can then consider alternate sign of X for each of the plotted points to see what is needed to produce a curve they obeys the ‘rules’ of real systems.

One solution, and the most likely solution is that all the points highlight in green are of the opposite sign. It is quite a different plot with those changes applied.


  • Reliably determining the sign of reactance from measurements of R,|X| is challenging.


Links / References