LP-100A impedance measurement

A correspondent wrote seeking clarification of the Telepost LP-100A claims re impedance measurement in the context of some of my previous articles on the sign of reactance.

I could see several mentions in the LP-100A manual and the LP_100Plot documentation and they do seem a little inconsistent.

The LP-100A manual states very clearly:

Note: The LP-100A cannot determine the sign of X automatically.


If you QSY up from your current frequency, and the reactance goes up, then the reactance is inductive (sign is “+”), and conversely if it goes down, then the reactance is capacitive (sign is “-“). A suitable distance is QSY is about 100 kHz or more. The LP-Plot program has the ability to determine sign automatically, since it can control your transmitter's frequency. When it plots a range of frequencies, it uses the slope of the reactance curve to determine sign, and plots the results accordingly.

The first part states clearly that the instrument cannot directly measure the sign of reactance, and presumably measures the magnitude of reactance |X|.

Lets explore the second part in light of the overarching statement of the first part.

Above is the calculated R and X looking into 7m of Belden RG58C/U with a load 25+j0Ω. Also shown is |X|(as would be measured by the LP-100A) and calculated magnitude of phase of R,X, |φ|.

The LP_100Plot provides a facility for correction of incorrect sign, the manual gives some further information:

Generally speaking, X or Phase never “bounce” off of zero or swing radically through zero (like from +20 to -20) over a 50 kHz span.

Generally speaking here appears to mean mostly rather than always, but then the use of never seems to impart the meaning of always. The meaning of “bounce off of” is less clear, but contrary to what was said, there can be a local minimum in |X| very close to zero and the appearance of such should not be assumed to indicate that X passed through zero.

Looking back at the graph above, the |X| curve suggests that X probably passes through zero and the rule given is that if X increases with frequency, then it is positive, and the converse also applies. So this rule is telling us that to the left side, X is negative (because it is decreasing), and to the right it is positive (because it is increasing).

This is the exact opposite of the true value of X shown and the rule as stated is clearly wrong.

The plots above are from the LP_100Plot manual, the right plot shows a solution to the very problem we have been discussing… in between 28.7 and 29.8MHz |X| decreases with frequency yet X is plotted as positive. The plots above don't reconcile exactly, and each seems sane, but they are in conflict with the rule given first up that if X increases with frequency, then it is positive, and the converse also applies

It would seem likely that the plots above were fixed by hand by a person with expert knowledge that went beyond the nonsense rule give in the manuals.

The manual would appear to be written and signed off by persons lacking an understanding of transmission lines… marketing hype over engineering accuracy.


  • Duffy, O. Apr 2008. Telegraphers equation. VK1OD.net (offline).
  • Telepost. Sep 2013. LP-100A digital vector RF wattmeter. Telepost Inc (accessed 02/01/2017).