Exploiting your antenna analyser #2

Reconciling the single stub tuner results

So having found that the length of the RG58 lines sections are both 1.98m (approximately 2m), let’s try to reconcile measurement and prediction of Zin at 9MHz.

The examples discussed in this series of articles are designed for the test jig as described at Exploiting your antenna analyser #1 with a pair of nominally 2m length RG58 patch leads, a pair of 50Ω loads and some tee pieces and adapters to connect it all up. If you duplicate it, you will experience the same learning opportunities (the examples are structured). If you presume to redesign the experiment, your outcome will probably be different.

Before you read on, take a measurement of Zin at 20MHz and write down the impedance value. Do whatever you do to determine the sign of the reactance. If your instrument displays the sign properly, use it, otherwise use the method in your user manual or whatever wisdom you trust.

Done that? If so, read on…

Here is the measurement again from my Rigexpert AA-600 which properly determines the sign of reactance.

Screenshot - 9_12_2015 , 7_19_21 AM

The graph above suggests Zin at 9MHz is around 6-j20Ω.

Check carefully, did you get the correct sign? Simple methods of finding the sign with an instrument that doesn’t actually measure the sign will probably give an error for this scenario though it is not a most challenging scenario (more in a later article.)

Now, using a good transmission line loss calculator:

  1. finding input Yin for the 1.98 series section with 25+j0 load, I get 0.011024-j0.004418S (you may need to convert Zin to Yin for lesser calculators);
  2. finding input Yin for the 1.98 shunt o/c section, I get 0.002766+j0.061603S;
  3. adding these (as they are in parallel) I get Yin=0.01379+0.05719S, Zin=3.98-j16.52Ω (close enough for the accuracy of components etc).

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