Check the Alt Z0 option button at the upper left of the SWR display. What
happens to the minimum SWR frequency? Then change the Alt SWR Z0 value in the
main window to some other value, say 300 ohm. What effect does that have?

Interesting, isn't it?

Roy Lewallen

Yes, changing the Alt Z0 makes a dramatic effect, and setting it to 9 ohm
obviously causes the minimum SWR point to shift from 29 to to 27 MHz (reaching
1:1).

Interesting to note that, using a 75-ohm cable, one can get a perfect match to
the simulated spiderbeam antenna in two possible ways:

- either cancelling the antenna reactance using a -32 ohm series-capacitor. One
then gets a (nearly) perfect match at 29 MHz, where antenna impedance is 76 +
j32 ohm

- or using a 9:75-ratio transformer. One then gets a perfect match at 27 MHz
(where impedance is 9 + j0 ohm)

Another interesting observation is that, at 29 MHz (i.e. where the antenna
impedance is 76 + j32 ohm and the SWR on a 75-ohm cable shows the minimum value
of 1.95) one can find a cable length at which the impedance appears to be purely
resistive and equal to 1.95*75 = 146 ohm (or 75/1.95 = 38.5 ohm). This fact is
deceiving as, seeing a purely resistive impedance, one could be led to
concluding that the real antenna resonant frequency is 29 MHz, whilst in reality
it resonates at 27 MHz (although knowing what is the real antenna resonant
frequency may not be so important).

I raised the above arguments just as a confirmation of the fact that
understanding what to do before attempting to adjust antennas is not that easy.

73

Tony I0JX