Antonio Vernucci wrote:
Reading here and there that the signals of the on-going DX-expedition to
Glorioso Island are generally very low, I got the curiosity to simulate
the so-called "spiderbeam" antenna they are using (sized for the
10-meter band) on EZ-NEC.

Doing that, I obtained an unexpected result. The simulated antenna shows
a clear SWR minimum at 29.0 MHz where impedance is 76 + j32 ohm.

I then checked SWR across the 24 - 34 MHz range with the following results:

- going up in range 29 - 34 MHz, the reactance steadily increases (+334
ohm at 34 MHz)

- going down in range 29 - 24 MHz, the reactance remains positive and
steadily increases up to 28.5 MHz, after which it starts to decrease,
until it becomes 0 ohm at 27 MHz, and negative below that frequency. At
27 MHz impedance is 9 + j0 ohm (hence it is the resonant point).

I knew that the resonant point does not precisely coincide with the
minimum SWR point, but I would not have suspected such a big difference
(2 MHz shift at 29 MHz!).

Any comment?

Tony I0JX
Rome, Italy

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