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