"Antonio Vernucci" wrote in message
.. .
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


that is not surprising for an antenna that has a very low or very high
impedance at the resonant point. The SWR depends on the magnitude of the
impedances not the angle, so you could have a minimum SWR with a big
reactance and small real component.

Dave wrote:

. . .The SWR depends on the magnitude of
the impedances not the angle, so you could have a minimum SWR with a big
reactance and small real component.

That's not true. For example, impedances of 50 + j0, 35.36 + j35.36, and
0 + j50 ohms all have the same magnitude (50 ohms), but a 50 ohm cable
connected to loads of those impedances will have SWRs of 1, 2.41, and
infinity respectively.

Correct formulas for calculating SWR can be found in the ARRL Antenna
Book, the ARRL Handbook, or any respectable transmission line text.
Incorrect ones can, I'm sure, be found on the Web and elsewhere.

Roy Lewallen, W7EL