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KG6BAJ > NWSGRP 20.09.09 02:24l 46 Lines 1642 Bytes #999 (0) @ WW
BID : 11560_N1OES
Read: GUEST
Subj: Re: Resaonance and minimum SWR
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Sent: 090919/1200Z @:KG6BAJ.#NCA.CA.USA.NOAM #:11560 [Grass Valley] InterGate $
** NEWSGROUP: rec.radio.amateur.antenna
** FROM : "Dave" <noone@nowhere.com>
"Antonio Vernucci" <vernucci@tin.it> wrote in message
news:4ab500ce$0$826$4fafbaef@reader5.news.tin.it...
> 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.
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