----- Original Message -----
From: "Bill VanAlstyne" <w5wvo@cybermesa.net>
To: <towertalk@contesting.com>
Sent: Monday, July 12, 2004 10:33 PM
Subject: Re: [TowerTalk] Stacking Distance for M2'd 6m7JHV?
> Jim Lux wrote:
>
> > The Drive both, drive one or the other, drive both out of phase
> > switching scheme would be a step in the right direction.
> >
> > It might be interesting to trade that scheme (which has 4 possible
> > settings) against a scheme where you combine the two antennas with
> > 0,90,180, or 270 degrees phase shift between them, which also has 4
> > settings. In this scheme, you've got twice the effective aperture
> > for all the settings, as opposed to the top/bottom/bip/bop scheme,
> > where the single antenna settings have half the aperture. For HF,
> > the change in aperture is probably a non issue, but for VHF and above
> > where the sky is quiet...
>
> Jim has articulated a very interesting approach to the question of
covering the
> range of elevation angles for 6M. I like the idea of retaining the
enlarged
> aperture of two yagis for the higher take-off angles. But how to model the
> effect of various phase shifts on take-off angle? Does anyone have a
handle on
> that? I don't know of any software that is set up to do the math involved
in
> this scenario. Any ideas or recommendations?
I'm working on an Excel spreadsheet to do this for the flat ground case. I
just need to check the output against some hand calculations to make sure
the reflection coefficients are of the right sign for how I'm doing the
calculation.
The basic technique is to calculate the sum of the contributions from all
antennas, in the direction of interest (which you then sweep from 0 to
whatever). You need to calculate the phase shift due to the position of the
antenna (= height * sin(elevation)) and add the phase shift you're
controlling. I usually do these calculations in complex numbers because
summing is easier...then you just take the magnitude at the end.
The tricky part here is taking into account the reflections from the ground.
Or, if you want to get really tricky, the other ground effects (diffraction,
etc.), as HFTA does. The HFTA documentation in the antenna book doesn't go
into much detail on how, precisely, N6BV implemented it, though.
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