[TowerTalk] Stacking Distance for M2'd 6m7JHV?

[Jim Lux]

At 09:30 AM 7/12/2004 -0600, Bill VanAlstyne wrote:
>To see how spacing affects take-off angle, install and run the YT 
>(Yagi/Terrain)
>software that comes on the CD-ROM with your ARRL Antenna Book. Basically,
>stacking couples two or more yagis such that energy normally released at 
>higher
>angles to the ground is radiated as part of the antenna system's primary
>(lowest-angle) lobe. Stacking doesn't lower the take-off angle of the primary
>lobe, but it does re-route the higher-angle energy that is radiated by even a
>very high single yagi. This is not necessarily an advantage on 6M, as I will
>discuss.
><snip>
>So on 6M it pays to try to cover as much of the useful elevation range (from 0
>to about 16 degrees) as possible. YT will show you that, for a flat terrain
>model in the far field, probably the best overall 6M yagi configuration is one
>which stacks two or more beams about 30 feet apart (with the lowest being at
>around 30 feet above ground level), AND which allows each antenna to be used
>separately as well as driven in phase. A 60-30 stack of two yagis with a
>StackMatch produces a pretty ideal coverage of all the useful elevation angles
>within around 3dB. Adding a third at 90 feet lowers the lowest possible 
>take-off
>angle a little. This would help for F2 propagation (next solar maximum), 
>as well
>as for tropo-scatter and ground-wave modes, but doesn't help much for most
>sporadic-E paths.
>
>Hope this info helps!
>
>Bill / W5WVO

This sounds like a useful strategy might be spacing them fairly far apart 
(to reduce mutual coupling) and some sort of relay box that allows 
switching in various lengths of coax to change the relative phase, might be 
a real handy thing.  Sort of a steerable interferometer.  30 feet spacing 
is a fair amount (1.5 lambda) so the coupling would be minimal. The reduced 
mutual Z  from spacing them this far apart would allow the use of switched 
coax for phasing without worrying too much about changing the relative 
drive current in each antenna.

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...