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RE: Cross boom length?


Thanks for the tip on the App Notes by Directive Systems (never noticed it
before).  Very timely for me as I plan on raising a tower with HF, 2x6m,
222 and 432 yagis all on one mast.  It didn't result in a change of
stacking plans, but did provide some reassurance that I am not far off the
goal.  The 17-foot mast will have to be sched-80, for sure.

At 09:02 PM 3/18/2006 -0600, Steve KA5YFC wrote:
>Directive Systems has a pretty good AppNote on stacking antennas. It is
>aimed at getting antennas set up on VHF/UHF but I think that is would
>apply to satellite antennas also. It does explain the interactions of
>the antennas on each other and how you can compromise to get the most
>bands on the shortest mast. 
>Look for the APP Notes on the left and then click on the link for how to
>plan multiple antennas on a single mast.
>I found it informative and it has given me several ideas as to spacing.
>I think that using circular polarized antennas, one would have the same
>stacking distance both in the H plane and the E plane.

Remember that if the circular polarized antennas are crossed yagis then the
circular aperture size is radius of the E-plane which is about 33% wider
than the H-plane.  Not all mfr's will display the E-plane stacking distance
since the assumption is stacking of horizontal yagis (H-plane stacking). 

My four 2m eme x-yagis are stacked 144x144 inches but since I do not
connect them for circular polarity, they suffer a slight loss in stacking
gain in one dimension in either H or V polarity.  That is because the
stacking distance is set for the E-plane and not the H-plane.  If the array
were only Horz yagis they would be stacked 144 wide and about 108 high;
like wise if they were Vert yagis they would be stacked 108 wide by 144
high.  Cross-pol eme arrays therefore end up with a slight compromise which
results in stronger sidelobes in the over-stacked direction (H-plane).  If
stacked 108x108 the array would suffer less than maximum stacking gain
(6-dB for 4-yagis) since the E-plane apertures would overlap too much.

But satellite antennas are not so critical to need squeeze the last few
tenths of gain since you should be designing to have an SNR from 10 to 20
dB.  If you lose 0.5 or 1 dB that will not be noticeable in real life use.

This is my reasoning about stacking a 144 and 432 circular pol yagi a bit
close.  My 33-inch dish presents much more disturbance than either antenna
to the other.  But it all worked fine on AO-40.  Results are the final test!
Ed - KL7UW  
http://www.qsl.net/al7eb - BP40iq 
144-EME: FT-847, mgf-1801/1402, 4xM2-xpol-20, 170w
432-EME: FT-847, mgf-1402, 1x21-ele (18.6 dBi), 60w
Sent via amsat-bb@amsat.org. Opinions expressed are those of the author.
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