Re: [amsat-bb] question regarding patch antennas

[Bruce Bostwick <lihan161051@xxxxxxxxxxxxx>]

> There have been many very favourable comments recently regarding patch
> antenna as feeds for parabolic dishes. All the construction articles I 
> have
> seen are for 2.4 GHz. Has anyone successfully scaled these antennas to 
> other
> commonly used frequencies? I was thinking of (say) 1691 MHz for the 
> NOAA and
> GMS satellites and 1296/1269 MHz for use in our 23cm band. Any 
> references
> would be appreciated. Scaling seems to work well for antennas like the 
> helix
> and loop yagi but perhaps patches are more 'perfinicky' and I don't have
> access to any modelling or analysis apparatus.

Scaling almost always works as long as you scale *all* dimensions.  Many 
people scale antenna designs and neglect to scale minor dimensions such 
as element thickness/diameter -- some antenna designs require elements 
to be a certain thickness to control skin current and frequency 
stability -- or use components that have unanticipated side effects on 
element behavior, such as insulated or coated conductors, which is why 
scaled antenna designs most often do unexpected things.

I don't *think* the thickness of a patch antenna element is critical, 
but you may want to experiment with different metal thicknesses just to 
be sure.  You *do* want to avoid using things like copper clad PC board 
because it will put a distributed capacitive loading on the element 
(just like insulation on a wire dipole! ;-) and I'd suggest low-loss 
standoffs like ceramics if you can get them.  With those caveats, you 
should be able to scale a patch pretty nicely.  Having an antenna 
analyzer like an MFJ-259 will help a *lot*.

The discussion of the AO-40 UHF patches posted a while ago (thanks 
KD4APP!) does have dimensions for 436, I believe, and you can scale 
those for most UHF/microwave frequencies if my instincts are right.  Try 
it and see, and let us know what you find out.

"Go ahead and do it, you can apologize later." -- RADM Grace Hopper, 
1906-1992
"The sunset is an illusion, but the beauty is real." -- Richard Bach

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