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RE: DH Satellite 32" dish, G3RUH patch feed, DEM preamp, TS 3733



Hi Mark -- nice setup. But let me offer a hint for the future.

> Can't wait for the next pass :)
>
> http://camel.campbell.edu/~hammond/ham/32dish/
>
> 73,
>
> Mark
>
> Mark L. Hammond  [N8MH]

You may not want to make the change but let me point out a hint that I
gleaned from the photo at
http://camel.campbell.edu/~hammond/ham/32dish/MVC-011F.JPG.

The tripod that you use attaches to the dish about half-way from the center
to the edge of the dish. As you can see from the photos, the feed support
legs shadow part of the dish. But you are taking a "double hit" on the
shadow. The first and most obvious comes from type of shadow you see in the
photo -- the blocking of a part of the aperture by the legs.
But when the support legs are as you have placed them, there is a more
serious shadow --  when seen from the feed, the legs cast a separate (and
larger) shadow onto the dish. To envision this, think of a light bulb at the
patch antenna's phase center. The support legs cast a wedge-shaped shadow
which renders a significant part of the dish surface without illumination
and hence unusable.

Even though your legs appear to be ~1 cm in diameter (i.e. 3/8" rod) and
don't seem to mask a significant area, at the 13 cm wavelength I'll bet that
the "shadow" is from an area at least a quarter- to half-wave in size (i.e.
1-2 inches wide). Near the edge of the dish, the shadow "wedges" are a
couple of inches wide.

The solution is that, when you mount the feed, make the support legs come
from a point near the rim of the dish and keep them out of the portion of
the dish you are trying to illuminate with the dish. Moving the legs out to
the dish rim, you only take the shadow "hit" once.

In practical experience (mine comes from Radio Astronomy), antennas with
"wide" support legs achieve ~5% higher aperture efficiency. A couple of
examples can be seen at http://lupus.gsfc.nasa.gov/brochure/bintro.html --
The Gilmore Creek 26m antenna is a "bad" design that was done in the early
1960's, while Kokee Park (see also the two antennas in
http://lupus.gsfc.nasa.gov/brochure/bheritage.html) is a more modern design
that is better because the support legs were moved outwards to minimize
blockage.


73 de Tom, W3IWI


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