[amsat-bb] no patches for offset dishes!!

[Achim Vollhardt <avollhar@xxxxxxxxxxxxxxx>]

Hi all,
I would strongly recommend to NOT use patches with offset dishes with a
f/D of 0.7 (more or less standard for offset dishes).

Those dishes as seen from the feed are far too small.. in other terms:
the patches will over-illuminate the dish, the pickup of the warm earth
behind the dish will increase. This is the effect many people see when
exchanging their helix feed of an offset dish to a patch: almost same
signal, more noise, less QSB. The last item is due to the better
circularity of the patch compared to helix..

There have been discussions about the circularity of the K3TZ patches
recently.. yes, there are indications that axial ratio increases when you
insert this nylon spacers to fix the patch over the reflector. My
solution: no spacers at all! Once soldered properly, this is not as
fragile as it seems. If really needed, you can fix it with a small screw
directly at the center point to the ground reflector: as this point should
be somehow neutral due to symmetry, this might work, but to my knowledge
never been tested.

Why has no one ever thout about the non-circularity of a helix? Just
because it is MECHANICALLY round, it is not necessarily electrical round.
See www.g6lvb.com/addendum.htm ... the less QSB reported by many patch
users speaks for itself.

I don't say that patches are the best solution. BUT: They are easy to make
and fit perfectly to a f/D 0.4 center fed dish as sold in Europe. Right
now, 0.4 dishes are even cheaper than the 0.7 offset dishes..

So compare the patch to the helixes, but bear in mind the different
characteristics of the combinded system patch-dish.

73s Achim, DH2VA

dh2va@amsat.org

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