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Re: no patches for offset dishes!!


A couple of quick comments.  I suspect that you are right about center fed 
dishes being cheaper in Europe and maybe Asia.  Except for 7-12 foot "BUDS" 
that is NOT the case in North America.

I am a big fan of using what you can for cheap.  Primestar dishes here are 
relatively cheap on e-bay and free if you can scrounge one (I did!) and make 
a whopping good AO-40 antenna for almost nothing.

This is okay even if it is not optimum.

Some people report that patches work better than helixes on their offset fed 
dishes and some report the opposite.  I think the distinctions blur a bit 
more on offset fed dishes because the offset
points the over-illuuination up at the sky instead of the horizon-- meaning 
that one can "get away" with more over-illumination of the offset fed dishes 
than with a prime focus-- defacto G/T increase.  This is exactly why the DBS 
companies use offset fed dishes.  I've read (and do not remember where) that 
on 12 GHz DSS system, tests were done that proved that the G/T on a 18 inch 
offset were better than a 24 inch prime focus.  Given both were properly 
illuminated.  But, this is why people are having successes even though 
theory might state otherwise.

Personally, my 26" prime focus with home brew CP patch is about the same as 
the Primestar with 5.25 turn helix on the bird.  (Both are good)... but you 
guys should hear the patch on the 4' dish I have..or on the 10'  
Wow...preamp?  We don't need no stinking preamp!! :-)  (It still helps!)

So I'd have to say, use what you have or can get cheaply.  All of these 
systems work, as witnessed by the many different designs out there.

Fred W0FMS

>From: Achim Vollhardt <avollhar@physik.unizh.ch>
>To: amsat-bb@AMSAT.ORG
>Subject: [amsat-bb] no patches for offset dishes!!
>Date: Thu, 11 Jul 2002 19:54:16 +0200 (CEST)
>
>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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>To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org




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