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Primestar Dishes: Cheap and Frustrating



I noticed something else this weekend (observations of a dish newbie)

When I had the 5.5 turn helix on the Primestar (90cm x 60cm), I had a broad
peak in signal has I moved the dish left to right
(turning it on its short axis)...  makes sense...  the helix is effectively
illuminating the middle half of the dish
(effectively, a 60cm offset fed  dish with extra area on the sides), and the
"ears" of the dish broaden the time that a 60cm section is looking at the
source. I move the dish left and right, and the "peak" is broad (but I am
only effectively using the same area as a 60cm offset dish at any one moment
in time, correct?). Tilt up and down, and the peak is narrow.

Take off the helix, put on the patch.

Now the peaks reverse...  left and right, the peak is narrow, up and down it
is broad. Again, this makes sense: the patch has a wider illumination, now
illuminating the entire width at one moment, but I am missing area on the
top and bottom.
Now I have an effective 90cm offset dish with the top and bottom edges
missing; I am seeing ground noise (I have the arm on top):

http://webpages.charter.net/acito/Patch/AO40setup1.JPG

I assume I would see the same if I put a 2.5 turn helix on it (Jim, K6CCC...
didn't you write once that you thought your Primestar performed better with
2.5 turns?)....  the 2.5 turn helix feed now sized for the long dimension
beam width vs. the short.

So theoretically and practically, what's a better situation for the special
case of the oblong offset fed dish?:

A) fully illuminate the center 60cm area, and have to move the dish less as
it move across the sky

or

B) fully illuminate a 90CM dish with missing chunks, and increase the noise
floor?

(what's the driving factor in performance (G/T?) in this case....  dish
area, or signal to noise?)


Also, can I assume that putting another 20db of gain (a DEM pre-amp) in
front of the AIDC 3731AA is of no use in either case, since I'm amplifying
the background as well as the signal (where is my copy of the receive chain
Excel spreadsheet?
:-)  ?

Bill (W1PA)
in search of the perfect, highly portable, AO-40 station.


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