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Re: Primestar Dishes: Cheap and Frustrating




>From my experimentation, the patch feed is a more efficient illuminator than 
the helix, but in the case of an offset fed dish, it picks up ground noise, 
negating that effect.

My opinion on this is the following, if your patch is tuned to be circular:

Helix (with it's nasty sidelobes) at 5.5 turns is equal to a properly tuned 
CP patch (with its nasty over illumination) on the P* oval dish.

The 2.5 turn helix has both nasty sidelobes and nasty over illumination.

Neither feed is ideal for this type of dish.  If it weren't for the CP 
aspects, the best feed I've seen for this dish is here:  
http://www.trevormarshall.com/biquad.htm

This might insinuate that two CP patches phased together (or two linear ones 
fed 90 degrees apart) might make the best feed for the P* oval.  To phase 
stuff together at 2400 MHz takes a lot of effort.

Fred W0FMS





>From: "Bill Acito W1PA" <w1pa@hotmail.com>
>To: <amsat-bb@AMSAT.Org>
>Subject: [amsat-bb] Primestar Dishes: Cheap and Frustrating
>Date: Mon, 4 Aug 2003 11:44:20 -0400
>
>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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