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Re: Primestar Dishes: Cheap and Frustrating



Bill,

Hi excuse the delay in replying, I was out of town last week and just
catching up on nearly 600 back e-mail ;-)

=======TOPIC #1========
At 11:44 AM 8/4/03 -0400, you wrote:
>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.

I use a 5.5 turn helix feed with my 85cm (33-inch) offset feed dish
(surplus Dish-Net ku-band).  It appears if the beam is not symetrical in
width then the dish area in use is not equal either (I am referring to
center fed dishes here).  My expectation would be that the beam would be
narrower in the plane of the wider dish dimension (just as the beamwidth of
a larger dish is narrower than a smaller dish).  So what can we determine
from your observations?

I am assuming that the Primestar dish is a parabolic-sector dish.  If so
then you will not get longer tracking time in the broad dimension.  If the
feed is effectively illuminating only the central "60cm" then the BW would
be equal.  If it over illuminates then the gain in the broader dimension
would  be greater and BW narrower.  Since you see the opposite effect, I
surmise that the Primestar is not exactly parabolic and was designed to be
broader in one dimension (perhaps to pick up more than one TV-satellite at
a time).
 
>
>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.

Yes!  I agree.  So maybe the 5.5 turn helix is too long (too narrow a BW)
for a Primestar?

>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):

This is why the arm is usually mounted on the bottom side so the feedhorn
looks up at "cold sky" in case of over illumination (spill over).

>
>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?:

OK, it looks like you almost understand the situation.  It is a tradeoff
between maximizing gain (illuminating the whole surface) vs. minimizing
ground-noise pickup (feed spillover due to over illumination).  You could
perform theoretical analyses to try to deternmine this but it will depend
on a lot of assumptions about the dish.

Instead why not do some observational tests:  compare feeds using the
beacon (MB) signal strength as reference.  Also compare mounting top vs.
bottom feed support postions.  (I think you will be quite impressed on this
effect).

You could also just do the comparisons using ground noise:  point the dish
at the horizon and change things.

In the first set of observations be sure to note the min s-meter level in
each case as this shows your background noise floor.

If you do these tests be sure to share them with us as this will be very
imformative to all.  (hint submit it to the Amsat Journal for
publication...always nice to see your call in print!)


>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?)

Actually the S/N will be affected by G/T so it is a comparison between G/T
and G.

Theory of weak-signal receiving implies that G/T is the ruling factor!

=====Topic #2=========

>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?
>:-)  ?

Uh No!  the function of a preamp for microwave earth stations is to lower
the system noise figure.  Gain is only needed to overcome the noise figure
of the following electronics (the AIDC3731 and the IF Rx, to a lessor
degree).  The AIDC and most ham VHF receivers have adequate gain.  The AIDC
is probably about NF = 1.2 to 1.5 dB (my guess).  The DEM preamp can be
tuned to NF = 0.6 dB with 16-18 dB gain.  The result on S/N in the
receiving system will be about 1.5 to 2 dB!  (You do not subtract the NF
ratings to determine this...S/N is affected much more than the change in NF
suggests).

Tsys is the total noise temperature:
Tsys = Trx + Tgnd + Tsky
Tsky is about 20K  {not much can be done about this one...it is noise from
the universe}
Tgnd is determined by your feed (we like to see the sidelobes down below 15
dB): goal = 29K  (ground is 290K)
Trx as low as you can afford  (max is 150K)   A preamp at NF= 0.6 dB has a
T = 50K.  If it has 20 dB gain everything after it is divided by 100 so a
Tconv = 150K would only contribute 1.5K so Trx = 50 + 1.5 = 51.5K
if you have a really good system then:
Tsys = 51.5 + 29 +20 = 100.5K

use the AIDC alone with a noise temp = 150K and see what happens to Tsys?
Tsys = 150 + 29 + 20 = 199K

Go try it on the AO40Rcv spreadsheet and you will see it right away!

General Rules for obtaining a "good" AO-40 receivning set up are:
1)  Adequate antenna gain:  24 dBi or higher
2)  Low Noise Figure:  Under 1.0 dB   {remember the conv and IF receiver
add to the overall NF of the preamp}

Bottom line....get the low noise preamp...you will be glad you did!  This
is especially true if your antenna is marginal!

73, Ed - AL7EB

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