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Re: primestar ??



At 09:21 AM 1/12/2003 -0800, w7lrd@juno.com wrote:
>Hello
>I am currently using the small (36X26) primestar dish with a 3731.  I
>have a 1.2M (54X48) primestar dish.  I am considering putting the 1.2 to
>use.  Questions are:
>Can I use the same 5.5 turn helix for a feed?
>How much more gain will I realize?
>Do any of you have the 1.2 in use?
>
>73...Bob...W7LRD
>Seattle

Hi Bob,

Well I'm going to guess a bit here, since I do not use the Primestar dish.
But my understanding is this is an offset feed dish that is broader in the
horizontal direction than the vertical one.  If so, that means using a
helical feed will only illuminate the roughly circular portion of the dish
in the center and not use the horizontal edges.

So the relative gain will be set by the vertical dimention, only.  Gain
increase will be in direct relation to the square of the ratio of
diameters:  G = [48/26]^2 = 3.41  and converting to dB:  GdB = 10Log(3.41)
= 5.3 dB.

I would guess you can use the same helix feed.  I hope those using
Primestar dishes will speak up and confirm this!

The 48-inch dish will have a gain = 10Log[4*pi*0.6*(48*2.54/wavelenth in
cm)^2] = 10Log[12.57*.6*(95.2)] = 10Log[718] = 28.6 dBi   {an efficiency of
60% is used for the offset feed dish vs 50% for center-fed dishes} 

More than you asked, I guess...should work at least an s-unit better!

Ed - AL7EB

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