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Re: Double Dishes



Scott:

Thanks a heap for noodling this over for me...

>A quick array calculation shows that you could just get away with it. 
>The edges of the two dishes will have to touch so that the focus-focus
>spacing is 60cm.

Why does the focus-focus spacing matter?  Couldn't I have the two dishes any arbitrary distance apart and still work just as well (ignoring 
additional feedline loss for the moment), assuming feedlines are of equal length, and the whole mess is on a swiveling boom so that they're 
exactly the same distance from the bird (when properly pointed)?

>You should be able to get +3dB out of the lashup.

That's what I figured - twice the antenna.

>You'd end up with a 2.8 degree half-power beamwidth (plus or minus from
>maximum)!
>You'll want an honest power divider to combine the feeds rather than a
>simple coaxial tee.

Yeah, I guess with a tee some of the power from the "left" helix would go to the "right" helix, which would defeat the purpose.

>The two feedlines from the two dish feeds to the power divider will need
>to be matched to < 30 degrees electrical length error to avoid squinting
>the beam more than 1 degree (which may be a bit high to design to).
>30 degrees at S-band with velocity factor=0.78 is .32 inches of cable.

Hmm...  (300/2401)*0.78 * (30/360) = 0.00812 m = 0.812 cm, 0.812/2.54 = .320 inches.

Yeah, good point.  I think I could get the cables well within that length tolerance (with a few tries, anyway...).

>Plus, any mechanical placement error in the phase centers of the feeds
>will...be bad.  Can't do *all* the engineering for you!

Why can't you? ;^)  

30 degrees at velocity factor = 1.0 would be ...
(taking the .78 out of the above...) 1.04 cm = 0.410 in.

Not much slop room for mechanicals.  At any distance apart, getting the Dish Twins within 1 cm bird-to-dish spacing would be, shall we say, 
dicey?

Thanks!  I think maybe I'll keep looking for an abandoned Primestar...

David, ky0dr

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