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Re: 23/13cm dual helix feeds - response




No, not really-- for reasonably sized dishes the depth is going to be 
relatively shallow versus the size increase.  A DH Satellite 1.2m
dish, according to their website, is 48" before spinning (They use a 
citrcular cut sheet of aluminum 48 inches in diameter) and after spinning 
(shaping the dish to the parabolic curve) the overall diameter is 46 1/4" 
with a dish depth of 6".  (They don't have the specifications for the 
smaller dishes, but you could guess 1/2 the size of all of these dimensions 
for a 24" dish and be in the ballpark)

So on a 24" dish, you *might* gain an extra 3-4"-- This isn't enough to make 
up 3 dB.  No Way.. But, you'd also lose 5-6" of depth from the 4' dish... so 
no, area is still less by 4x... 10 log (4) = 6 dB

Also, my understanding is that most commercial dishes are sized like that, 
i.e. the 1.2m 4 "footer" is actually a 46 1/4" dish.

The 2 foot Andrews (I think) surplus dish I'm using now is actually 26" in 
diameter.  So I think "close enough" applies here.

More likely Gunther's S-meter is 6 dB a S-unit at the range he was measuring 
rather than 3 dB..But if he can hear the transponder noise floor, thats all 
he needs no matter what levels the signals are.
(If it ain't broke, it ain't broke..)

Fred W0FMS


>From: "Don Woodward" <dbwoodw@earthlink.net>
>Reply-To: "Don Woodward" <dbwoodw@earthlink.net>
>To: "Frederick M. Spinner" <fspinner@hotmail.com>
>Subject: Re: [amsat-bb] 23/13cm dual helix feeds - response
>Date: Tue, 30 Oct 2001 21:19:56 -0500
>
>Fred:
>
>Dish depth is included in the area to calculate the gain right? Could that
>explain Gunther's difference in gain figures between his 1.2m and 60cm?
>
>73's
>
>Don Woodward
>KD4APP
>AMSAT 33535
>dbwoodw@abraxis.com
>sales@sunsunsun.net
>www.sunsunsun.net
>
>
>----- Original Message -----
>From: "Frederick M. Spinner" <fspinner@hotmail.com> wrote...
>
>This is rather amazing in and of itself, because gain in a dish antenna is 
>a
>function of area and not diameter...
>
>This means going from a 120cm dish to a 60cm dish you should have seen
>levels go down 6 dB.  It's true, I'm not making this stuff up...
>
>3 dB would be more like going from a 4 footer (120cm) to a 3 footer
>(90cm)...
>
>Fred W0FMS
>
>
>
>
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