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Re: IC-910 / 3731



Tony,
My $0.02 is that I don't agree with either of your points :-)
1. With a 4" diameter ground plane, the blockage on a 60cm dish is only 
(2/12)^2=1/36~3% of the total dish aperture.
2. A properly wound 5-turn helix has very little sidelobe 
structure.  Front-to-back may not be the greatest, but it doesn't look 
anything like a longer helix where the 1st sidelobe is only 8 to 10dB down 
from the main beam.  Besides, what's the alternative for a dish with a f/D 
of nearly 0.6 (most offsets are around this value)?  The patch is clearly 
too broad-beamed.

And 1a. there is a way to feed a prime-focus dish with nearly zero feed 
blockage...I just haven't written it up yet (but I am using it!).


At 17:19 2003-11-13, Anthony Monteiro wrote:
>At 06:02 PM 11/13/2003 -0500, Andrew Glasbrenner wrote:
>
>
>>What sets the gain characteristics apart from a 600mm center fed pay tv dish
>>and G3RUH's 60cm center fed dish? Also, why are offset feed dishes not good
>>below 30 degrees south latitude? Is the converse true for 30 degrees north?
>>
>>73, Drew KO4MA
>
>Hi Drew,
>
>Nice to hear from you (enjoyed the symposium!)
>
>I don't see how a center fed 60cm dish could work very
>well since the feed blockage is so severe at 2.4 GHz
>whereas a 60cm offset-fed would seem passable if
>fed correctly. Don't you think?
>
>I think most of the problems with offset fed dishes
>is that people try to feed them with a helix which
>picks up too much ground noise.
>
>Any comments?
>
>Tony AA2TX
>
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Scott Townley NX7U
Gilbert, AZ  DM43di
http://members.cox.net/nx7u

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