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Re: IC-910 / 3731

At 08:02 PM 11/13/2003 -0700, Scott Townley wrote:

>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!).

Hi Scott,

The blockage loss doesn't work that way. It is much
worse than just the area blocked because the feed
signal is at its max in the center and tapers off
at the edges. In other words, blockage in the
center results in a lot higher loss than blockage at the
edges for the same size obstruction.

I realized when I sent this out though, that I was thinking
of an 18" dish where the feed blockage is horrendous
on a prime focus dish. For a 60cm dish it is only about .7
to .8 dB - not that bad.

I agree with you that a patch would be a poor feed for
an offset fed .6 f/d dish. A better feed would be a horn.

1a. - OK, I'll bite! How do you get nearly zero feed
blockage on a 60cm prime focus dish?

Tony AA2TX

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