Re: [amsat-bb] TVRO Dish Feeds - Patch vs Helix

["Frederick M. Spinner" <fspinner@xxxxxxxxxxx>]

A 1m dish is about as small as I'd like to use, though many have used 
smaller.  For the Ku-band type dishes, a helix of about 5.25 turns
with a slightly narrower pitch than normal seems like the best type of feed 
to use for that type of dish.

However, a 1m prime focus with a patch would likely work better.  Offset fed 
dishes are technically better for satellite work, but small helixes
suffer from much larger sidelobes, picking up much more ground and 
atmospheric noise.  But for your dish the above feed is about the
best compromize and will work quite well if you have a good low noise 
receiving setup.

Fred W0FMS


>From: "David Beach" <VE3STI@rac.ca>
>To: "amsat-bb" <amsat-bb@AMSAT.Org>
>Subject: [amsat-bb] TVRO Dish Feeds - Patch vs Helix
>Date: Fri, 27 Dec 2002 13:25:44 -0500
>
>I see that many hams are using 60-100 cm satellite TV dishes with homebrew
>feeds for AO-40. What puzzles me are the seemingly conflicting comments
>regarding patch vs. helical feeds.
>
>I would like to try constructing a receiving dish for 2.4G using an old
>offset feed dish that appears to be about 1 m in diameter. I haven't chosen
>this particular dish for any reason other than it happens to be available.
>If such a dish is a lousy choice, let me know!
>
>I have read a fair bit of stuff on the net at various websites but still
>have a few questions:
>
>For this type of dish (~1 m dia, offset feed) what are the pro's and con's
>of using a patch vs. helical feed? Both seem fairly easy to construct from
>website pictures/instructions I have seen.
>
>Is the f/d ratio important for choosing between these differing feeds? (I
>have not measured the f/d for this dish so don't know what it is.) One
>website said the ratio is important for choosing the feed type, others are
>'mum' on the subject.
>
>Does a patch or helical feed 'fit better' with a specific type of dish -
>offset vs central placement of the active element?
>
>Thanks again for your comments. I don't wish to be a pest but I have looked
>already (!) and am still a bit puzzled. Good URLs would be fine if you want
>to keep your reply short and the info is available but I missed it in my
>searches.
>
>David Beach
>VE3STI
>
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