Re: [amsat-bb] Helix Feed for a Dish

["Edward R. Cole" <al7eb@xxxxxxxxxxxxx>]

>From: "John Stephensen, KD6OZH" <kd6ozh@gte.net>
>I tried the reverse today. I have a 3 turn helix (0.75" turn spacing) with a
>square "cup" feeding a 33" 0.3 F/D dish. With the 4" x 4" x 2.25" cup I
>measured 1.9 dB of Sun noise. After removing it the difference between Sun
>noise and noise when pointed straight up decreased to 1.2 dB.
>
>73,
>
>John
>KD6OZH

Yep, doesn't matter what the order you try it.  I ran the G/T math and
determined that I will not be effective in seeing sun noise until I have a
lower NF {preamp}; 0.5 dB vs 2.5 dB.  So bidding my time until then.

It appears the experience so far with the cavity-helix, or "cup", is very
positive toward better performance.  I would say this is due to improved
antenna temperature {less ground pickup from sidelobes}, rather than any
gain enhancement.  But then that is the purpose of the "cup".  Nice how it
actully turns out.  This may become a standard feed for 2.4G!

Now has anyone experiented with a circular horn {coffee can} feed, for
comparison?  I would expect less performance due to higher illumination
angles and sidelobes, as well as, polarization loss {its linear if you use
only one coax probe}.  Still it might be usefull with offset dishes where
there was no dish blockage issue.  If a scalar choke ring were added {VE4MA
style horn}, it might even outperform the cavity-helix.

Dishes are fun :>)

Ed

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