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Re: Helix Feed for a Dish

Thank you very much for reporting your observations.  That is indeed encouraging.  While I don't have a regular dish feed to compare it to, I did listen to UO-11 over the weekend and can report comparative signals to my 16-turn helix:  the helix provides about 1 S-unit above the noise, whereas the small 2' K5GNA dish with cup feed produces 5 S-units above the noise (S7 on a 60 degree pass).  I've never heard it this loud before.  FWIW, my RHCP 16-turn helix with flat reflector gives almost identical signals on UO-11 as my LHCP 10-turn helix with AF9Y ring reflector--making me suspicious circularity of the signal is far from perfect.

Pix of the beast are at:

After taking those measurements, I took the feed off the dish and put it on W4MVB's bridge analyzer.  It was a terrible match (not as good as the 16-turn).  I had not done any of the conventional matching techniques such as the flattened last 1/4-turn or the W0OQC grounded feed.  I just wound it on a piece of pipe and stuck it in the coffee can on an N-connector :-)  I will improve it this next weekend and see how that affects performance.  I had not previously worried about the match because I (maybe erroneously) assumed since I had no feedline losses, it would not matter much.  Some comments on the BB recently about the sensitivity of the preamp to the antenna feedpoint impedance makes me want to revisit the wisdom of my assumptions.

On a related topic:  in playing around with models of the short helix feeds, I note NEC4Win reports a significant inverse correlation between helix conductor size and feedpoint impedance.  To test this phenomenon (I don't entirely trust models as I have built many paper-perfect antennas that are junk in the corner of the garage now), I built an exact-dimmension replacement for my "original" #10 AWG (0.102" dia) 3-turn helix except substituted 1/4" Cu tubing (flattened to about 0.375" when wound).  This showed about 1/2 the mismatch (of the #10 wire) on Jess's bridge, but still far from a null.  Testing on local MMDS, the picture was slightly improved, but the result was impossible to quantity with these crude measurement techniques.  If my AMSAT-UK noise source kit ever shows up (hint-hint), I will have a consistant source for making comparative measurements.
Jerry, K5OE

In a message dated Tue, 5 Jun 2001  2:15:12 AM Eastern Daylight Time, "John Stephensen, KD6OZH" <kd6ozh@gte.net> writes:

<< 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.



----- Original Message -----
From: <K5OE@aol.com>
To: <fspinner@hotmail.com>; <amsat-bb@AMSAT.Org>
Sent: Friday, 01 June 2001 23:09 UTC
Subject: Re: [amsat-bb] Helix Feed for a Dish
> It would be very educational if someone who currently has a "regular"
> feed with reflector could take noise measurements and then install a cup
> around the feed and observe/report the difference.
> 73,
> Jerry, K5OE

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