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Re: Helix Feed for a Dish

I have 3 x 16-turn helix array that should have 19 dBic of gain. The 33"
dish should have about 33 dBic of gain. However, the difference in SNR when
listening to the AO-40 beacon is 6-7 dB instead of the 4 db expected. I
suspect that this is due to the elimination of sidelobes with the shielded
heical feed and the fact that the dish has a 0.3 F/D so illumination is
probably 12-15 dB down at the edges.

The triple helix array is roof-mounted while the dish is sitting between the
house and the garage. The dish is much more likely to pick up extraneous
thermal noise from sidelobes but it doesn't.

The copper tubing probably shows a better match because it has more
capacitance to the reflector at the feedpoint and is therefore a lower
impedance. I used 3/16" tubing and flattened it over the last 1/4 turn so
that the impedance slowly goes from the 150 ohms of the helix to 50 ohms.
The flat portion sits on top of a Teflon pad and should form a 50-ohm
microstrip line. I measured the return loss 5 or 6 years ago and it was
about 15 dB.



----- Original Message -----
From: <K5OE@aol.com>
To: <kd6ozh@AMSAT.Org>; <amsat-bb@AMSAT.Org>
Sent: Tuesday, 05 June 2001 11:35 UTC
Subject: Re: [amsat-bb] Helix Feed for a Dish

> John,
> 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:
> http://members.aol.com/k5oe/helix_cup04.jpg
> http://members.aol.com/k5oe/helix_cup03.jpg
> http://members.aol.com/k5oe/helix_cup02.jpg
> http://members.aol.com/k5oe/helix_cup01.jpg
> 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
> 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.
> 73,
> 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.
> 73,
> John
> ----- 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"
> helix
> > 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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