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Re: mode-s helices: Network Analysis



RE: W0OQC Helix.

Yup, thats the one.  I got mine to match by bending up the brass
strip as far as it will go from the reflector, lengthing it a bit,
and playing with the tap point slightly.  All in all it wasn't to
hard to do.  He also has a nice weak signal source on his page too
using a 50 MHz clock oscillator.  I built one with an 80 MHz source,
it has been invaluable on getting the old DEM mode-S converter functional.  
I agree that a DC ground is a good idea in the matching
network, that is why I tried this helix.

Howeve, I do wonder if the "traditional" brass strip feed couldn't be made 
"static safe" by adding a RF choke of some sort to ground, or even a high 
value resistor to ground.  Will try some experiments in the next round.

I won't be able to do any range testing (Its like -10 F here now), but the 
gain is to be expected, all three designs use the same pitch and spacing, 
and thats close to theoretical.

I still believe that a 3:1 match is probably good enough for 3D reception, 
though some sources state that circularity suffers with
a mismatch.

Hope to get pictures and data on my web site in the next week or so.

73,

Fred W0FMS



>From: W1BFN@aol.com
>To: fspinner@hotmail.com
>Subject: Re: [amsat-bb] mode-s helices: Network Analysis
>Date: Mon, 24 Jan 2000 09:07:23 EST
>
>Some of us down here in the wilds of North Carolina built the W0OQC helix,
>the original version with the matching tab, and bootlegged a test on a 
>local
>company's antenna range. Never did check the SWR, but I have copies of the
>gain tests, which came out at 18.5 dBi. The advantage of the shorted half
>wave feed is protection from static discharge, and I personally find that 
>to
>be reassuring. Probably if I ever replace my loop yagi, I'll use Bill's
>design, and tweak the tap for best receive strength from my test beacon.
>
>Mike

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