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Re: Mode-S Helix Design

fspinner@hotmail.com writes:<snip>

> This is a very good dialog... According to my personal correspondence

I agree.  In a few months, everybody will be clamoring for a mode S antenna of some kind.
> with AF9Y, he believes that the tapering is important to the match.
> I still think it'd be valuable to come up with an optimized and
> reproducable design, as this could become the de-facto P3D antenna.
> I'm considering in doing a direct and accurate scale of the AF9Y
> design for 2.4 GHz (and especially 1269) but this would require
> getting rid of the PVC pipe support, greatly adding complexity
> to building of the design.
> I find you comments of the reflector diameter to be critical interesting-- I
> suspected that, but didn't have time to try too many
> different sizes.  Is the 1.5" diameter you have posted the ultimate?

I did not physically try different diameters, that comment was based on my modeling results only.  I scaled from the 1.4 nominal circumference of the 1" PVC (1.32") + 10 AWG wire (.102") and added 5 percent (same as I did for 70 cm).

I just realized I may have left out an important construction note on my web page discussion of the 2.4 gHz version.  When I "played" with the 70 cm version, I determined the distance between the feed point and the end of the first 180 degree point needed to be elongated.  When I built the 13 cm version, I made the first turn 1-1/4" spacing as opposed to the designed 1-1/16" spacing for the remaining turns.  This "mod" brought the 70 cm version SWR down from 2.5 to right at 1.0.  If you look closely at the pictures, you can see the slightly wider spacing.  Sorry--I'll correct it tonight.

The tapered AF9Y design clearly (from modeling) has improved gain and much reduced side lobes, especially the nasty ones at 45/135 degrees.  The challenge at 13 cm will be to replicate the very small differences in size from loop to loop.  Therein lies the beauty of the G3RUH and W0OQC designs--the average ham (like me) can build one around a piece of garden-variety PVC pipe and get it to work.
Jerry, K5OE
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