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Re: JavaScript Axial-Mode Helix Calculator

I am not an expert at this, but in my experience the theory matches with 
reality fairly well.

Feeding at the center "a traditional helix" produces a 140 Ohm feed.

Simply perhiperally feeding the helix lowers the impedance somewhat--
maybe 100 Ohms.  Especially if there is almost no pitch to the first 1/4 
turn (1/4 wl)

Adding a brass strip, like the G3RUH antennas (ftp.amsat.com site) 
"fattening up" the 1/4 wave impedance transformer gets you to about 50 Ohms 
with tweaking.

I imagine that on 2400 MHz receive, the SWR will not make too much of
a problem with gain, but I have read from Ed Krome, K9EK that
the pattern suffers if there isn't a match.  Again, I don't think
that will be too much of a problem on AO-40.  Maybe Ed can comment again.

One web site I saw states that a ham had success leaving the first 1/4
turn at the same pitch, but then he had a tunable brass strip protruding 
from the reflector, and a brass screw tapped for adjusting the spacing 
between the brass strips, and the one on the first 1/4 turn.  He then 
adjusted for maximum signal.  Alternatively, after you've formed the helix-- 
don't mount it permanently to the support structure-- push in on the first 
1/4 turn with the brass strip until you get maximum signal (or best SWR if 
you could measure).  I think this would work as well.  On receive, max 
signal is max signal.

On the 1270 MHz transmit side, I plan on using a directional coupler
I have and a Boonton RF Millivoltmeter (w/50 Ohm adapter) to tune the helix. 
  A diode detector and a homemade directional coupler could be made fairly 
easily.  Tune for minimum reflection, and you've got it.

Fred W0FMS

>From: Don DeGregori <don1mh@loop.com>
>To: "Frederick M. Spinner" <fspinner@hotmail.com>
>Subject: Re: [amsat-bb] JavaScript Axial-Mode Helix Calculator
>Date: Tue, 12 Dec 2000 07:22:33 -0800
>"Frederick M. Spinner" wrote:
> > The pitch is set at 12.8 degrees, which is (I guess) close to
> > optimal for gain.  The circumference is calculated by the program to be
> > optimal with the pitch.  In other words, it is based on a program
> > that is less NEC like.. it generates "optimal" 12.8 degree pitch
> > helicies.
>Thanks for the info. I should have figured you held 1 parameter constant 
>(pitch). Anyway,
>I think your program gets people going to start building something. Choose 
>freq, select
>turns, and "DO IT"! One other question: Is reducing the turns spacing near 
>the reflector
>end of the helix the normal way of reducing the feed impedance? I guess 
>this more
>important in transmitting than receiving? Most people don't have of a way 
>of checking
>this. So, how much change?

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