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Re: JavaScript Axial-Mode Helix Calculator

Thanks for the brass strip tuning idea, or just nudging the 1st quarter turn of the
helix. Looks like the problem can be solved, at least for receiving. Move it for best
signal. Too bad the bird doesn't  have as beacon on 1269. Then, we could tune up our
uplink antenna the same way, and then transmit on it. Oh well!
Now to find a 2400 downconverter. And blue Drakes out there gathering dust?

"Frederick M. Spinner" wrote:

> 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.
> >
> >Fred
> >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?
> >73...Don
> >
> >
> _____________________________________________________________________________________
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