[Date Prev][Date Next][Thread Prev][Thread Next] - [Date Index][Thread Index][Author Index]

Re: JavaScript Axial-Mode Helix Calculator



Hi all

I've been following this thread with some interest.

I'm interested to know what kind of methods are used to measure the
matching - I've had an S-Band helix for some time but haven't used it
because the one time I've had to check its matching on proper equipment I
was told it was a bit of a hopeless case, maybe needing the brass strips or
other adjustments. The gain was only 2dB on a 20 turn helix, so obviously
something was wrong!

So are there ways of checking matching on S Band without investing heavily
in Hewlett Packard equipment?

73 Howard G6LVB
----- Original Message -----
From: "Don DeGregori" <don1mh@loop.com>
To: "Frederick M. Spinner" <fspinner@hotmail.com>
Cc: <amsat-bb@AMSAT.Org>
Sent: Wednesday, December 13, 2000 6:16 AM
Subject: Re: [amsat-bb] JavaScript Axial-Mode Helix Calculator


> Fred
> 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?
> Don
>
> "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
> > >
> > >
> >
> >
____________________________________________________________________________
_________
> > Get more from the Web.  FREE MSN Explorer download :
http://explorer.msn.com
>
> ----
> Via the amsat-bb mailing list at AMSAT.ORG courtesy of AMSAT-NA.
> To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org
>

----
Via the amsat-bb mailing list at AMSAT.ORG courtesy of AMSAT-NA.
To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org



AMSAT Top AMSAT Home