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Re: JavaScript Axial-Mode Helix Calculator



Fred, et al:

I have built a couple helical antennas.  In 1996 I built an eight foot long
437 MHz helix for receiving MGS.  It was modeled after the design by AF9Y.
It tuned perfectly without adjustment.  I use a quarter wave copper strip
line from the connector to the beginning turn.  It was very close to the
ground plane [a 15 inch pizza pan].  The helix was stepped down in diameter
to approximate a taper.

This fall, I used NEC4WIN to model a 3-turn dish feed Helix for 1420 MHz.
The feed used a 1/4 WL stripline, also.  I haven't a signal source to
measure return loss, but it is detecting solar noise and celestial sources
OK on my 2.5m dish.

I will use this design as a starting point for a 2400 MHz feed antenna for
my 18 inch DSS offset dish.  Will share it with all of you when I get it
done!  Helix length, diameter, spacing, feed Z, all effect both impedance
and resonant freq.  It take a bunch of iterations to get it where you want
it :-)

Ed

>From: "Frederick M. Spinner" <fspinner@hotmail.com>
>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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