[amsat-bb] Re: Quadrifilar Helix antenna question

i8cvs domenico.i8cvs at tin.it
Mon Jan 23 14:38:32 PST 2012

Hi Norm, N3YFK

Are you sure that your Antenna Analyzer Times T-100 is reading a correct
VSWR and R+/-jX impedance at 435 MHz ?
My MFJ-269 connected to a professional 50 ohm termination goog up to
18 GHz reads VSWR = 1.2 :1 but unfortunately the MFJ-269 do not read
 R+/-jX impedance above 144 MHz

Do not forget that to get RHCP the helix must be winded lefthand and on
reverse to get LHCP the helix must be winded righthand.

In addition,depending on how you connect the four terminals of the loops
into the infinite balun you can get or End-Fire radiation or Back-Fire

I am interested on your experiments using your data to put it into the Smith
Chart and check

BTW on 70 cm using half-turn quadrifilar (two half-turn bifilar loops).
with dimensions that yield the correct phase relationship with a loop wire
diameter of 0.0088 wavelenght as follows and suggested by Walter
Maxell W2BU

and using:

Smaller loop:
D = 0.156 wavelenght
lenP° = 0.238 wavelenght
Perimeter= 1.016 wavelenght

Larger loop:
D = 0.173 wavelenght
lenP° = 0.260 wavelenght
Perimeter = 1.120 wavelenght

I got a return loss of -20 dB about a VSWR of 1.22 : 1 at 435 MHz as
measured on my HP8555A Spectrum Analyzer usind a well calibrated
HP11692D directional coupler.

73" de

i8CVS Domenico

----- Original Message -----
From: <normn3ykf at stny.rr.com>
To: "AMSAT-BB" <amsat-bb at amsat.org>; "i8cvs" <domenico.i8cvs at tin.it>;
"William Leijenaar" <pe1rah at yahoo.com>
Sent: Monday, January 23, 2012 3:03 AM
Subject: Re: [amsat-bb] Re: Quadrifilar Helix antenna question

> Gentelmen:
> Thank you for your input. I've measured 2.4:1 at it's intended design
frequency, using a half wave stub.
> The antenna is essentially two loops (one capacitive and one inductive) in
parallel, giving an intended 50 Ohm resistive impedance at resonance. I'm
measuring 25 Ohms and a bit inductive. Do have more data if anyone is
interested. (smith chart, return loss)
> Tomorrow I'll build both versions, lhcp and rhcp. Tonight's live demo on
so50 shows promise, though I'll need two and have to switch polarity.
> 73 de Norm n3ykf

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