[amsat-bb] Re: Quadrifilar Helix antenna question

i8cvs domenico.i8cvs at tin.it
Sat Jan 21 15:01:32 PST 2012

Hi William, PE1RAH

In a separate email I have sent to you copy of my article "Experimental
Investigation of Quadrifilar Helix Antennas for 2400 MHz that was published
into the AMSAT Journal May/June 2004 particularly covering the infinite
balun theory and applications if you need a End-Fire or Back-Fire radiation
from the Antenna.

Also I added an article written by Walter Maxell W2DU one the well know
guru about the Helix Antennas.

Finally I have added an artiche that I have written for Radio Rivista about
a QFA for 70 cm but sorry it is in italian but you can read the drawings.

Best 73" de

i8CVS Domenico

----- Original Message ----- 
From: "William Leijenaar" <pe1rah at yahoo.com>
To: <amsat-bb at amsat.org>
Sent: Saturday, January 21, 2012 11:19 PM
Subject: [amsat-bb] Quadrifilar Helix antenna question


Because I can not set up my rotor/elevation system at the new QTH, I try to
get some omni antenna's to be active on the OSCAR's.
I made a 2m version and 70cm version from some calculation website
(www.jcoppens.com) on the internet. Unfortunately both were too low resonant
in frequency. Cutting the lengths shorter, (to get a higher resonance
frequency) didn't work well, as I got too short elements at a certain time
and the mechanical construction didn't allow more shorter elements.

So I started all over again, this time taking the well known Rothamel
antenna book as example. There is an GPS example which I re-calculated to a
70cm version and combined it with an infinite balun that is also described
one page further. For the elements (one element as infinite balun) I used
Aircell-7 coaxial cable. When I measured this antenna it surprisingly had
also a too low resonance frequency, some 60MHz lower then the expected
435MHz... Another thing I noticed is that the return loss is around 10dB
best case for this kind of helix antenna (that's at 375MHz resonance). It
gets better when I touch the elements, but that should not be the way it
should work...

Does anyone has experience with quadrifilar helix antenna's on 2m and 70cm
for TX ?

When using the infinite balun principle, should it be a number of 1/4
wavelength ? or should it fit the antenna element length ?
Because the quadrifilar element length (=infinite balun length) was shorter
than 3/4 lambda length. Due to this fact I actually expected a higher
resonance frequency, but it went out the opposite way.

FYI: The supporting of the elements is all done with PVC tubes, to avoid any
metal influence.

Kind regards,

73 de PE1RAH, William Leijenaar
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