[amsat-bb] Quadrifilar Helix antenna question

William Leijenaar pe1rah at yahoo.com
Sat Jan 21 14:19:11 PST 2012


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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