----- Original Message ----- From: "Spectrum International, Inc" [email protected] To: "John Hackett_LA2QAA" [email protected] Cc: [email protected]; "AMSAT-BB" [email protected] Sent: Wednesday, August 25, 2010 4:49 PM Subject: [amsat-bb] Circular Polarized Antennae
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There is however one (and only one that I am aware of) antenna design that is circular polarized over its entire radiation pattern. I refer you to the Q_uadrifilar Helix Antenna_ described by Dr. C.C.Kilgus in IEEE Trans., Vol. AP-16, July 1968, pp. 499-500. Also Bricker, R.W. and Rickert, H.H., in RCA Engineer, Vol.20, No. 5, February/March 1975. There is an excellent review by Walter Maxwell, W2DU, at http://www.IAG.net/~w2du/quadfinal.pdf.
When installed pointing to the zenith, the "ideal,theoretical" Quad Helix has 360 degree coverage in the azimuthal plane and 90 degree coverage in the elevation plane. It is circularly polarized over the entire upper hemisphere. There is no radiation in the lower hemi-sphere; the energy in the lower hemi-sphere of an isotropic radiator is uniformly distributed over the upper hemi-sphere. Hence the gain of an "ideal" Quad Helix is 3.01 dBi. However you can modify the elevation pattern to give more gain at the horizon and less gain overhead by adjustment of the overall length to diameter ratio. It is possible to adjust this ratio to give constant signal amplitude, at an earth based receiving station, from a satellite in a circular orbit where the range ratio (and hence signal path attenuation) between AOS and the zenith can be significant. This results in a little radiation below the horizon and also avoids the nasty mathematical boundary value problem at the horizon in the ideal case.
Regards, Spectrum.
Hi Spectrum International,Inc
What you stated is absolutely correct and in my opinion the best comprehensive article written over the Quadrifilar Helix Antennas by Walter Maxell W2DU in his book "REFLECTIONS Transmission Lines and Antennas" is the following one:
Best 73" de
i8CVS Domenico