[amsat-bb] Circular Polarized Antennae

Spectrum International, Inc spectrum.ma.ultranet at rcn.com
Wed Aug 25 07:49:25 PDT 2010

                John,  LA2QAA

                           That was an interesting message you posted to 
the -bb this morning.  It has many true observations; however there is 
one serious mistake, an unforgivable error.

                ......... particularly by the newcomers - who 
apparently, and mistakenly, think that high power is necessary.
 It's  *NOT* . . . nor is an EME class aerial array necessary to work 
the LEO satellites. Ditto circular polarisation.

FACT:    polarisation is  *ONLY* circular along the boresight of an aerial.

                    Your polarization statement is not 100% accurate. 
Unfortunately most antenna manufacturers, who supply/sell "alleged 
circular" polarized antennas do not state that their antennas are 
circular polarized ONLY along the boresight. They conveniently (?) do 
not mention that their alleged "circular polarized" antenna is only 
circular polarized on boresight, that it is linear polarized at 90 
degrees off of boresight and is elliptical polarized of varying ratio at 
all angles in between!

              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.

               Quadrifilar Helix antennas are used on many LEO 
satellites  for VHF, UHF, L-band and S-band communication.  One of 
their  parameters of interest to satellite builders is that they do NOT 
require a "ground plane" provided they are at least a quarter wavelength 
above the satellite structure. Hence there is negligible critical 
location requirement and they do not occupy satellite surface area that 
is required for solar cells.

                The Quadrifilar Helix antenna is popular with earth 
based receiving stations for receiving the VHF image data signals from 
the NOAA weather satellites. Right-hand circular Quad Helix antennas for 
the 137MHz NOAA weather image signals and 137MHz left-hand circular Quad 
Helix antennas for receiving the ARGOS  programme signals, are available 
from Spectrum International, Inc. Spectrum also supplies both right-hand 
and left-hand versions for the 2M and 70cm Amateur radio bands.

                May we suggest you sprinkle some "Grow More fertilizer" 
around the base of your "chopped down satellite array" every Sunday 
morning and offer a few words of wisdom while so doing. With a little 
help and the dregs of Saturday night's Black and Tan, your mini array 
might grow.

                Regards,     Spectrum.


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