[amsat-bb] Circular Polarized Antennae
Spectrum International, Inc
spectrum.ma.ultranet at rcn.com
Wed Aug 25 07:49:25 PDT 2010
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
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