Satellite Antennas Part 5 by GM4IHJ
Circular polarisation was reported in Pt 4, as a means of automatically
cancelling natural rotation of signal polarisation. It is a sad fact that
the frequency on which we amateur satellite users most want to operate ie
145 MHz, is unfortunately too low in frequency = too long in wavelength
for really successful deployment of circular polarisation antennas on the
satellites themselves.
On Oscar 10 and 13, the satellite is simply not big enough to accomodate a
useful 2m helix. So a compromise antenna using 3 whips and reflectors
spaced at 120 degree intervals around the satellite's body, is used to
provide a modicum of circular polarisation. That it does not do this very
competently is evidenced by the spin modulation present on the downlink
signal audible on both satellites as a product of the uneven antenna polar
diagram, and, a second more elusive problem whereby the 2m antenna does
not produce circular polarisation unless your ground station antenna is
seeing the satellite " face on", looking directly at the whips mounted
around the flat face. If you are seeing the satellite from an angle to one
side of the normal to its flat face, you can get elliptical or even
reversed circular polarisation depending on your angle to the face.
Fortunately Phase 3D is a much bigger bird. So some improvement to the 2m
antenna platform design should be possible. BUT as Amsat designers have
been saying for years - and most radio amateurs steadfastly refuse to hear
, the sooner we start using higher frequencies , the better.
Up at 70cms Faraday is still a nuisance , but the flat face carrying the
three dipoles set 120 degrees apart is "relatively bigger" at this higher
frequency = shorter wavelength. So the antenna pattern is better than 2m.
However at L band 1269 MHz and S band 2401 MHz satellites can deploy
genuine helical antennas if required, with a resulting alround improvement
which will be discussed in Pt 6.
Meanwhile how can a ground station user cope with the odd behaviour of
Phase 3 2m antennas presently in space? Some people (IHJ included) use an
XY yagi mounted with the elements on the same shaft in the X configuration
45 degree polarisation. Two separate feed lines are run one from each yagi
both lines the same electrical length minus an allowance on one for their
physical distance apart on the antenna centre shaft ( distance between
dipoles multiplied by cable velocity factor Eg 5cm separation times .75
velocity factor on some not all coax requires 3.75 cm less cable from
rearmost dipole). The two coaxes are then coupled in a switch box using
several switchable delay lines to achieve LHC RHC H or V polarisation.
RSGB old VHF UHF Handbook page 7.24 Figure 7.50 refers. There was a
manufactured " Polariser or similar named device " on sale a few years ago