Satellite Antennas Part 6 by GM4IHJ
When building a ground station for satellite operation at L band 1269 MHz
or S band 2401 MHz, the dipole or yagi antenna is not necessarily the best
answer. Several RSGB and ARRL publications feature yagis where the
elements are in the form of loops. These can work well but they need very
careful construction particularly in respect of accurate physical
dimensions - near enough is not good enough. Equally important is the fact
that you need to rescale the dimensions from the terrestrial examples
given for 1296 MHz , to the satellite frequency of 1269 MHz.
If you do not like the idea of building a tight tolerance loop yagi, a
helix is a much simpler solution at both L and S band . No rescaling is
necessary , a helix for terrestrial freqencies is wide band enough to
cover satellite frequencies. L band antennas pose few further problems
because the band is used only for Phase 3 uplinks. But S band is used for
downlinks on both LEO Low earth orbit satellites which have huge doppler
shift and cross your sky very quickly, and it is used for Phase 3
downlinks with limited doppler shift and almost no azimuth movement. At
IHJ these quite dissimilar sets of requirements were solved by building a
simple twin helix the same size and general shape as a pair of binoculars
for chasing the high speed tracking of Dove, Packsat and other LEO birds
where very little antenna gain is required. This twin helix mounted on a
camera tripod pointing out of a roof window also worked on Oscar 13 S band
CW signals, but it lacked the required gain for reception of the apogee
SSB signals. So a bigger long helix was built for Oscar 13 S band, where
its high gain gave better signal copy and its narrow beam was not a
disadvantage when following the slow moving Oscar 13.
At both L and S band IHJ has also tried two dishes, a 90cm TV dish and a 2
metre dish normally used on radio astronomy targets and near horizon
geosats. With these dishes several types of antenna feed have been tried.
QST for August 90 showed a very useful multi band feed which is built on a
PCB card ( power limit about 15/20 watts max). This is now available
commercially at UK rallies. Or you can use a cavity resonator , RSGB ARRL
books have several designs which are easy to build. At IHJ several sizes
of powdered milk can do service at various frequencies including L and S
bands . Indeed antenna feeds of this type can be changed around easily
allowing perusal of some of the commercial satellite signals you may wish
to check out equipment on ( Eg the QPSK comms circa 1500 MHz )
At still higher frequencies amateur satellites are presently rare. But
with the 90 cm dish it has proved possible to explore around 10 GHz and
also try reception on the TVSAT band above 10 GHz using feedhorns and
polariser obtained from SATTV specialists
Before closing this antenna series it is perhaps important to remind would
be users of these microwave antennas that the weather in Northern UK and
elsewhere, is a great wrecker of antennas (and people). Big dishes need
very heavy strong mountings , and tracking screws on polar or AZEL mounts
must be very strong. Fragile cavity resonators simple cave in when covered
in ice , and fragile operators should not get in the way of a big dish if
the wind has decided to move it . IHJ had a badly bruised foot for weeks
when he tried to alter the azimuth of a 2 metre dish in "a very gentle ?