Satellite Antennas

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


Back To Main