Satgen 479 Circular Polarity ?? by GM4IHJ (BID SGEN479) 1998-05-30 It seems to have become a long accepted, but very questionable "truth" of satellite operations, that circular polarisation of antennas is the optimum mode of construction for both up links to and downlinks from satellites. But, a study of operating with, current and past antenna outfits on satellites, suggests that what may be optimum for bigger satellites which can carry large helical antennas , is far from optimum on small amateur satellites. The main problems arise with the way in which so called circular polarisation is achieved on amsats. Very few amsats have sufficient size for genuine circularly polarised arrays. So we have to make do with either spaced dipoles on a flat surface wired say with 120 degree phase shift or, monopoles placed on each of the corners of one face of the satellite. A situation which results in a very coarse version of circular polarisation towards stations normal to the plane carrying the dipoles or monopoles, but, a rapid shift through elliptical, reversed circular and near linear polarisation, as the line of sight to ground stations moves off the normal to the antenna plane. "So What" many Pacsat, Lusat, Uosat 22 (and dear departed A013 ) users will say . "We get super results most of the time with our circularly polarised ground station antennas , all the way, as the satellite goes from horizon to horizon". They do so however because of the massive excess in signal margins which these low fliers normally provide. Shift the situation to a day disturbed by Sporadic E, and they do not hear a thing for half the orbit pass, or if they do hear it , it suffers the ignominy of requiring reverse polarisation for reception. Indeed , in practical examples of a typical Es afternoon in late May, a station with reception capability RHCP/LHCP/H/V, will get copy which peaks if the operator is prepared to chase polarisation changes every ten seconds or so. Worse still , those who dwell above latitude 50N or 50S, get this effect Es or not on most orbits as their target satellites come down from or go up to the pole , whereby the Polar Auroral Front produces a very sudden polarisation change, not just on amsats but also on LEO Wefax and Orbcom sat signals. Anything that in any way reduces the signal margins , will reveal polarisation mismatches. Fortunately A010 never goes high enough in latitude to be seriously affected. But Ao13 positively required that IHJ by capable of reception in all polarisations, although a straight switch between H or V yagis was adequate for almost all eventualities. Such that at this present time the IHJ antennas for 2m and 70cm feature quadrafiliar helixes for all round operation , and linear H and V yagis for most satellite tracking. The facts outlined here are not new but they does seem to need repeating every 5 years or so. PS. Contrary to some Amsat reports Ao10 is not out of action at present. Since it stopped spinning fast , it no longer looks like obeying the old routine that suggested it would be QRT until Aug 98. Indeed as of last week it was still getting stronger but had some FMing, whereas for the past few days it has been very strong even at apogee and it is not FMing. So JA and others are booming in to IHJ.