Satgen 566 SatSignal Reception 2 by GM4IHJ (BID SGEN566) 2000-01-29 The problems encountered by an HF satellite signal transiting down through the ionosphere were discussed in Satgen565. In consequence the arrival of Oscar 7 Mode B 432MHz up 145 MHz down , was anticipated with much pleasure, and for the most part this optimism turned out to be justified. It was no longer necessary to worry about signal attenuation in the ionosphere, but there was some Faraday rotation of the plane of signal polarisation. As a rough rule of thumb if you go 5 times higher in downlink frequency you get 5 times less Faraday fading , ie 15 fades on a 29 MHz downlink pass , but only 3 on a 145 MHz downlink. Which is important - BUT the fades last longer at 145 MHz. Oscar 7 used permanent bar magnet stabilisation, coupling to the earths magnetic field to keep it pointing at the earth as it orbited. It was not spin stabilized as future satellites would be. Even at launch it only rotated once every 2 minutes and this soon reduced to more than 10 minutes per revolution. So physical rotation of its satellite antennas with respect to your ground station antenna was not very noticeable. A simple switch betwen a horizontal 5 element 2m yagi , and a 5 element vertically polarised yagi sufficed even when the DX was weak , down in the noise. The arrival of Oscar 10 ( 1983), changed all that. The satellite was designed from the start to be spin stabilized. So it mounted simple circularly polarised antennas, and was therefore best received on a similar circularly polarised ground station antenna. Which would have been prohibitively massive at 29 MHz, but at 2m , was easily accomodated. Which should and did solve most of the Oscar 10 problems. But when Oscar 13 arrived in 1988, it presented new problems. The simple circularly polarised antennas on both Oscar 10 and Oscar 13 , had to be a compromise between less than perfect circularity , and the limited space available on the satellite body. Normally, if the satellite was pointing its antenna mounting face straight at you, this did not matter. The circularity of the satellites transmission seen straight on was reasonable, although in the case of Oscar 13, a small antenna for a higher frequency mode clearly distorted the pattern producing spin modulation of the received signal. But at regular intervals Oscar 13s heavy power demands necessitated pointing it , not at earth, but at an offset angle which maximised the amount of sunlight falling on its solar power panels. Such that at times the offset to earth pointing was so great that the simple antennas on the satellite , producing acceptable circular polarisation head on, produced ,as the offset increased, first elliptical polarisation, then as the offset increased further, the opposite direction of circulation ie left hand circular polarisation as seen from earth way off the centre of the satellites antenna cone , instead of the right hand circular polarisation encounter with no offset. A problem some afficionados tackled by using two quite separate ground station antennas . One right hand circular, the other left hand switching between them as necessary.