Satgen463 Space Propagation Pt2 by GM4IHJ (BID SGEN463) 1998-02-07 In satgen 461 , the problem encountered by a signal coming up or down , through the ionosphere was discussed. Whereby a linear polarised signal invariably became elliptically polarised , and, maximum signal was received in a plane different from the one transmitted. In practice there are lots of other phenomena which affect the signal. So experimenting using a signal from a moving satellite can be very confusing. Some of this confusion can be reduced by using a geostationary satellite as your target. The table below shows what happened to a linearly polarised signal at 137 Mhz coming from a geostationary satellite during daylight hours in summer. The signal was received on a large XY yagi antenna with separate feeds coming from the X and Y sections to a two way selection switch and thence to the receiver, with receiver audio feeding an oscilloscope displaying signal amplitude and signal to noise ratio. Table 1 One Days observation of 137.08 MHz signal from Meteosat Time UTC Horiz cms Vert cms Sig/noise dBs Remarks 0640 2.3 2.8 12.66 Sunrise was at 0523ut 0733 2.9 2 10.88 0801 2.1 3 11.12 0820 1 3.4 10.88 0928 3 1.1 10.1 1028 1.3 3.2 10.88 1208 1.1 3.1 10.36 1242 1.3 3.7 11.82 1405 2.8 2 10.6 1503 3.1 1.6 10.88 1615 3.8 1.3 12.25 Aurora began 1648 2.8 1.9 10.63 1725 in noise 3.5 10.88 1826 1.4 3.8 12.04 Aurora ends 1945 1 3.8 12.04 Deep fading (sunset 1907) 2017 in noise 4 12.04 Deep fading 2039 3.1 in noise 9.82 Deep fading 2122 2.9 2.2 11.12 Fading decreased Summary. Though the path is fixed, the ionosphere along the path is constantly changing, being unsettled early in the day ,then stabilizing a little, albeit with continuous shifts in received polarisation angle. Until, at 1615 an aurora starts north ( behind the antenna), and there is a momentary increase in signal to noise, where signal is steady but noise level drops. Aside from this, the aurora continues with no obvious effect. But at sunset the signal starts to arrive via several paths ( established later by using antenna space diversity reception), so severe fading results as signals alternately add in phase or subtract when out of phase. Some time after sunset the situation again stabilises and fading gradual decreases. It is clear from this experiment that changes in the ionosphere through which the signal descends must be allowed for in any assesment of satellite signal propagation. Most of these changes seem to be related to variation in solar insolation levels on the ionosphere. Chronological List title - Space Propagation Pt2 Suggested Index references - Faraday : Propagation