Satgen 465 Space Propagation Pt4 by GM4IHJ (BID SGEN465) 1998-02-21 Propagation features such as Faraday Effect and Multipath reception ( as described in Sgens 461, 463 and 464 ) are almost always present on satellite uplinks and downlinks, in some form or another. But there are other forms of satellite propagation disturbance which occur only spasmodically, but which when present can completely block communication from your station , to or from a particular satellite. Recognition of the presence of these spasmodic effects can be difficult , however, there are several techniques which provide usefull warning of their presence. Considering these Spasmodic effects in order of importance :- 1. Sporadic Ionisation of the E layer of the Ionosphere (SpE) This propagation problem is caused by localised , vertically thin patches of dense ionisation located in the E layer at altitudes aroumd 100 kms . SpE occurs mostly in daylight, peaking in the local afternoon,. It is an all year round phenomenon over tropical regions . Whilst over temperate latitudes it is present strongly in the local summer and weakly in the local winter, but mostly absent in the local spring or autumn. SpE is of course familiar to terrestrial VHF DX amateur radio operators, as an aid to long range ( up to 2500 kms ) propagation in the 28MHz and 50 MHz bands and less frequently in the 144 MHz band. So it should be no surprise that what refracts and forward scatters terrestrial VHF signals to provide occasional over the horizon contacts, actively prevents or severely limits the passage of VHF signals up to, or down from, mode A, B or J satellites. The effects of SpE are probably at their worst on mode J satellites. Whereby the 145 MHz uplink to the satellite , never gets through the SpE and onwards up to the satellite. In typical cases from Scotland , satellites coming above the horizon as they overfly Italy are not accessed until 5 or 6 minutes later , by which time the satellite has overflown most of central Europe and has got as far north as Scandinavia. Noting that temperate latitude SpE is most frequent over the latitude band 30 to 50N . The location of SpE clouds is rarely completely fixed, they do drift in position over a time period of an hour or so. But there are location over which SpE is frequent , and other locations at similar latitude where it is very infrequent indeed. Such that SpE can be loosely considered to be regularly located over the same places. In Europe typical examples of this are locations over Nantes in western France , Bonn in Germany and , again in Germany further north and east over Bremen. There are also frequent SpE clouds over Southern England, with SpE over Nantes or Southern England being 27 times more frequent than say SpE over Southern Norway. The presence of SpE is most obvious when you cannot access, and you hear no one near you accessing, but, stations elsewhere in Europe are heard weakly by you on the downlink, when they are giving one another 5 and 9 reports. Warning systems and Experiments in SpE will be discussed in Pt5. Chronological List title - Space Propagation Pt4 Suggested Index references - Propagation : Sporadic E