Satgen 567  SatSignal Reception 3  by GM4IHJ  (BID SGEN567) 2000-02-05

By 1978 conditions on the 2m amateur radio band , were, in some
countries , becoming impossibly crowded, making it very difficult for
satellite users trying to hear 2m mode B satellite downlinks. So the
first mode J, 2m up 70 cm down, Oscar 8 satellite, was built. At first
mode J had almost no users. You could easily get a 2m transmitter for
the uplink, but there was no 70cm SSB CW receiver on the market. A
difficulty solved by the arrival of the Microwave modules 70cm/10m
receive converter. Soon followed by the Jamsat low noise converter.

So all should have been fine. But it was not. The first problem was
break through into the receiver of the 2m transmissions third
harmonic. Solved by those radio amateurs who could build a 70 cm
cavity resonant filter into the antenna/receiver line. This enabled
the small band of users to enjoy a clear wide QRM free band all to
themselves. But before the celebrations went too far , a further
problem began to emerge. Whereby access to the mode J satellite was
interupted sometimes for more than half the satellites pass . Users
simply could not get a clue as to why they could not hear themselves
on the downlink. Eventually it was realised that the problem was a
seasonal , daylight, phenomena affecting passes over mid latitudes.
Clues which pointed to Sporadic E, a point proved when bad or no
satellite access coincided with reception of DX 49 MHz TV from as far
away as Russia. Mode J suffered because its 2m uplink was much more
succeptible to attenuation by E layer ionisation than the 70 cm uplink
of mode B Oscar 7.

So at least the winter season free of Sporadic E , should be good for
mode J. Unhappily this proved to be less than true. Some days were
good but others were very bad, particularly on DX routes Europe to
North America. The trouble there being that the great circle paths to
low earth orbit satellites providing Europe to America links, cross
quite high latitudes. Eg just think of Titanic and the iceberg. Ships
too, follow relatively high latitude paths across the Atlantic, and
these latitudes feature arctic conditions. Not just icebergs but also
the polar ionospheric disturbances we associate with aurora. 

Examination of the downlink signal when conditions were bad, revealed
that like the little star in the nursery rhyme, it was twinkling. The
scientific name for this condition is scintillation. The satellite
signals were scintillating both in amplitude and in phase. With this
loss of phase coherence, being particularly damaging. Other
authorities and services apart from amateur radio satellites were
suffering from scintillation when the satellite was near or inside the
polar auroral arc. Some days were much worse than others as the polar
arc expanded south in disturbed magnetic conditions. Such that
Navigation satellites , Weather satellites and special military
satellites were affected. NASA therefore set up a satellite experiment
called P76. This satellite transmitted a series of cw signals between
low VHF and the microwaves, all signals phase locked to a common
oscillator. Unfortunately the experiment proved that even at 2GHz ,
scintillation was a problem . So the next move seemed to be, put your
satellites higher to get clear of this reception problem.