Satgen 453  Predict SubHorizon DX   by GM4IHJ   (BID SGEN453) 29Nov 97

One of the more exciting, (and least understood ), modes of satellite
signal propagation occurs when the satellite is far below your
horizon. On a mode A satellite you only hear the beacon and the users
on the downlink. But on mode K you can usually access the satellite on
21MHz up and get super DX QSOs. Presently we have no mode K sat, but
hopefully that may soon change. Meanwhile the solar flux is rising
gradually and contrary to RSGB forecasts there are signals on 28 MHz
coming from all sorts of places.

How can we get clues as to when and where to expect sat subhorizon
signals?. The answer as GM4JJJ has pointed out, is via the world wide
pattern of beacons established by the North California DX Foundation
( http://www.ncdxf.org ) Listen on 28.2 MHz . Every 10 seconds a
different beacon comes on in a cycle which repeats every 3 minutes.

There are also transmissions on 14.1; 18.11; 21.15; 24.93 MHz
On some days this month the Australian 28.2 beacon VK6RBP has come up
regularly in its 1min30sec slot in the cycle with 4X6TU also there in
the 2min40sec slot both around 0930ut. Later by 1030 the ZS6DN signal
has been audible in the 2min20 slot and by 1200 ut OA4B Peru and YV5B
Venezuela in the 0:20 and 0:30 slots have been audible. So 10metres
was clearly open with indications of where to point your antenna.

The next move in this game is to study RS12 and RS15 orbits and find
locations where they intersect, a great circle extended from Scotland
through the beacon being heard and going between 2000 and 4000 kms
beyond the beacon.

Given that the satellite, the zone in the ionosphere propagating the
beacon to you, and your station, need to be on a great circle, you
have to be reasonably precise about when your satellite will be in the
favoured position where it can see across the earth to the UNDERSIDE
of the ionospheric region which is propagating the beacon to you.It is
clearly easier to do this when the satellite is actual approaching or
opening directly from your station Eg best results when South African
beacon is heard and sat is south or near south of the beacon and
coming towards GM. It is more difficult when the satellite is crossing
the great circle track at nearly 90 degrees, giving far less time ,
perhaps only 1 minute in the propagation zone. This latter short event
is typical for sats over the Pacific beyond Peru as seen from GM.

So please try this method. Remember RS12 and RS15 are not Sun
synchronous. So their orbit path shifts with time. Sometimes they are
favourably placed. In the right spot and the right time for several
days ( usually about 5 at most) before they slip in time and or orbit
to a less favourable line up. Please also note that while this scheme
is performing reasonably well on signals out to about 7000 kms from
GM, it is quite hopeless in locations approaching the station
antipodes. In these cases there is still satdx, but it does not seem
to follow any pattern of great circle propagation. Several really long
range DX events recorded were invariably far from great circle paths. 
PS. The AF9Y FFTDSP display is great for painless monitoring of the
NCDXF beacons