Satgen 571 RsDX Theory v Practice by GM4IHJ  (BID SGEN571) 2000-03-05

After several months with solar flux gently oscillating between a low
around 130 and a high of 200, with a 27 day sawtooth waveform. 
Everything changed recently. First the sawtooth peak failed to get
anywhere near 200 , then suddenly, completely out of phase with the
previous pattern, the flux level shot well above 200, and has been
there for several days. Will this help improve RS13 sub horizon DX
possibilities ? If that is, it lasts or goes higher. 

In theory things should get better , and they certainly need to.
Signals into Scotland from RS13 overflying the Antartic ice cap have
continued to be just about useful, but there are no licensed penguins.
Meanwhile signals over Australia have been very diffuse and spasmodic.
While over South America they were too low for anything useful to
results. But 3 days ago this began to change and signal levels from
all these areas began to rise.

However as radio amateurs are aware, Murphy ensures it is never that
simple. Just as the solar flux reaches semi respectable levels , the
satellites orbit plane begins to get close to directly coinciding with
the day night terminator solar illumination line. This severely
restricts DX opportunities. The point being that when the satellite
orbit plane is say 3000 kms from the terminator , you get super DX
from satellite passes over the NIGHT SIDE. Sat in darkness but able to
illuminate the underside of the ionosphere 4000 kms away , that is a
sunlit ionosphere capable of onward propagating the signal. The mode
of propagation is chordal hop with signal coming straight from the
satellite skimming someway above its horizon at 3000 kms plus, then
carrying straight on to meet the ionosphere, as earth and ionosphere
curvature bring it down to intercept the signal path. From where the
signal can propagate at least 1500 kms further or if your lucky pick
up another chordal hop and go more than 4000 kms further.

Please , Please if you try to sketch this, do not fall into the radio
amateur magazine trap ( ie Radio Communications Jan 99) of drawing the
thing to the wrong scale portraying the ionosphere at 5000 kms
altitude rather than it true situation as a narrow 500 km band around
an earth diameter of 6378 kms. Chordal hop propagation does not need
to touch the surface of the earth . It can skim tangentially just
above earth and waves going straight on for another 1500 kms before it
meets the underside of the ionosphere. Also please note that if you
have to draw things to a false scale to achieve clarity , you must
clearly mark the sketch "NOT TO SCALE". 

Temporarily we have lost the optimum situation, where RS orbits well
over on the dark side of earth. But in a couple of months the
situation should begin to improve and by the time we get to September
if solar flux levels hold up , some record breaking sub horizon DX may
be possible. But even then please remember that if someone is 11000
kms from you it is almost certain that you will hear the satellite
over his station when he is fast asleep. Just keep trying however .
Sooner or later maybe, you will find an insomniac out there who is
happy to talk to you.