Monday, January 02, 2006

HF Radio Science + QSK = Radar!

When operating QSK at 15-20 wpm, I am running into echoes of my transmissions. These occur on certain azimuth bearings at certain times of day, most often to the SE, which is over water until hitting S. Africa or Antarctica from here. I've seen this from 20 M to 15 M, at least.

Rarely, I think I've seen long-path echoes that come back to me from the opposite azimuth. (The SteppIR bidirectional mode picks them up.) More often, the return bearing is the same as transmitting. I haven't been able to measure the delay time accurately, but it is roughly 2 dit (element) times at 25 wpm (about 50 msec), indicating a 10,000 mile roundtrip.

It seems to be a real effect. I can get rid of it by changing azimuth or using a dummy load.

My question is whether other ops see this and whether it has been written up anywhere in "ham space". These are not the "long delay echoes" that people have claimed to see. The radio science community does run HF radar to study fluctuations in the ionosphere, and this phenomenon is probably well known to them.

The Orion makes a pretty fair radar set, as it turns out.

Note added (1/8/2006): The "echo" appears to be an example of the more general phenomenon called "backscattering" in which the ionosphere returns a certain amount of power back in the direction of the transmitter. See, for example, "Radio Amateur's Guide to the Ionosphere" by Leo F. McNamara (Krieger, 1994). Backscatter ionograms are one method of probing ionospheric conditions. Ionospheric scattering modes (normally in the forward or near-forward direction) are sometimes used for ham DX communications via non-great-circle routes when the great circle route is not open.

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