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Author Topic: Measuring the ”Take off angle”  (Read 3987 times)

Posts: 3

« on: February 07, 2017, 02:13:34 PM »

Has anybody here been successful in building a DIY system for measuring the angle of incidence of incoming shortwave skywave?

73, Roland SM0BRF


Posts: 16170

« Reply #1 on: February 07, 2017, 09:12:17 PM »

I've designed a scheme for measuring both elevation and azimuth, but have never tried
implementing it.

Imagine two omnidirectional antennas mounted some distance apart - perhaps 5m, depending
on the desired frequency range.  (A practical distance to rotate.)  If these are fed with equal
lengths of cable they will have a bidirectional pattern with nulls off the sides.  But if the cable
lengths are different by a small amount (perhaps 1m) then the pattern is slightly uni-directional,
and the position of the nulls varies with the vertical angle.

To take a bearing, one would find the two directions where the incoming signal is nulled out.
The midpoint would the direction to the signal, and the difference would tell you the elevation
angle at which the signal was arriving.

This would require a fair bit of calibrating on known stations to get a feel of how accurate it is.

The best antenna I've come up with for this application is a 2-turn small horizontal loop, likely
an active receiving loop.  Two turns give a more omnidirectional pattern than just one turn in
this application.  An amplified loop also permits a better impedance match to the coax over a
wide frequency range.

Would it work?  I don't know.  It will require some care to make sure the two sides were
reasonably symmetric, and a readout with sufficient accuracy to measure the angles of the
nulls.  But it might...

Posts: 3

« Reply #2 on: February 09, 2017, 01:35:53 PM »

It’s an interesting approach using the envelope of the tilted vertical lobe. I haven’t made any calculation of the possible accuracy but I think it has been used so it probably works, but I’ve failed to find any source. Probably described by P J D Gething in his book “Radio Direction-finding and the Resolution of Multicomponent Wave-fields” but I hesitate to buy it due to the high cost and that it is very theoretical.

Obviously there are three ways to go: .

Another practical solution was used by Dr Harry Whale in New Zealand (and probably others) in the fifties and sixties. There were two short dipoles at the tip of a rotating bar and the system was an Interferometer. Described here . I’ve failed to find any description of the system. Of course one thinks of the possible use of an electronically rotating antenna like in Doppler RDF systems.

A third system is using three Orthogonal loops described here . No description of the “inner workings” found.
And of course the Rohde & Schwarz way:

73, Roland SM0BRF
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