Dear Mal, LF Group,
I'm afraid there isn't much chance of getting the 136kHz Loran problem
sorted due to potential interference to commercial users - The main users of
this part of the spectrum seem to be the tele-switching stations like DCF39,
and the Datatrack beacons. The operators of these systems have to make sure
their signal levels will reliably overcome the QRN, not to mention the local
man-made QRM in buildings and vehicles where the receivers are located,
hence their huge ERP. The presence of Loran sidebands is highly unlikely to
be a factor affecting the overall system reliability. So the Loran noise is
not really a problem to anyone apart from amateurs, who have, from other
peoples viewpoint, a perverse insistence in trying to utilise signals that
are only marginally and fleetingly above the natural band noise. This is a
bit different from the potentially real safety issue of interference to
Navtex broadcasts in the North Sea.
The Loran transmitters have to conform to particular standards for signal
quality, which they apparently do. The standard is based on the pulse shape
of the emitted signal, which has to be closely specified so that the
receivers can extract the correct timing information, and thus get an
accurate position fix.. The sideband levels at 136k are largely fixed by the
pulse shape. Modifying the TX signal to generate lower sideband levels at
136k may be feasible, but would have consequences throughout the system.
Modifying a whole network of multi-100kW transmittersand possibly thousands
of receivers too would be a massively expensive undertaking, unlike the
531kHz broadcast TX, which just needed someone to go and turn the modulation
down a bit.
I think it is more interesting to try and do something positive with Loran,
rather than just saying it shouldn't be as it is. DF6NM's propagation
studies using Loran have been very interesting. Several methods to overcome
the QRM have been tried - some even work quite well! I have had quite good
results with phased antennas with steerable nulls. It is frustrating
sometimes, but also the challenge makes life more interesting.
Cheers, Jim Moritz
73 de M0BMU
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