Dear LF Group,
Dear old BT seem to have it in for LF amateurs - after all, it appears it
was they who operated the 73.25kHz "grinder", which made operation on 73k so
much more difficult.
I spent some time last year trying out various noise blanker ideas for LF,
without a lot of success. It is quite possible to gate out the impulses you
are trying to get rid of, but in the process you add modulation sidebands to
the powerful narrow-band signals adjacent to the amateur band, which
effectively puts the impulses back in again (building a circuit which did
that was quite annoying...). Filtering out the adjacent signals to reduce
the amplitude of the sidebands has the effect of lengthening the impulses so
that longer blanking periods are needed, increasing the duty cycle of the
blanking gate, which also puts you back where you started more or less.
Noise blanking is a non-linear process, so there are always going to be some
distortion products generated.
An alternative would be a cancellation scheme, which might work like this:-
Receive the Loran signal (a simple TRF RX would be ideal), square up the
pulses to regenerate the Loran sidebands locally, then pass them through a
phase shifter and attenuator before summing with the 136kHz RX input, as in
"noise cancelling" antenna schemes. The amplitude and phase of the
regenerated sidebands could then be adjusted to null out the off-air Loran
noise. It would probably be quite fiddly to operate, since any changes to
the RX antenna, operating frequency, or the skywave component of the Loran
signal would require re-adjustment of the null.
I don't think a Loran station in the central UK would be a total disaster
for LF; you could still use narrow-band modes like QRSS provided the
frequency was selected carefully, and in many locations you could get a
directional null on the Loran without eliminating all the signals you were
trying to receive, but it certainly would make life more difficult,
especially for CW operation... and especially if you are located where G3YXM
is!
Cheers, Jim Moritz
73 de M0BMU
-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of John Rabson
Sent: 02 November 2004 08:03
To: [email protected]
Subject: Re[2]: LF: Rugby Loran station
On 02/11/2004 at 02:49 Stewart Bryant wrote:
How diffiicult would it be to make a Loran gate to put in front of theRX?
After all the station is fixed so you must, in principle, know when the
pulses are due.
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