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Dear LF Group,
Since the new Rugby Loran renders all except local
136k amateur signals inaudible at my QTH, I have been trying some experiments to
see what can be done to improve the situation. I have used loop antennas to null
the noise from Lessay coming from a rougly SW direction for a long time, but the
direction to Rugby is roughly NW, so using a loop it is only possible to null
one Loran at a time - plus, with a null in a NW direction, the other null
is SE, which reduces many European signals too.
What is required is an antenna with two nulls which
can be "steered" so that the nulls have an arbitrary difference in angle between
them, rather than being fixed at 180degrees. A unidirectional cardioid pattern
with a single null can be obtained by summing equal signal levels from a loop
antenna and a vertical, as used in D/F. If the signal from the vertical is
attenuated, you get a pattern intermediate between a cardioid and a
figure-of-eight, i.e. an asymetrical "8" with one lobe smaller than the other,
and the null angles skewed away from 180 degrees, by an amount which depends on
the relative signal levels from the two antennas.
To implement this, I used one of the "bandpass
loops" (see G3YXM's "features" page). In principle, the TX vertical antenna
could be used, but at my QTH this picks up a lot of mains noise, plus it has a
narrow bandwidth which would cause the amplitude and phase to vary a lot across
the band, so I built a seperate "bandpass vertical", with a double-tuned
input similar to the loop, and a bandwidth of about 15kHz. Each antenna has a
preamp, and the signals are combined using an RC amplitude/phase adjusting
network not unlike a "noise canceller".
To set this up, I aligned the loop so its maximum
direction was mid-way between the bearings to Lessay and Rugby - about E-W at my
QTH. Then I adjusted the signal levels from each antenna individually so
that the Loran noise was about the same. Then both signals were added together ,
and the phase control adjusted for minimum Loran. Then amplitude, phase and the
position of the antenna were iteratively tweaked for the best
results.
The results are better than I expected - see the
attached SpecLab spectra - the audible Loran chatter is virtually eliminated ,
and only very slight tweaking is required from one end of the band to the other
to maintain the null. As a bonus, it also nulls the "Watford Whistle" power line
carrier on about 136.645kHz, which is always annoying here. I was able to
copy DL1DQZ, DK7SU and F6BWO this morning, which with nulling disabled were
completely inaudible. It proved important to de-tune the TX vertical to prevent
it "sucking out" signal from the RX vertical - this is much more pronounced than
with the RX loop antenna. I think the antenna pattern should give good coverage
of Europe and Scandinavia and beyond, although not very good for the western UK
and GI/EI, or North America. But with the very narrow band modes used for
transatlantic DX, the Loran noise is less of a problem. So once I get the
current lash-up sorted out, with some T/R switching, I should be back in
business again :-)
Cheers, Jim Moritz
73 de M0BMU
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