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
