Dear Larry, LF Group,
Getting the "wrong frequency right", and being able to identify the
dot length with reasonable accuracy is a pretty tall order to be just
a coincidence - so I think that must have been VA3LK last night. I
will dig out some screen shots when I get home.
The most important thing regarding readability of the signal is the
frequency - if the two carriers on almost exactly 137.790 turn out
to be a permanent fixture, it is obviously important not to use this
exact frequency; 0.6Hz low seemed to be in the clear. It is clearly
neccessary to check how this compares with what people see at
different QTH's before making great pronouncements on the
subject, though.
I will have a look at the frequency calibration again - I had to allow
for around 2Hz error in the indicated frequency on the spectrogram,
which is much larger than that due to the receiver, and clearly very
significant for this type of use. However, drift appears to be
insignificant. Measuring frequency to +/-0.1Hz in this way seems to
be perfectly realistic.
As far as the dot length goes, anything over about 60s should be
fine for me, since the 42mHz resolution is as low as DL4YHF's
Spectrum Lab goes, as far as I can tell. I think Spectran has
narrower resolutions, but my aged PC doesn't seem to be able to
cope with it. Spectrum Lab also has nice image recording facilities.
I don't currently have any specialised DSP hardware. Some sort of
distinctive pattern would be nice; but the overall pattern duration
should be less than, say, 15 minutes in order to take advantage of
the shorter lived propagation peaks.
As far as loading coil shelters go, if made of metal then the
individual parts should be connected together and to ground; but
there is probably something to be said for not having a continuous
screen around the coil in order to minimise eddy currents, shorted-
turn effects, etc. The text books say that the distance from the
surface of the coil to the screen should be at least the coil
diameter, if Q is not to be significantly reduced. Steel is to be
avoided due to high losses. Shielding would probably be only a
benefit where the coil is located near objects with high dielectric
loss - eg hedges, fences, reinforced concrete bunkers, the ground.
If the loading coil is well clear of such things, screening is probably
unneccessary.
I will set the gear up to record tonight, using the same settings as
before. let's see if it's more than a one-off!
Cheers, Jim Moritz
73 de M0BMU
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