Dear LF group,
If you have a receiver and signal source which can be tuned to
198kHz or one of the other stable broadcast frequencies, you can
compare the frequencies with parts-per-billion resolution quite
easily using spectrogram. Couple a sample of the signal source
into the antenna circuit when the receiver is tuned to the broadcast
frequency, and juggle the level of source and/or broadcast signals
so they are approximately equal. Hopefully both signals will be
within a fraction of a hertz of one another, so will appear
superimposed as a single line on the spectrogram. However, as
they drift in and out of phase, there will be peaks and nulls in the
combined signal level. The frequency difference is the reciprocal of
the period in seconds between 2 successive peaks or nulls, which
can be measured with the cursor. If you choose an appropriate
spectrogram time-base, and leave the set-up running for 1/2 an
hour or so, it is easy to measure difference frequencies of the
order of 0.001Hz, which is 5 parts in 10e9 at 198kHz. The only
slight problem is that it does not tell you whether the difference is
positive or negative, but that can be found by trial and error
adjustment.
I have tried this with 198kHz; also the Loran "carrier" at exactly
100kHz works, and might be more convenient if your reference
signal is at a fixed frequency like 10MHz. Another possibility is to
use a decade divider chain to obtain 1kHz from the reference; the
harmonics in the LF range will be strong enough to make the
comparison to an "odd" broadcast frequency.
I was interested to see Rik's plots of the results from the CFH
logging session (http://www.qsl.net/on7yd/t181200.htm) - as
G3NYK suggested, deep fades do occur at different times at the
different locations. The relative signal levels between different
receive locations vary rapidly, so it is very likely that one person
will be seeing nothing whilst their neigbor is getting good copy of a
transatlantic signal. This suggests to me that, with marginal signals,
there is not much point in going to very long dot lengths, or using
very long "grab" sequences with BPSK software, since the signal
will probably fade in and out in the time it takes to send a callsign.
The propagation on LF seems to get more and more volatile as the
distance increases
It might be interesting if Alan were to make some plots with his
logging gear while CFH is off-air (or on a different frequency when
it comes back on again) of the noise level for comparison; after all,
it is the signal to noise ratio which determines whether a signal can
be received or not.
Cheers, Jim Moritz
73 de M0BMU
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