Dear LF Group,
Last Friday I was QRV on 73kHz between about 2100 and
2300 utc - didn't hear/see anyone else though. This was a pity
because the Rugby TX was transmitting only two unmodulated
carriers, spaced by 85Hz, which considerably reduces the amount
of QRM at 71.8kHz. This was also the case the week before, and
also when I listened briefly last night. Obviously, it isn't guaranteed
when these windows of opportunity will occur, but it seems worth
having a go. Anyway, I intend to be on 73kHz again tonight from
about 2000 onwards - see you there?
Also last Friday, I measured G3XDV's field strength at the
University of Hertfordshire Telecomms Lab, on 135.9kHz. Using the
HLA6120 loop antenna, and HP8591 EMC analyser, the equivalent
electric field strength was 42.9dBuV/m (140uV/m). The distance
between the University and Mike's QTH is 7.5km, from the
Ordnance Survey map. Mike's ERP from these measurements
works out to 22mW.
I also simulated Mike's antenna using EZNEC, using the details on
his web site, in order to obtain the radiation resistance. This
comes out to about 50milliohms, compared with 64milliohms as
calculated using the simple vertical monopole formula. The exact
value of Rrad depends on the loading coil inductance, spacing
between wires & masts etc., but the differences are not major,
leading to perhaps 1dB variation in results. Using 50milliohms, and
an antenna current of 1.5A as quoted by Mike, the calculated
radiated power is 113mW, and the ERP 203mW. This should give
a field strength at 7.5km of 421uV/m.
There is therefore a discrepancy of -9.6dB between expected and
actual ERP - even greater than the -6.4dB difference that was
found for the M0BMU measurements a few weeks ago (calculated
E = 408uV/m, measured E = 195uV/m). It was suggested at that
time that part of the difference might be due to the short distance
between transmitter and receiver not being enough to achieve 'far
field' conditions, however, Mike's QTH is nearly twice as far as
mine, so would be expected to show less loss from this cause.
I have been searching for information on LF propagation - some
interesting results were obtained by Dr. J.H. Causebrook in the late
70's, who was working for the BBC at the time. He started from the
observation that the ground wave field strength of medium wave
broadcast stations was often much lower than expected in built-up
areas. He did some detailed measurements on the signals from
Brookmans Park (small world....) in the London area, which
confirmed this. He showed that the usual ground wave propagation
data for 'poor ground', 'good ground', and sea-water were not
adequate to describe propagation in urban areas. He developed a
theoretical model in which low density buildings, lamp posts, and so
on were modelled as grounded, lossy monopoles. Heavily built up
areas were described by a high conductivity layer (the buildings)
on top of the lower conductivity ground. This approach was
persuaded to give quite good agreement with the observed results.
This work is not directly useful to LF amateurs, since the frequency
range is different, and the analysis does not yield any simple
formula for the signal losses. However, it does show that in urban
or suburban areas, LF amateurs can expect to see increased
losses in signal strength than occurs in open countryside over the
same distance. The path of G3XDV's signal to the receive site
goes through Welwyn Garden City and Hatfield town most of the
way, whereas my signal goes mostly through a 'semi-rural'
environment. This could account for the increased loss in Mike's
signal. Another factor is that the amateur's transmitting antenna is
often in an urban area too, which is not often the case for
professional LF users. Since the 'near field' of an LF antenna must
extend at least for hundreds of metres, the radiation from the
antenna must be greatly influenced by the surrounding scenery,
especially when the features in that scenery include many things
that are bigger than the antenna itself.
So the conclusion so far is that, if you live in an urban area, your
ERP is probably a lot lower than you would expect from simple
calculations for a given antenna and power. This is in rough
agreement with the PA0SE & DK8KW results. For ultimate DX
results, operate /P from a small, flat, uninhabited island surrounded
by sea!
Obviously, more measurements are required to get a better
picture, so my project for this summer is to put together a portable,
calibrated field-strength measurement set. The
HLA6120/HP8591EM setup is a bit too bulky to drag round the
countryside, so I am working on a calibrated ferrite rod antenna of
the type used by PA0SE, for use with an old Rycom selective
voltmeter I have.
Hope to see someone on 73kHz...
Cheers, Jim Moritz
73 de M0BMU
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