Message-ID: <29412.200002251259@gemini>
From: "James Moritz" <j.r.moritz@herts.ac.uk>
Organization: University of Hertfordshire
To: rsgb_lf_group@blacksheep.org
Date: Fri, 25 Feb 2000 12:59:50 +0000
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Subject: LF: 73kHz on Friday? / Field strength
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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