Dear LF Group,
Thanks for the comments about field strengths, propagation modes,
etc. I have done a bit more work on the subject:
I have revised my estimates of the radiation resistance of my
antenna, using EZNEC. If zero losses and a perfect ground are
assumed, EZNEC gives a value for the radiation resistance of the
antenna as 19 milliohms, as against the original estimate of
21milliohms, for my 8.1m high antenna. I also tried estimating the
effects of objects near the antenna, ie. house and shed, by adding
grounded wire boxes of aproximately the right size and shape. This
reduced Rrad to 17milliohms. There don't seem to be any other
objects nearby that could radically affect the radiation resistance. I
also used G4FGQ's TANT136 program to estimate the radiation
resistance. This yielded a value of 17milliohms also.
Using this new value, and 1.4 A antenna current, radiated power
comes to 33mW, and ERP is 60mW. The expected field strength at
4.2km is 408uV/m, against the measured value of 195uV/m. The
discrepancy is reduced a bit, from 7.8 to 6.4dB.
HB2ASB points out that field strengths are subject to quite large
measurement errors. Measuring vertically polarised fields at LF is
regarded as relatively reliable; from the equipment manuals, the
loop antenna calibration is to within 1.8dB, and the analyser within
1.1dB. Allowing a further 1dB (about 10%) each for measuring the
antenna current and dimensions, the total uncertainty is about 5dB.
This is still not quite enough to account for the difference, and the
errors are unlikely to all add together anyway, as the statisticians
will tell you.
Most Antenna text books contain graphical data on ground wave
propagation. This is usually a graph of field strength vs. distance
for various types of ground, normalised to 1kW power radiated
from an electrically short vertical. Reading off at 4.2km, and scaling
for 33mW radiated power gives about 400uV/m, the same
estimated field strength as above  the ground losses are small
over such a short distance. OH2LX also kindly sent me some
computer calculated data, indicating that for 30mW radiated power,
field strength at 4.0km should be 385uV/m, which is also close. The
estimated field strength seems to be a good consensus.
Of course, a single set of measurements is not enough to build
elaborate theories around  what is needed are some more
measurements, with different antennas and grounds, etc.
Measuring over a short distance is important, because it
effectively eliminates other propagation modes, such as
ionospheric reflections.
What use is all this? well, 2 things. First, it makes the field strength
measurements made over larger distances much more useful 
since you then have a good idea how much power was radiated in
the first place, as well as how much arrived at the other end.
Therefore, we can learn more about different types of propagation,
effectiveness of antennas, and so on.
The other important thing is to determine what ERP your station
has  especially in countries where the 1W maximum ERP limit
applies. The results I have obtained so far seem to show that
calculated ERPs are rather over optimistic. A field strength
measurement may be more realistic, and isn't very difficult to do
(eg. see PA0SE's design for a field strength meter on G3YXM's
web page  http://www.picks.force9.co.uk/pa0se.htm).
The HP8591EM/HLA6120 measuring combination I used is rather
deaf  the noise floor is about 26dBuV/m  and I'm not sure if the
university would let me borrow it  which limits the possibilities for
doing more measurements with it. G3XDV's signal could probably
be measured from within the university though  how about it Mike?
Cheers, Jim Moritz
73 de M0BMU
