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Dear Chris,
Interesting to see your field strength measurements
- would also be interested to know how this compares to any calculated value you
have for the ERP of your station.
A few years back, I made several hundred FS
measurements on my 136kHz and 73kHz station, looking at the effects of antenna
configuration, distance etc., using a mobile measuring set-up with the measuring
antenna mounted on the roof of my car. I found that one could expect
"random" variations within a range +/-3dB of ERP at different sites. These
variations were not just random errors; one could return to the same site months
later and measure the same variation in field strength. I came to the
conclusion that things in the environment were causing some enhancement or
reduction in field strength; presumably conductors such as wire fences, buried
cables, etc. acting as parasitic antenna elements. Some sites produced much
larger variations up to +/-10dB or more. On inspection there was usually a
fairly likely-looking cause for this, such as overhead power lines, or large
metal structures such as buildings and bridges, but there was not always
anything immediately obvious to be seen. One road leading away from my house
gave wildly varying results along its length - it turned out that the overhead
telephone wire from my house ran along this road, and high and low FS readings
were separated by about the right distance to be interpreted as a lambda = 2200m
standing wave on the overhead wire. But provided a substantial number of
measurements are available, it is easy to spot which measurements are suspect
and check them, and to get a good average value. Something worth checking is
that you get a deep directional null with a loop or ferrite rod antenna; if
not, the measuring site is probably dubious. I expect if one were only
to use measurement sites that were in the middle of open ground, a good distance
from any large metallic objects, variations between measurements would be
reduced greatly. Unfortunately, there seems to be a shortage of accessible
sites like this near me...
Regarding the letter in RadCom, the difficulties of
field strength measurements depend a lot on the frequency. I have some
experience of HF FS measurements - in this range, connecting cables and any
other fair sized bit of metal can become resonant, and can produce huge
peaks and troughs in the results without careful attention. At VHF and
above, multipath propagation becomes a headache, with multiple unwanted
reflections from the ground and any nearby objects. I think LF/MF is relatively
straightforward in comparison - the main difficulty being that you need to be at
least several hundred metres away from the TX antenna to get good measurements.
But once the problems have been identified, measurement results are usually very
consistent.
Cheers, Jim Moritz
73 de M0BMU
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