Dear Stewart, LF Group,
G3YSX> I thought that the Puckeridge test showed this, ie that your
smaller antenna had a higher field strength than you expected.
Yes it did, but only by about 1.5dB as I recall - this would be within
the limits of experimental error. The thing about Puckeridge was
that it was a near ideal site - moist soil, open fields for miles and
so on - where you might expect good results. Indeed, when I put
the same antenna up at home, and repeated the experiment, the
field strength was down on the calculated value by about 4dB, so
there was 5dB or so lost just by moving the antenna to a less ideal
location. A loss of several dB compared to calculated values is
typical for field strength measurements on "back garden" antennas,
such as mine and G3XDV's, and some bigger ones too, like the
MB2HFC antenna at Windsor.
There are some plausible explanations as to why the field strength
of the small antenna at Puckeridge could have "higher than
theoretical" efficiency, such as increased effective height due to
the skin depth of the ground being significant compared to the
physical height of the antenna at LF. There are also plausible
reasons why antennas in built up areas give lower field strength,
such as absorbtion by surroundings. High ground wave losses in
urban areas are well known to MF broadcast people. Unfortunately,
the losses seem to win over the gains for most of us. However,
there is not really enough experimental data of high accuracy to
say much more than that.
Cheers, Jim Moritz
73 de M0BMU
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