Dear Rik, Brian, Andy, LF Group,
As Andy says, it is difficult to seperate out the directivity (i.e. gain due
to the shape of the radiation pattern of the antenna), the gain reduction
due to antenna losses, and the losses due to propagation over lossy ground
(which probably isn't really part of the antenna gain, but in practice will
always be present to some extent). When you make field strength measurements
to determine ERP, you are effectively measuring a combination of these
things, but by keeping the measuring distance reasonably short, you can
reasonably expect the ground wave losses to be negligibly small. This is
fine as far as ERP is concerned, because ERP is a measure of the overall
strength of the signal being radiated. In principle, one could determine
directivity by measuring the radiation pattern of the antenna, but of course
this is practically very hard to do at LF and MF at anything other than zero
elevation angle, unless you happen to have a suitably equipped aircraft
handy!
An important thing to remember when looking at the results of antenna
simulation is that what it gives is the far-field radiation pattern. This is
effectively the radiation pattern measured "as distance tends to infinity".
Infinite distance means infinitely more ground loss for the ground wave
signal at zero elevation as compared to the signal propagating away from the
ground, when anything other than "perfect" ground is simulated. So there is
always a null in the radiation pattern at zero elevation, and some reduction
of the lower angle radiation. But the reality is that field strength is not
measured at infinite distance, and the ground wave signal is not attenuated
to an infinitesimal level - in fact, over practical distances it is hardly
attenuated at all. The consensus seems to be that the radiation pattern at
these relatively short distances is the same as the "perfect ground" case.
It obviously isn't easy to check this. But in my experiments with LF/MF
antennas in an "open field" site last year, the FS measurements agreed
almost perfectly with the values calculated assuming the "perfect ground"
4.77dBi gain value.
The far-field-with-lossy-ground radiation pattern concept is probably fine
at HF, because the ground wave is heavily attenuated even at short
distances. But at MF, and especially LF, we are often communicating over
distances where the ground wave is the major component of the received
signal at distances of many hundreds of km. Since the effective parts of the
ionosphere are only of the order of 100km high for these signals, and the
curvature of the earth is also a significant factor affecting ground wave
strength, the situation that is being simulated using NEC consisting if an
antenna with flat ground plane and space above going off to infinity is not
at all realistic. Also, mine and other peoples' experiments have shown that
the trees and other objects around the antenna at short distances often
dominate the antenna performance, something that is hard to model. So I
think simulations of LF/MF antennas with ground losses are not very helpful
at all in predicting antenna performance.
Cheers, Jim Moritz
73 de M0BMU
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