Dear Jim and LF
Thank you for your reply.
Rik's comments stimulated my interest and curiosity and the high ground
to the East
spurred me into including high level (altitude) measurements. I was
extremely
surprised when I included these in the survey and noticed the correlation
between
the calculated ERP and altitude. I plotted ERP against take off
angle from -1º to about 6º. This
might(?) even throw doubt on ""but by keeping the
measuring distance reasonably short, you can
reasonably expect the ground wave losses to be negligibly
small"" As the plots I attached
did not make it through the reflector filtering system back to me I am
also sending this direct
and attaching the XLS file.
73, Brian CT1DRP
At 13:18 05/12/2008, you wrote:
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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