Dear Walter, LF Group,
As far as the practical question "how far away do things have to be
to make no difference to the antenna loss" goes, my answer would
be "when they are much further away than the other things which
are causing losses". Everybody's antenna has ground under it, and
it is known that LF fields penetrate some distance into the ground.
Most soil is not a great dielectric, therefore, the losses in the
ground will be considerable, and if there is nothing else around, the
ground losses will dominate. This certainly seemed to be the case
at Puckeridge, where the "small" antenna had only moderately less
loss in open fields than when set up in my garden, surrounded by
trees.
Therefore Rik's comments of about twice the antenna height seem
a reasonable rule of thumb. I found moving my antenna so it was
several metres, rather than a few metres from trees, buildings, etc,
made a significant improvement - over a few incarnations it has
gone from about 60ohms to 30ohms loss resistance. Bear in mind
the feed point has just as much voltage on it as the rest of the
antenna, so keeping this (and obviously the loading coil as well)
well off the ground will help too.
A tuned TX antenna will obviously suffer losses when used for
receive in just the same way as when it is used for transmit,
although because of the high noise levels, this probably will not
make a noticeable difference to the overall SNR. The situation with
small loops and whips will be a bit different, because the external
signal/noise to internal preamp noise margins are smaller. I did
some experiments with my FS measuring gear on the DCF39
signal - the ferrite rod showed variations of only about 3dB in a
wide variety of surroundings - trees seemed to make things no
better or worse. Tuned loops do not seem to suffer significant Q
reduction so long as they are about a loop diameter or so from
surrounding objects. I have not yet done quantitative
measurements with an E-field whip, but data from EMC sources
suggests that this will suffer much greater screening effects - the E-
field is more seriously affected than the H field, and 10dB or more
variations can be expected.
An object of a few wavelengths or less between TX and RX in size
will not cast a radio "shadow" extending many wavelengths
beyond the object because the wavefront will diffract around it, like
waves passing a ship in the sea. Close up to the screening object,
there may well be a screening effect - the only place this is likely to
happen on LF is in a narrow valley, or in a gap between large
buildings.
Cheers, Jim Moritz
73 de M0BMU
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