Hello Walter,
As far as I understand trees (dry or wet) do not shield on LF but they
absorb RF energy. So instead of comparing them with a metal plate (as on
VHF and up) one can better compare them with a lossy capacitor.
Regarding the distance : the further the better, but if you can keep the
trees further away than 2 times the hight of your antenna you might avoid
the worst losses (the largest capacitance should be between the antenna and
ground, not between the antenna and the trees).
If you have trees close to the antenna try to keep the wire as far as
possible away from the greenery. Close to the antenna (near field) the
electric field decreases with the square of the distance, so despite a
wavelength of over 2000m every inch can count !
Since antennas work reciprocal I would expect that trees will also absorb a
part of the 'incoming' EM-signals, but that should be a much less problem
as not only the usefull signals will be absorbed but also QRM/QRN, leaving
the SNR unchanged (unless you come close to your RX noisefloor, rather
unlikely on LF).
73, Rik ON7YD
At 14:15 15/10/01 +0100, you wrote:
At 73/136 khz how far away from a wet tree do you have to be not to see
any RF loss? At one extreme if your ant runs through it you will lose a
lot; at the other if it is several kms away you won't lose anything. Is
there a critical distance? And the reverse - if there's a wet tree close
to you in the same direction as a transmitter, will you receive a much
weaker signal than you ought? Does it act like a metal plate does at
microwaves?
Walter G3JKV.
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