Dear LF Group,
Very interested to read the discussion on antennas - a couple of
things:
Ground planes: it seems to me that the ground isn't much like a
plane at all; at LF, the skin depth of the ground is from several
metres to tens of metres, depending on the type of ground - similar
to or larger than the height of most amateur antennas. Therefore, a
considerable part of the field of the antenna must be within the
ground itself. It might be better to think of such antennas as
asymmetrical dipoles, one side of which (the ground system) are
embedded in a lossy dielectric (the ground).
The other thing is about ground waves and sky waves; OH2LX's
comments about how much of each are produced by a given
antenna, and whether anybody knows, are most fascinating. It
certainly seems to me that ground waves are a different thing to
waves in free space; looking at the literature, there are whole
classes of surface wave propagation modes where the surface of
a conductor, or the interface between two dielectrics, acts as a
sort of one-sided wave guide. There are many types of microwave
antenna that use these modes, and a copy of the RSGB handbook
from the 1960's features a single-wire low loss UHF transmission
line using a surface wave mode. Then again, the field strength
produced by the ground wave, at least when the ground is good,
and the distance fairly short, is supposed to be close to the value
arrived at for the 'free space' antenna. Apparently, measurements
confirm this. Software such as EZNEC gives the result that there is
zero propagation along the horizontal with anything other than
perfectly conducting ground, but we all know this is not so.
I would be very interested to see the article Vaino refers to, if he
could tell me where it can be obtained.
Hope this makes some sense....
Cheers, Jim Moritz
73 de M0BMU
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