"Richard Fry" wrote in message
...
"Richard Harrison" wrote:
The earth`s attenuation of low-angle radiation from a 1/4-wave
vertical
antenna has a significant effect on the vertical radiation
pattern. ee
Fig. 54-1 on page 465 of B. Whitfield Griffith`s "Radio-Electronic
Transmission Fundamentals". This figure shows field intensity
curves
versus vertical angle from a 1/4-wave vertical antenna radiating 1
kilowatt over earth of average conductivity. Anything below about
5-degrees is gone, eaten by the earth`s losses.
________________

This certainly is not true for frequencies below about 2 MHz. If it
was
true, MW broadcast stations would have no groundwave coverage --
which of
course is the only useful coverage they _do_ have in the daytime.

A monopole vertical radiator of any length up to 5/8-wave, when used
with a
ground system of ~120 buried radials each ~1/2-wave long, radiates
its peak
field very nearly in the horizontal plane regardless of the
conductivity of
the ground in which the radials are buried. This gain is within a
few
percent of the theoretical peak gain for these radiators when
working
against an infinite, perfectly conducting ground plane, as was
demonstrated
by the field tests of Brown, Lewis & Epstein in 1937. This
principle has
been accepted and used by the FCC and other regulating agencies, and
has
been field-proven in thousands of installations going back many
decades.

Once "launched," the groundwave signal is affected by ground
conductivity
along the propagation path, earth curvature, obstructions etc.
Groundwave
path loss increases with increasing frequency, and above some
frequency in
the low HF range, the groundwave is unable to serve a practical
purpose.
But that doesn't necessarily mean that the transmit antenna did not
generate
the groundwave in the first place, ie, that it radiated zero field
in the
horizontal plane and at very low elevation angles.

==========================================

Rich, all what you say is quite true - except that groundwave is
radiated at ALL frequencies from a vertical of 5/8-wave or shorter.

Useful propagation occurs at 30 MHz and below. But loss in the ground
and loss due to obstructions above 1/4-wave in height is high. Solid
ragchews across town and small city are quite possible on the 10m
band.

For predicting groundwave propagation from VLF to HF, download program
GRNDWAV3 from website below.
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Regards from Reg, G4FGQ
For Free Radio Design Software go to
http://www.btinternet.com/~g4fgq.regp
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