Dear Group:
The use of one number is crude and suitable only if one has imbibed
enough grain alcohol or one does not care about Lord K's admonition or both.

The preferred scheme is to model the expected virtual heights at each
ionospheric reflection (note that the apparent TOA (angles) involved are not
likely to be the same at each of these reflections) and then to use the
angles involved to note the expected incidence angle of the intermediate
"ground" reflection. The coordinates of the expected "ground" reflection
areas are calculated and note is taken as to whether those areas are
predominately salt-water, ice, or neither. A heuristic algorithm is applied
that depends on type of "ground" and that weakly depends on the expected
incidence angle. The result is a statistically significant reflection loss.
[Some iteration is needed, which computers are very good at doing.]

It is important to note that the apparent TOA of the major mode between
two points is unlikely to be the same at both ends of the path because the
virtual height is unlikely to be the same near both ends.

For HF paths greater than something like 12 or 14 Mm predictions become
more complex. In all cases of multiple hops (real DX), the TOA at which the
gain of the antennas is effective varies between about 12 degrees and 2
degrees.

If one wishes a simple predictor, one could find it in a paper published
in the UK in the transactions on a conference on HF propagation in the 80s
(as I recall) by a BBC engineer (IEE was the publisher). The entire
computer code is given in the paper. It uses a simple heuristic model that
was found by the BBC to give satisfactory results over the paths that their
World Service used. Note that the BBC used paths that did not include much
of the polar regions.

To digress: The worst path to a DXCC entity from the Upper Midwest is
over the magnetic pole to VU4. 100 watt transmitters, ice reflections,
heavy absorption, and roughly 10 db more noise at the far end make the need
for power and high antennas almost necessary.

73 Mac N8TT

--
J. Mc Laughlin; Michigan U.S.A.
Home:

"Richard Harrison" wrote in message
...

snip

It could be right for some reflections.

E.A. Laport was Chief Engineer of RCA International when he wrote "Radio
Antenna Engineering". On page 236 Fig. 3.17 shows the effect of ground
conductivity on maximum field strength from a horizontal dipole antenna
versus its height in vavelengths.

Optimum height would be about 0,50 wavelength to most concentrate energy
at a certain vertical angle, 30-degrees according to the RAF Signal
Manual quoted by Laport. 30-degrees might hop 1000 kilometers.

At the antenna earth reflection point, frequencies between 2 and 16 MHz
are reduced to 95% of their prereflection field strengths by ordinary
soil from a dipole at 1/2-wavelength height. A reduction to 70.7% of
prereflection strength would represent a 3 dB power loss. So no harm
done yet by the reflection from an antenna over good soil. Lower antenna
height and poorer soil would attenuate more.

The angle at which rhe signal strikes the earth in subsequent
reflections should be the same as the first reflection from the antenna.
Conductivity and dielectric constant at subsequent earth reflection
points are what they are.

Shortwave broadcasters use vertically stacked horizontal elements to
concentrate the vertical beam to avoid multipath interference. They also
prefer targets reached on the first reflection from the ionosphere..

Best regards, Richard Harrison, KB5WZI