"Richard Harrison" wrote in message
...
Dan, KB0QIL wrote:
"From a practical perspective it would seem to me that building a 40
foot crnter loaded dipole and putting it in the sttic or on the roof
would probably perform somewhat better."

The roof or attic may be noisy receiving locations.

The ionospheric spot which effectively reflects a high frequency
signal
to a point beyond the horizon is variable so that the received
signal
direction varies from the true bearing of the transmitter, The
received
signal elevation angle also varies from that predicted by the
assumed
layer height for any given path length, and may change from instant
to
instant.

The differences between predicted and actual azimuth and elevation
angles may at any momement be several degrees. These differences
make
high frequency direction finding complicated, but results may be
good
enough for some pracical purposes. Optimum vertical and horizontal
angles are sought in directional antenna design but enough beamwidth
is
needed to accommodate
the angular variations which occur.

Over sea water, ground wave propagation is good and loss is low as
compared with propagation over earth. Frequencies up to about 5 MHz
are
used for communications beyond the line of sight between ships and
between ships and shore. These frequencies are also used for
tropical
broadcasting among islands.

For ionospheric reflection to near spots beyond the line of sight,
near
vertical incidence reflections are used. The frequency must be below
the
maximum usable frequency for vertical incidence at the transmitting
site.

For ground wave propagation a vertical transmitting antenna is used.

Horizontally polarized antennas are often used for sky wave signals
because reflection from the ionosphere makes equal strength
components,
horizontally polarized and vertically polarized, from the incident
wave,
regardless of its initial polarization.

Most disturbing noise is that generated within ground wave range of
the
receiving antenna. It is vertically polarized.There is no ground
wave
propagation of horizontally polarized waves. Thus, a horizontally
polarized receiving antenna ignores much of the available noise.
However, it receives as much signal from the sky wave as a
vertically
polarized antenna would.

If a single antenna is to be used for both transmitting and
receiving a
shy wave, a forizontally polarized antenna may be the better choice
due
to its noise rejection. See "Radio Antenna Engineering" by Edmund A.
Laport for details.

Best regards, Richard Harrison, KB5WZI

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

Richard,

I am impressed by your logical descriptions and explanations of
skywave and groundwave propagation. You are more than convincing. No
doubt reinforced from practical experience. It all makes sense.
Something much needed on these newsgroups.

I notice you do not treat the works of so-called 'experts' as bibles
but as a means of further study.
----
Reg, G4FGQ