Richard Fry wrote:
"Richard Clark might have allowed for systems operating above 30 MHz.,
which include several ham bands, I believe."

I believe so too.

From page 810 of Terman`s 1955 edition:
"Space-wave Propagation. At frequencies above about 30 MHz, the
ionosphere is not able to refract energy to earth, while the ground wave
attenuates to negligible amplitude in a relatively few hundred feet."

On the preceding page, 809, Terman presents Fig. 22-4 which gives among
other things the magnitude of the reflection coefficient for a
vertically polarized wave versus the incident and reflected angle
(symbol = psi, and they are identical). At zero-degrees, the coefficient
is 1.0. It falls to about 0.17 at 10-degrees, which is the minimum or
Pseudo Brewster Angle. From there, it rises to about 0.5 at 20-degrees,
and about 0.7 at 70-degrees incidence. The nip in vertically polarized
radiation does not exist at zero-degrees, the takeoff for ground wave
propagation. The ground wave is interactive with the surface all along
its path.

The space-wave does not depend upon interaction with the earth all along
its path, as the ground wave does. The wave along the earth at 30 MHz
and above "attenuates to negligible amplitude in a relatively few
hundred feet." It depnds upon reflection from the ionosphere or or
something else for popagation beyond the horizon.

Best regards, Richard Harrison, KB5WZI

"Richard Harrison" wrote:
The wave along the earth at 30 MHz and above "attenuates to
negligible amplitude in a relatively few hundred feet."
________________

However this not mean that a direct ray at VHF and above cannot propagate
well on a line-of-sight path near the Earth that is tens of miles in
length--especially if a Fresnel clearance of 0.7 or better exists for the
path. Such service is produced by radiation from the transmit antenna
typically at elevation angles from zero to several degrees BELOW the
horizontal plane.

If otherwise, the service areas of TV and FM broadcast stations would be
very small.

RF

Richard Fry wrote:
"Such service (VHF and above) is produced by radiation from the transmit
antenna typically at elevation angles from zero to several degrees BELOW
the horizontal plane."

A broadcaster doesn`t want to skip over his station`s nearby customers.
His distant customers likely receive the station from a grazing
incidence due to earth curvature.

I initially set dishfeeds on point to point microwave systems with a
carpenter`s level for the right elevation angle. I seldom was able to
improve the signal using on-the-air adjustment of antenna vertical
elevations.

In Scotland a few weeks ago I noticed the Satellite dishes aimed at
birds parked over the equator. Scotland is so far north that the
dishfeeds seemed horizontal. I saw some that dipped below the
horizontal. I doubt those were adjusted for best results. The beamwidth
is probably enough to get a picture anyway.

The point to point paths we designed had enough clearance to allow
anomalous atmospheres making the earth appear half again its actual
size. To that grazing point clearance, we added 0.6 1st Fresnel zone
clearance.,

Then we produced paths with 40 dB fade margins and limited the paths to
22 miles. Where we could, we produced redundant paths through looped
systems or space diversity.

High fade margins help when there are no fades by suppressing system
noise, a must in long systems with many hops.

Best regards, Richard Harrison, KB5WZI

OOPS!

Half again normal size? No, 2/3 normal size. It requires taller towers..

Best regards, Richard Harrison, KB5WZI

In message , Richard
Harrison writes
OOPS!

Half again normal size? No, 2/3 normal size. It requires taller towers..

Our graph paper was drawn to 4/3 earth radius.

Mike
--
M.J.Powell

Mike Powell wrote:
"Our graph paper was drawn to 4/3 earth radius."

VHF and higher frequencies are generally said to require a line-of-sight
path.

Terman writes on page 820 of his 1955 edition:
"For the standard atmosphere, where dM/dh=0.036, the equivalent radius
is 4/3=1.33 times the natural radius of the earth.

There are times when the atmosphere doesn`t bend radio waves toward the
earth and even bends them away from the earth. If you need to build a
high reliability chain of microwave relay stations, you must provide
enough path ground clearance so that you`ll have paths when the radio
waves bend up as well as when the waves bend down. You pick the excess
you can afford for the statistical reliability this excess has been
shown to provide. I recall picking k=2/3 actual earth radius. This lead
to purchase of this type of graph paper to plot the paths on showing
land contours and possible obstructions.

Terman also om page 820, gives formula (22-7b) for:
Radio horizon in miles=aq rt (2h). where h is the height of the antenna
in feet. You can approximate many of these in your head.

Best regards, Richard Harrison, KB5WZI

Richard Harrison wrote:
Richard Fry wrote:
"Richard Clark might have allowed for systems operating above 30 MHz.,
which include several ham bands, I believe."

I believe so too.

From page 810 of Terman`s 1955 edition:
"Space-wave Propagation. At frequencies above about 30 MHz, the
ionosphere is not able to refract energy to earth,

It is comforting to note that the ancient ones were wrong about some
things. It may not happen daily, but it happens a lot at 6 meters,
often at 2 meters, and sometimes even higher.

tom
K0TAR

"Tom Ring" wrote
From page 810 of Terman`s 1955 edition:
"Space-wave Propagation. At frequencies above about 30 MHz, the
ionosphere is not able to refract energy to earth, "
It is comforting to note that the ancient ones were wrong about some
things. It may not happen daily, but it happens a lot at 6 meters, often
at 2 meters, and sometimes even higher.
____________

The term "space wave propagation" does not necessarily mean that the wave
has to be reflected from the ionosphere (or anything else) to be useful.
Microwave systems, FM and TV broadcasting, aeronautical communications,
satellites, and many other point-to-point systems at VHF and above all use a
space wave. Reflection/refraction of it whether from the ground,
structures, or (occasionally) the ionosphere are not always beneficial to
those systems.

RF