"Roy Lewallen" wrote:
As I'm sure you know, AM broadcast antennas intentionally radiate
very little sky wave, and that's what amateurs need for communication
beyond a few miles. Some care must be used in comparing MW
broadcasting requirements and characteristics with amateur
HF communications.
__________________

AM broadcast station verticals have very significant energy at elevation
angles that can be propagated by skywaves. As I'm sure you know, Class A AM
broadcast stations have an extended geographic service area served
exclusively by their nighttime skywave--many times more area than is served
by their surface wave, in fact.

Richard Clark's statement did not limit his conclusion to amateur HF
communications.

RF

Richard Fry wrote:
"Roy Lewallen" wrote:

As I'm sure you know, AM broadcast antennas intentionally radiate
very little sky wave, and that's what amateurs need for communication
beyond a few miles. Some care must be used in comparing MW
broadcasting requirements and characteristics with amateur
HF communications.

__________________

AM broadcast station verticals have very significant energy at elevation
angles that can be propagated by skywaves. As I'm sure you know, Class
A AM broadcast stations have an extended geographic service area served
exclusively by their nighttime skywave--many times more area than is
served by their surface wave, in fact.

Richard Clark's statement did not limit his conclusion to amateur HF
communications.

RF

I'm sorry, I stand corrected. Extended coverage AM stations do indeed
produce significant sky wave as you've pointed out. I was thinking only
of suppression of high angle sky wave radiation to avoid fading.

And you're also correct about Richard Clark's statement. Perhaps he is
indeed attempting to do some MF broadcasting to his local area -- I just
assumed he wasn't.

Roy Lewallen, W7EL

"Roy Lewallen" wrote
And you're also correct about Richard Clark's statement. Perhaps he is
indeed attempting to do some MF broadcasting to his local
area -- I just assumed he wasn't.
___________________

I did, too. But antennas carefully developed and documented for good MF
broadcast performance also are useful for 160 meter ham applications.

And as far as significant radiation at elevation angles below 15 degrees,
Richard Clark might have allowed for systems operating above 30 MHz, which
include several ham bands, I believe. Use of these bands often is
line-of-sight between terrestrial endpoints, and would not be very
successful if all antennas had low relative fields near the horizon--as
implied by Richard Clark's post. FM and TV broadcasting and public service
radio (police/fire etc) use such low-angle radiation successfully, and so do
ham radio operators.

RF

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

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