On Tue, 06 Jan 2004 22:32:30 GMT, Gary Schafer
wrote:

On Tue, 06 Jan 2004 22:11:54 GMT, Shortwave Sportfishing
wrote:

On Tue, 06 Jan 2004 18:17:04 GMT, Gary Schafer
wrote:

TV and FM stations do not depend on ground wave propagation. It is
line of sight. Nothing to do with polarization or ground wave.

Gary, maybe you can help me out with something here, because either
I've forgotten or I was taught incorrectly.

Over short distances (Up to five to eight miles), the ground wave
component of any VHF signal is actually the primary method by which
VHF signals are received and not by Line Of Sight.

I don't think so. There is some bending, refraction of the signal that
makes the VHF radio horizon slightly longer than line of sight but it
is due to the atmosphere and not ground wave propagation.

If you have a VHF station on a high mountain peak or a very high tower
there could be no ground effect at all.

You are making me unpack my reference books now. :)

Later,

Tom
S. Woodstock, CT
----------
"My rod and my reel - they comfort me."

St. Pete, 12 Lb. Test

Some clarification of "ground wave" is in order here. The term ground
wave is sometimes lumped into several different modes that are not
"sky wave" modes.

SURFACE WAVES:

The ground wave that I am referring to here is sometimes called
"surface wave". It is in the ground wave family.
A surface wave travels close to the ground actually touching the
ground. AM radio stations depend on this method of propagation. At AM
frequencies it is good out to about 100 miles. The signal strength is
quite weak at that distance. But in the shorter distances it provides
a very reliable and predictable constant coverage. Unlike sky wave
signals that come and go between night and day.

At higher frequencies that distance is much shorter as the signal gets
highly attenuated. So "surface waves" are not very useful at
frequencies much above the broadcast band for any significant
distance.

The old 2 MHZ marine band used to depend on "surface wave"
communication. With good conditions range can be better than VHF but
usually quite noisy on those frequencies. Also a more complicated
antenna system was needed and higher power than what is used on VHF.

DIRECT WAVES:

Line of sight propagation is also a form of ground wave propagation in
that it does not become a sky wave and reflect off the upper
atmosphere. It may follow the earths curvature and may or may not
touch the earth. This type of propagation is commonly called "direct
wave".

As you asked about "the ground wave component of VHF signals being the
primary method by which VHF signals are received and not by line of
sight".

The primary method is by line of sight which is technically a form of
ground wave as explained above. But usually when someone mentions
"ground wave" they are usually referring to "surface wave".
Line of sight is usually used when referring to "direct wave" VHF
communications.


In line of sight communications there are reflections from the earth
or other objects that can add or subtract to the signal that is being
received directly. The reflected signal gets shifted 180 degrees as it
is reflected. At VHF the reflected wave (because it is rather short
compared to the distances involved) can shift rapidly in phase. This
is what causes flutter on the signal at times.

With horizontal polarization at low HF the wave length is long
compared to the distances involved in the direct and ground reflected
waves. They arrive almost always 180 degrees out of phase. That highly
attenuates the direct wave signal.

NO SURFACE WAVE FROM HORIZONTAL ANTENNAS: from ARRL antenna handbook.

With a vertical antenna the electric field is perpendicular to the
ground. It is a law of electromagnetics that electric lines must touch
a conductor (the earth in this case) perpendicular or else they would
have to generate infinite currents in the conductor. The earth is a
rather good conductor below about 10 mhz so waves at these frequencies
are mainly vertical when traveling close to the ground. If they were
horizontal they would try to induce large currents and be absorbed by
the earth. They get shorted out.

This is why a horizontal antenna has little ground wave or surface
wave. It dies out very rapidly. In addition horizontally polarized
direct waves at low HF get canceled by ground reflections. (see above)

Thus the reason for vertical AM broadcast antennas.

At VHF antenna heights are usually several wavelengths above ground
and polarization does not matter. Plus the earth is not a good
conductor as to short out the horizontal waves as it does at low HF.

The reason for vertical polarization being mostly used at VHF is it is
much easier to obtain omni coverage with simpler antennas. Especially
portable and mobile antennas.

Regards
Gary

On Wed, 07 Jan 2004 02:31:17 GMT, Gary Schafer
wrote:

Some clarification of "ground wave" is in order here. The term ground
wave is sometimes lumped into several different modes that are not
"sky wave" modes.

SURFACE WAVES:

The ground wave that I am referring to here is sometimes called
"surface wave". It is in the ground wave family.
A surface wave travels close to the ground actually touching the
ground. AM radio stations depend on this method of propagation. At AM
frequencies it is good out to about 100 miles. The signal strength is
quite weak at that distance. But in the shorter distances it provides
a very reliable and predictable constant coverage. Unlike sky wave
signals that come and go between night and day.

At higher frequencies that distance is much shorter as the signal gets
highly attenuated. So "surface waves" are not very useful at
frequencies much above the broadcast band for any significant
distance.

The old 2 MHZ marine band used to depend on "surface wave"
communication. With good conditions range can be better than VHF but
usually quite noisy on those frequencies. Also a more complicated
antenna system was needed and higher power than what is used on VHF.

DIRECT WAVES:

Line of sight propagation is also a form of ground wave propagation in
that it does not become a sky wave and reflect off the upper
atmosphere. It may follow the earths curvature and may or may not
touch the earth. This type of propagation is commonly called "direct
wave".

As you asked about "the ground wave component of VHF signals being the
primary method by which VHF signals are received and not by line of
sight".

The primary method is by line of sight which is technically a form of
ground wave as explained above. But usually when someone mentions
"ground wave" they are usually referring to "surface wave".
Line of sight is usually used when referring to "direct wave" VHF
communications.

~~ snippage ~~

Thank you. :)

Later,

Tom
S. Woodstock, CT
----------
"My rod and my reel - they comfort me."

St. Pete, 12 Lb. Test