hmmm, this stuff I have read on NVIS says 0.1 to 0.2 wavelengths. I can find
the source if anyone is interested (I am not at home right now). I am
certainly not arguing (I have no problem arguing, but this is one of many
subjects about which I have very little knowledge), Richard, just mentioning
a difference in what I have read.

Paul AB0SI


"Richard Harrison" wrote in message
...
Max, VE3TMT wrote:
"I have come to the conclusion that the only antenna I am going to be
able to use is a ground mounted dipole or random wire."

Don`t expect much NVIS broadside to a dipole on the ground unless it`s
on deep dry sand.

For NVIS the ideal height over the effective earth reflector is about
1/4-wave. This is because the earth is 90-degrees away from the
radiator. This makes a round-trip for the signal of 180-degrees. This
added to the 180-degrees of phase inversion produced in reflection gives
a total of 360-degrees so that the reflected wave when it returns to the
radiator is back in-phase with the new emissions which are headed in the
same direction, that is toward the zenith (straight up).

Lowering the height of the radiator shifts the phase between the two
signal components from directly in-phase at 1/4-wave distance between
radiator and reflector sites to out-of-phase at zero distance between
their sites.

At zero height the radiator and earth behave as a lossy single-wire
transmission system where opposing polarities nullify radiation
broadside to the radiator.

Hope for effective radio communications using a near to the earth
antenna comes in the form of a Beverage antenna. I`ve used a Beverage
antenna very successfully for sky-wave tnans-Atlantic reception in
Portugal of WCBS in New York, and WWL in New Orleans.

The 1945 War Department book, "Electrical Communication Systems
Engineering" on page 317 says:
"Insulated wires of sufficient length laid on the ground, or better yet
on vegetation just above the ground, will provide ground-wave
transmission with vertical polarization in line with the direction of
the wire, that is, off either end of the wire, and for moderate
horizontal angles from this direction. Such low antennas have the
advantage of being easily concealed. A length of about 600/F feet (where
F is the frequency in megacycles), or somewhat longer, is suitable, if
the length is adjusted by trial to permit good transmitter loading. A
100-foot on-ground antenna attached to Radio Set SCR-536 (handie-talkie)
gives at least as good transmission in such directions as the whip
supplied with the set, and can be more easily concealed. Half-wave
on-ground antennas---can sometimes be used for short distance sky-wave
reception or transmission."

Stealth has a price to pay in efficiency when low height must be used.

Best regards, Richard Harrison, KB5WZI