a.
Short distance (0-250 miles). For stations within 25 miles
of one another, ground wave can be used.
In order for good ground
wave to be used, the intervening terrain should be flat with no
mountains or thick foliage. For short distance sky wave, intervening
terrain has little or no effect because of the high take-off angle.
The exception are trees, separation from trees, for the transmitting
antenna must be maintained if communication is to be maintained. To
make the signal stronger, a counterpoise or radial ground should be
used.
The only requirement for short distance sky wave is having
room enough for the antenna and being away from reflecting objects.
Antenna heights should be less than a quarter wave, 10 to 30 feet
high.
Assuming the correct frequency was used, transmit antenna
heights of a 10 to a /4 can be used without noticeable loss of
signal at the receiving station(s).
(See figure 88 on page 113.)
Notice transmit antenna heights of 2 feet at 6 MHz produced a 8 dB
loss of signal at the receiving station. Transmit antenna heights of
10 feet will work with some loss of received signal when using
frequencies between 2 and 9 MHz, while frequencies above 9 MHz suffer
less loss. As the transmit antenna is lowered, the antenna must be
made longer because of the ground effect.
The loss of received
signal is like cutting your transmitter power output in half for each
3 dB loss in received signal. For an 8 dB loss in received signal it
is like cutting your transmitter power from 100 to 12 watts.
b. Medium distance (250-1500 miles).
There is no ground wave
possible at this distance, unless transmitting over water. The same
requirements are needed as there were for short distance, except that
the antenna should be a half-wave high.
c. Long distance (1500 miles and over). The same requirements as
for medium distance, except that the antenna should be as high as
possible.
Ground Conductivity For Various Terrain
101
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