Propagation as a Ground Surface Wave

If the transmitting and receiving antennas are close (in comparison to a wavelength) to the ground, the wave propagates bound to the ground, as a

Figure 10.16 Wave propagation via the ionosphere.

surface wave. The electric field strength of the wave decreases rapidly as the distance from the surface increases. At low frequencies the attenuation of such a ground wave is small, and the wave can propagate beyond the horizon thousands of kilometers, especially over seawater. However, the attenuation of a ground wave increases rapidly with frequency. Therefore this propagation mechanism is useful only below 10 MHz.

Attenuation of the wave depends on conductivity and permittivity of the surface (land, lake, or sea). Table 10.1 shows some typical characteristics at frequencies below 30 MHz. At microwave frequencies these characteristics change strongly as a function of frequency. ITU-R publishes field strength graphs for different surface types. Figure 10.17 presents the electric field strength versus distance when the wave propagates along the surface of a medium dryland and sea [14]. The transmitting antenna is a vertical monopole, and the power transmitted is 1 kW. The graphs show that at 10 kHz the electric field strength decreases as 1/r (as in free space) up to a distance

Table 10.1

Electrical Properties of Different Ground Surfaces (f < 30 MHz)

Table 10.1

Electrical Properties of Different Ground Surfaces (f < 30 MHz)






Fresh water


3 X 10"3

Wet land



Dry land


10-- 4 -

Ice on a lake



Figure 10.17 Electric field strength of the ground wave when the transmitting antenna is a vertical monopole and the power transmitted is 1 kW: (a) over a medium dry land, a = 10"3 S/m, er = 15; (b) over sea water, a = 5 S/m, er = 70. (After: [14].)

of 1,000 km. At higher frequencies the attenuation is much higher. Over the sea the wave attenuates much slower than over medium dry land. In practice the electrical characteristics of the surface may vary a lot between the transmitting and receiving stations, and therefore a prediction of the power to be received is difficult.


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