4.4.2 Frequency Of Operation

Figure 4.16 illustrates that the effectiveness of surface-wave propagation decreases as frequency increases. For this reason, surface-wave systems are typically confined to the LF and MF bands, and rarely operate above approximately 2–5 MHz. At higher frequencies, raising the antenna height and employing space-wave propagation generally provides better performance.

Although attenuation decreases as frequency is lowered, atmospheric noise increases at lower frequencies, particularly in the LF and MF bands. An operating frequency must therefore be selected as a compromise between propagation range and noise environment.

4.4.3 Other Factors

Type of terrain. Surface waves follow gradual variations in terrain with little additional loss. However, large or abrupt changes in elevation can introduce screening effects that reduce range. Vegetation cover also contributes additional attenuation, particularly when wet. Sparse bushland introduces modest additional loss, whereas dense jungle can severely attenuate surface-wave signals due to absorption within the foliage. In practical link assessment, ground type, terrain profile, and vegetation must therefore all be considered.

Polarization. Surface-wave propagation is effective primarily for vertically polarized waves. A horizontally polarized electric field is strongly attenuated at the ground boundary because induced currents in the surface effectively short-circuit the field component parallel to the ground. Consequently, surface-wave systems employ vertical polarization, requiring vertical antennas for both transmission and reception.