The frequency or wavelength of radio waves is different, and their propagation characteristics are not completely the same or even very different. For example, the frequency bands currently used in GSM and CDMA mobile communications are 890-960 MHz, 1710-1880 MHz, and 806-896 MHz, respectively. The frequency range of 806 to 960 MHz belongs to the ultra short wave range; The frequency range of 1710-1880 MHz belongs to the microwave range.

　　The line of sight propagation of ultra short waves and microwaves: Ultra short waves, especially microwaves, have high frequencies and short wavelengths, and their ground waves decay quickly. Therefore, it is not possible to rely on ground waves for long-distance propagation, as they are mainly propagated by space waves. Space waves can generally only propagate in a straight direction to directly visible areas. The propagation area of ultra short waves and microwaves within direct viewing distance is commonly referred to as the "illumination area". Ultra short wave and microwave receiving devices can only stably receive signals within a direct viewing distance.

　　Due to the influence of the curvature radius of the Earth, the relationship between the maximum direct viewing distance R and the height of the transmitting antenna HT and the height of the receiving antenna HR is R=3.57 (√ HT+√ HR) (km)

　　Due to the refraction effect of the atmosphere on radio waves, the maximum direct viewing distance is R=4.12 (√ HT+√ HR) (km)

　　The frequency of electromagnetic waves is much lower than that of light waves, and the effective direct propagation distance is Re=0.7 R.

　　For example, if HT and HR are 49 m and 1.7 m respectively, the effective direct propagation distance R is 24 km.

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