The very-high-frequency range is defined as between 30 MHz and 300 MHz. The ionosphere can reflect radio signals on frequencies of > 50 MHz but over most of the VHF range the main effect of the ionosphere is to scatter radio energy by irregularities. Such irregularities exist in the D region (turbulence), in the E region (meteors and sporadic E), and in the F region, especially near the dip equator and in high latitudes (plasma instabilities). On lower frequencies (e.g. less than 50 MHz) propagation effects such as Faraday rotation, angular refraction, and time delay can be important. The value of Y(=f_H/f) is small (<0.05), and under most conditions the propagation is essentially quasi-longitudinal (e.g. on 60 MHz for θ ≲ 89.28° with f_H = 1.5 MHz) and the ray path is almost straight. The maximum electron densities in the ionosphere are rarely sufficient to reflect radio waves on frequencies much above 30 MHz except for relatively short periods (e.g. in the equatorial regions and in higher latitudes near the maximum of the sunspot cycle). For communications on the ground the main ionospheric mechanism is scattering by plasma irregularities. The chief irregularities in the ionosphere are: (i) turbulence in the D region; (ii) ionized trails due to the passage of meteors through the atmosphere; (iii) spread-F (field aligned); (iv) sporadic E; and (v) artificial irregularities. This chapter deals with very high frequency radiowave propagation in the ionosphere with particular focus on scattering phenomena including D-region scatter, meteorscatter, auroral scatter, and equatorial scatter. VHF propagation by ionospheric layers is also discussed.

Inspec keywords: ionospheric disturbances; aurora; ionospheric electromagnetic wave propagation; electromagnetic wave scattering; meteors; D-region; E-region; VHF radio propagation; F-region

Other keywords: auroral scatter; ionized meteor trails; Faraday rotation; equatorial scatter; ionospheric VHF radiowave propagation; angular refraction; time delay; artificial ionospheric irregularities; ionospheric turbulence; sporadic E layer; E-region; D-region scatter; F-region; plasma instabilities; meteorscatter; very high frequency radiowave; ionospheric layers; ionospheric electron density; field aligned spread-F

Subjects: Ionospheric electromagnetic wave propagation