A Review of VHF Ionospheric Propagation

Although the very high frequencies (vhf,30-300 mc) are allocated for utilization almost entirely upon the premise that propagation will be tropospheric, there are very definite ionospheric effects with which one must reckon. These are: (1) regular F2 ionization, (2) sporadic E ionization, (3) scattering from regular ionization, (4) auroral ionization, (5) meteoric ionization. The first of these is predictable with reasonable accuracy and provides phenomenally good communication over great distances and with extremely low power. The second is far less predictable, but it also can provide good communication with extremely low power. The third is mainly a newly discovered phenomenon; although it offers intriguing possibilities for regular communication, it presents at the same time large problems in achieving that end. The fourth and fifth are so far only of nuisance value to the communicator. However, they are phenomena of great interest to the geophysicist, as indeed are the other phenomena as well. In this paper, the nature of vhf propagation arising from these sources of ionization is discussed. The maximum distance between path terminals for single-hop propagation is, of course, about 2,400 km for reflection from E region effects and about 4,000 km for F region effects. Multiple hops are rarely significant except from the regular F2 ionization at frequencies only slightly above 30 mc. A considerable amount of information is included on the nature of the ionization as deduced from the propagation effects. The work of individual investigators is extensively referenced.