The application of characteristic-mode techniques to vehicle-mounted NVIS antennas

Radio communications over relatively short distances, but beyond line-of-sight, present few problems nowadays, except in certain situations when no suitable, artificial means of relaying the signals exist. A mode of propagation, known as NVIS, or Near-Vertical-Incidence Skywave, which relies only on the ionosphere, provides a solution. However, to exploit it requires careful choice of operating frequency, as well as the use of appropriate antennas. If the terminals are mobile, then the antennas, which are usually electrically small, can be the limiting factor. A method of optimizing antenna performance is presented that exploits the natural or characteristic modes of the structure on which the antenna is mounted. Extensive use is made of the Method of Moments, in the form of both the NEC and MININEC codes, to compute these modes, and to design and test antenna systems that exploit them. Measurements, made using an instrumentation package flown from a tethered balloon, compare very favorably with computed results.