The Double-Helix Antenna and Its Variants, a New Class of Tunable Endfire Antennas

Endfie antennas develop maximum gain when the phase velocity of the surface wave traveling along the structure is adjusted to its optimum value determined as a function of antenna length and operating frequency. These antennas usually have a rela- tively small pattern bandwidth and, if maximum gain is desired, can be used over only a small frequency range. The antennas described in this paper inaugurate a new class of antennas that are tunable for maximum gain in the endfire direction over a wide frequency range; tuning is accomplished by changing the phase velocity continuously or in prescribed steps. Such antennas include certain configurations of the double helix (a novel type of endfie antenna) and its artificial and natural dielectric variants. Use- ful structures are obtained through parallel displacement of two juxtaposed elements or angular displacement of a scissors arrange- ment. Model measurements show that maximum gain can be obtained over a frequency range of more than 2:l. Tuning effects are illus- trated in detail by means of near-field plots of a tuned dielectric antenna. 1 XTRODUCTION