Polyrod Antennas

A U N I F O R M rod (or "wire") of dielectric material without metallic ·*· boundaries is a well-known type of single conductor transmission line. In this kind of waveguide, a portion of the energy travels along in the s j ^ c e outside the rod. At discontinuities, including those caused by proximity to other objects, radiation takes place. For this reason, the dielectric waveguide has n o t become generally useful as a transmission medium, this need having been satisfied by the hollow metal pipe. T h e tendency toward radiation inherent in the dielectric guide is turned to a d v a n t a g e , however, in a new form of radio antenna. Here the objective is to encourage radiation from all p a r t s of the dielectric rod. In progressing along the rod, therefore, power is gradually transferred from within the dielectric to the space outside. At a point where the transfer has been effectively completed, the rod can be terminated abruptly. By proper design, this radiating structure is an endfire antenna. Since it has been most often fabricated from polystyrene, it has become known as the polyrod a n t e n n a . It is especially useful for microwaves. We must now review and examine certain features of dielectric rod transmission and of endfire a n t e n n a theory, for their bearing on polyrod design and performance. 2. DIELECTRIC W I R E TRANSMISSION 1