An investigation of periodic rod structures for Yagi aerials

Abstract
The periodic structure of conducting rods, normally used for Yagi-type aerials, is investigated from the point of view of a guide for surface waves. In a series of resonator experiments it is found that a non-radiating plane surface wave of the HE11-mode may be guided along the structure, and that radiation occurs only from a discontinuity. The guiding effect is found to exist only where the rod lengths are less than λ/2. The structure is thus analogous to a capacitively-loaded transmission line. The propagation coefficients are experimentally determined for structures of different rod length and spacing. These propagation coefficients are used to predict the radiation patterns of Yagi aerials in conjunction with the theory outlined in a previous paper. There is quite good agreement with experimentally-observed radiation patterns of long Yagi aerials. The side-lobe structure is explained by interference with direct radiation from the driven element caused by inefficient launching of the surface wave.

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