Near-field investigation of uniform periodic monopole arrays

Measurements of amplitude and phase of the fields close to a number of driven periodic arrays of monopoles have been made to determine frequency dependence of the propagation constant for such structures. The results have been analyzed, where possible, in terms of a single traveling wave along the structure, and dispersion curves are given which include attenuation as well as phase shift. These curves clearly illustrate the possibility of obtaining leading phase shift along the periodic structure that will produce backfire radiation. Experimental radiation patterns corroborate the near-field results. A simple theory is given for the backfire phasing in terms of the space-harmonic content of a Fourier series expansion for the fields of periodic structures. This theory predicts beam scanning for the periodic structures which is readily observed in the experimental models. The measured data are approximately applicable to tapered versions of the periodic structures which have log-periodic geometry. Examples of periodic structures that have frequency independent log-periodic counterparts as well as some that do not are discussed.