Waveguide-fed log periodic antennas

Abstract

Abstract : Almost all log periodic antenna designs incorporate a parallel or tapered-vee transmission line, as the feeding mechanism for the radiating elements. These lines propagate the TEM mode. Techniques whereby log periodic antennas may be fed with waveguides are introduced in this report. In this case the dominant TE sub 10 mode of the waveguide is used in the feeder transmission line application. Methods of coupling energy to the radiating elements of the log periodic structure by probes inserted in the waveguide are presented. It is shown that an array may be fed either on the broad wall or narrow wall of the waveguide. The useful operating frequency range of these antennas is limited to the normal waveguide bandwidth. The waveguide-fed method is primarily useful for monopole arrays although dipole arrays can also be fed in this manner. Results of measurements performed on various antenna models are discussed. It is shown that VSWR responses below 2:1 and absolute radiation pattern gains above 10 db can be achieved. The effects of different design parameters on antenna characteristics are discussed. In addition, a requirement is presented in which either the element lengths or element spacings must be compensated to achieve constant H-plane radiation patterns. (Author)