A monopole antenna fed by a rectangular waveguide

Monopole antennas are widely used in communication systems because of features such as pure vertical polarization and omnidirectional radiation patterns in the horizontal plane. Conventionally, monopole antennas are fed through a ground plane by a coaxial probe. Recently, monopole antennas fed by printed transmission lines have attracted increasing attention. However, to the best of our knowledge, monopole antennas are seldom fed by a rectangular waveguide. We consider the problem of this type of feed because of our interest in designing an omnidirectional antenna at millimeter wave bands. At millimeter wave frequencies, rectangular waveguides are preferred to coaxial or microstrip lines because they exhibit very low loss, have a large power handling capacity and other waveguide components are readily available. The paper presents a design for monopole antennas fed by a rectangular waveguide. In order to achieve a good transition from a rectangular waveguide to the monopole antenna, a ridged waveguide section is introduced, and the vertical monopole is attached to the ridge through a circular aperture opened on the upper broad-wall of the rectangular waveguide, which is part of the ground plane. Two examples are provided and experimental tests of one of these antennas shows that the monopole can achieve about 24% bandwidth and 5 dBi gain.