Effect of Structural Parameters on Dispersion Behavior of Stub-loaded Ridge Waveguides for Use of Leaky-wave Antennas

1. Introduction Leaky-wave antennas based on an open-waveguide structure without partly physical cut become a candidate of millimeter-wave antennas because asymmetry of the structure can easily produce a radiation along its length due to a leaky mode. One of these open waveguides for antenna purposes is the stub-loaded ridge-rectangular waveguide, which consists of a ridge-rectangular waveguide with an asymmetrically located stub guide[1][2]. This guide radiates in a single polarization (with negligible cross polarization at all scan angles), possesses flexibility in beamwidth, and scans the angle of maximum radiation by changing the frequency. Its guide width can be reduced to less than a half wavelength by the ridging effect, so that for use of a scanning array antenna, extra main beam due to a grating lobe can be avoided. However, this guide structure developed by us has an inherent drawback that the channel-guide leaky mode[3] propagates simultaneously together with the desired leaky mode and affects the radiation field. Therefore we have proposed the new stub-loaded ridge waveguide for a single-mode operation in which only the desired leaky mode propagates and the channel guide one is below cutoff[4]. This guide has both a ridge and a stub guide on the same side, different from the conventional guide that they are set each other on the opposite side. In this paper, we investigate the dispersion characteristics for various structural parameters of the single-mode operation guide shown in Fig. 1, and then find out the structural parameters that are effective to construct a leaky-wave antenna with low sidelobes. The guide structure includes many conductor edges at which the fields diverge, so its leakage characteristics are accurately calculated by the mode-matching method incorporating the singular field behavior into the aperture-field expressions[2][4].