Loss Factors in Parallel-plate Slot-array Antennas on a Copper-clad Dielectric Substrate in the Millimeter-wave Band

A post-wall waveguide-fed parallel-plate slot array antenna [1–3] has been proposed as a highgain, high-efficiency and mass producible planar antenna in millimeter-wave applications. The authors are developing post-wall waveguide slot arrays using a copper-clad dielectric substrate which reduces the cost of waveguides structures by printed circuit board (PCB) fabrication technique. The transmission loss reduces the antenna efficiency, particularly in millimeter-wave bands. In order to accurately estimate the antenna efficiency, loss in every part of the antenna should be carefully evaluated. The precise theoretical estimation of the losses including the effects of surface roughness is difficult [4,5]. The authors use the Whispering Gallery (WG) mode resonator method [6,7] to measure not only the complex permittivity of a dielectric substrate but also the effective conductivity considering the rough interface between the copper and the dielectric over a wide frequency range. A discussion [8] of the measured loss of microstrip lines has been shown to support the property of the effective conductivity obtained by this method. In this paper, the authors investigate the efficiency of the post-wall waveguide-fed parallel plate slot arrays at 76.5GHz. Loss factors, such as the transmission loss (the dielectric loss and the conductor loss both in the feed and the parallel plate waveguides), the aperture illumination efficiency, the reflection and the insertion loss of the input transition have been taken into consideration. The transmission loss is evaluated in the sense of perturbation theory using the macroscopic model where the fields are decaying exponentially due to the presence of coupling windows in the feed and slots radiation in the parallel plate. The loss associated with the currents locally enhanced by slot radiation is also evaluated using a microscopic model with one slot pair.