A millimeter-wave antenna with wideband mode-transition between microstrip-line and waveguide

A millimeter-wave antenna directly fed by waveguide is proposed. The antenna has a simple dipole element fed by a microstrip-line (MSL) which is connected to a WR-15 waveguide via a wideband mode-transition. The antenna element and the feeding MSL are structured on a low-loss single-layered liquid crystal polymer (LCP) substrate. In order to connect the MSL to the WR-15 waveguide with a wideband operation, a metal T-shaped back-short block is mounted over the terminal of the MSL which is surrounded by via-holes and an aperture is opened on the ground plane of the MSL. The aperture is attached by a metal two-staged aperture block which is connected to the waveguide. This structure provides a wideband mode-transition between the MSL and the waveguide. Testing devices for evaluating the mode-transition and the antenna combined with the transition are fabricated and the performance is measured. An impedance bandwidth of 16.7% (55-65 GHz) is obtained for the mode-transition and that of 18.0% (58-69.5 GHz) is obtained for a bow-tie-shaped dipole antenna combined with the transition. The antenna shows a broad-beam radiation pattern and can be used for short-range wireless communication in 60 GHz-band applications. The mode-transition can be applied for other types of antennas fed by MSL thanks for its wideband operation.