A microstrip array antenna with 45-degree inclined linear polarization is proposed for automotive radars. The proposed antenna has the advantages of high aperture efficiency, low profile and ease of manufacture. The rectangular radiating elements inclined at 45 degrees to the straight microstrip line are directly connected to it at their corners in the proposed array antenna. The radiating element has a feature that radiation conductance for co-polarization is controlled widely enough to set desired amplitude distribution keeping excited mode for cross-polarization negligibly small. The feed line loss of the linear array antenna having 15 wavelengths is estimated 0.9 dB in the design taking the loss of the microstrip line into account. The performance of two types of developed antennas, for electrical and mechanical scanning radars, is presented. The fan beam subarray antenna for electrical scanning radars has an aperture efficiency of 53% with gain of 22.5 dBi at 76.5 GHz. For mechanical scanning radars, the two-stage series feeding circuit is also proposed for lower feed line loss and setting desired amplitude distribution. The pencil beam array antenna has an aperture efficiency of 39% with gain of 32.2 dBi at 76.5 GHz.

We propose a new type of microstrip line to waveguide transition fabricated on a single layer dielectric substrate. Impedance matching of the transition is achieved by controlling the size of a matching element and the length of an inserted microstrip line across a waveguide. As a result of experiments, low transmission loss of 0.4 dB is realized at the design frequency of 76.5 GHz. Bandwidth of the transition is numerically investigated by the finite element method. It is clarified that the bandwidth of the transition becomes wider as the cross section of the waveguide becomes smaller and twice as wide as that of a conventional microstrip patch antenna element fabricated on a dielectric substrate with the same parameters. In addition, the effect of errors in relative position between the dielectric substrate and the waveguide is also investigated. It becomes clear that degradation of transmission characteristics is caused by the shift of resonant frequency and keeps less than 0.1 dB for a manufacturing accuracy within 0.1 mm.