Compact wide scan -angle antennas for automotive applications and RF MEMS switchable frequency -selective surfaces.

Abstract

The objective of this thesis is to develop antenna systems for automotive radar applications at 24 and 77 GHz, separately and simultaneously, and which combine good performance with wide scan-angle, compactness, and low cost of production. Tapered-slot antennas (TSAs) are used as feed antennas to a single homogeneous spherical Teflon lens. This is a low-cost solution, and due to the small physical cross-section of the feed antennas, a very wide field of view is possible. The dependence of sidelobe level, directivity, and efficiency on the feed location is investigated in simulations and experiment, and a maximum achievable gain of around 38 dB is found. Using a lens with a radius of 6.42lambda₀ at 77 GHz (25 mm), a sidelobe level of -20 dB and a gain of 29.4 dB is demonstrated which corresponds to a total system efficiency of 49%, not taking the losses in the TSA into account. Next, antenna arrays using a spherical Teflon lens are designed at 24 and 77 GHz with a scan-angle of up to 180&deg; while maintaining a low sidelobe level of -16 to -20 dB. The size of this multibeam antenna design is reduced using a hemispherical Teflon lens with backside metallization instead of a sphere. Further, the feed antennas for 24 and 77 GHz are integrated into a single aperture, using frequency-selective surfaces (FFSs), thus further increasing the compactness of the system. In a related project, this thesis demonstrates the first RF MEMS Switchable frequency-selective surface. A polarization independent FSS using four-legged loaded elements is designed using 2.5 and 3-dimensional electromagnetic solvers and equivalent circuit models. High resistivity bias lines are avoided by connecting the elements through air bridges. The impact of the bias circuitry for normal and oblique incidence is explained and the loading MEMS bridges are placed accordingly. An FSS consisting of 909 unit cells with 3,636 movable membranes is built on a 3<super>&Prime;</super> glass wafer using standard micromachining processes. The measured insertion loss at 30.2 GHz in inidband is 2.0 dB in transmit mode (bridges up) and 27 dB in reflection mode (bridges down). The FSS can also be tuned in an analog fashion with a pass-band center frequency of 30.2 to 29.4 GHz. The power handling of the switchable FSS is estimated to be >60--100 W before any of the bridges is actuated by the RF power for <italic>V_p</italic> = 20--30 V, or before any thermal limits are reached at the center of the array with active air cooling provided.