Compact Multibeam Antenna Array Facilitated by Miniaturized Slow Wave Substrate Integrated Waveguide Butler Matrix

This communication presents a compact multibeam antenna array facilitated by a $4\times 4$ Butler matrix with substantially miniaturized dimensions based on a slow-wave substrate integrated waveguide (SW-SIW). It is well known that the conventional Butler matrix is bulky as the beamformer occupies most area of the multibeam array. In this work, the miniaturization of the Butler matrix is realized by loading slow wave (SW) structures in the form of loaded patches and non-inclusive via-holes into substrate integrated waveguide (SIW). In this manner, the effective permittivity and permeability of the SIW are increased. Consequently, both the guided wavelength and the cut-off frequency of the SIW are largely reduced. This SW-SIW technology enables the designs of miniaturized couplers, crossovers, and phase shifters with substantially reduced longitudinal and lateral dimensions. With these miniaturized components, a Butler matrix with excellent performance is developed, which is 75.5% smaller compared with the conventional SIW-based Butler matrix. The miniaturized $4\times 4$ Butler matrix is then employed to design a compact four-beam slot antenna array. The measured results agree reasonably well with the simulations, demonstrating the feasibility of the miniaturized SW-SIW Butler matrix in designing a compact multibeam antenna array.