Monopulse Radial Line Slot Array Antenna Fed by a 3-D-Printed Cavity-Ended Modified Butler Matrix Based on Gap Waveguide at 94 GHz

This article presents the design and fabrication of a monopulse radial line slot array (RLSA) antenna excited with a circular cavity connected to a modified Butler matrix at 94 GHz. The antenna consists of two pieces: the RLSA antenna implemented over a low loss dielectric substrate and a 3-D-printed Butler matrix based on gap waveguide. The interface between both parts is a circular cavity that couples two different modes into the RLSA through a coupling slot in the dielectric substrate. The coupled mode depends on the input excitation of the Butler matrix for the generation of simultaneous sum and difference radiation patterns. This interface is deeply detailed in this article, together with the design of the RLSA antenna and the integration of the full monopulse antenna. The measured S-parameters fit very well with simulations. The operation is inherently narrowband with a realized gain of 27.8 dBi at 94 GHz, a radiation efficiency of 70%, 0.3 dB axial ratio, and a maximum null relation between sum and difference patterns higher than 50 dB.