Miniaturized 3D directional coupler for compact monostatic microwave imaging systems

The potential utility of real-time handheld microwave imaging systems is significant in wide range of applications including nondestructive testing of aerospace composites. In such applications, monostatic imaging systems, where the structure-under-test (SUT) is inspected from one side using transceiver antenna arrays for irradiation and reception as well, are highly desired. This stems from the fact that access to both sides of the SUT is commonly limited in practice and monostatic imaging systems yield higher resolution than bi-static systems. To realize a monostatic system, a directional coupler is typically used to drive each antenna array element with the illuminating wave which impinges upon the SUT, and also delivers the reflected wave to the receiver for detection. Due to the space limitation in typical imaging arrays, the size and weight of the utilized directional coupler become critical for the realization of handled imaging systems. Many of the conventional directional couplers cannot fulfill the size requirement or otherwise show weak coupling and/or isolation coefficients at high frequencies. In this paper, we present a miniaturized 3D directional coupler design on multi-layer printed circuit board (PCB), which satisfies the size requirement and has a good performance in terms of coupling and isolation coefficients at 24 GHz.