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The novel application of microwave directional couplers to develop angular‐displacement microwave sensors is reported. The proposed sensor approach employs as stator a branch‐line‐type coupler arranged in transversal mode by loading its direct and coupled ports with two distinct‐length open‐ended stubs. Thus, by taking the isolated port of the coupler as the stator output node, a bandpass filtering transfer function with transmission zeros (TZs) is created. Then, a rotor made up of an angularly‐moveable open‐ended stub is attached to a curved section of the longest loading stub of the stator through physical contact, so that their interconnection point varies with the angular‐displacement of the rotor. In this manner, the sensor transfer function is altered with the stub rotation through TZ reallocation, angular‐displacement sensing capabilities are achieved. The theoretical operational foundations of the conceived branch‐line‐coupler‐based microwave angular‐displacement sensor, which features single/multi‐band sensing properties in terms of inter‐TZ spacing and stop band attenuation levels, along with design examples and curves are provided. The extrapolation of this sensor principle to other classes of power‐distribution circuits, such as the rat‐race‐type directional coupler, is also demonstrated. Finally, for experimental‐validation purposes, two 920 MHz microstrip prototypes of the conceived branch‐line‐coupler‐based angular‐displacement microwave‐sensor approach are built and measured.

A differential planar microwave resonator permittivity sensor for material characterization is proposed in this paper. The sensor is based on asymmetric terminated cross-shaped resonator (TCSR) to provide multi-band notch frequency characteristics, allowing permittivity measurement of a small dielectric material under test (MUT) with a single resonator. Differential sensing is robust against varying ambient factors that cause frequency variations in the measurements. The dielectric properties of the MUT can be measured from the difference in notch frequencies with the reference material. To illustrate the technique, a tri-band sensor is prototyped using the proposed resonator configuration to measure the permittivity of both solid and liquid samples. The empirical equations for the determination of the MUT permittivity in relation to the notch frequencies have been derived. The sensor operates at 0.97, 1.69, and 2.91 GHz with an average sensitivity of 1.18, 4.45, and 1.22 MHz, respectively. The measured results are in a good agreement with the theoretical analysis.