A miniaturized spot-focusing microwave sensor for broad-band reflectivity measurements

Abstract

This study presents a novel spot-focusing microwave sensor designed for on-site monitoring of absorbing materials and coatings, a critical process in assessing the performance of equipment in service. The sensor addresses the challenge of integrating miniaturization, wide bandwidth, and focused radiation-requirements that are typically at odds in microwave sensor design. The developed approach utilizing a multi-layer lens nesting structure to achieve narrow-beam radiation over an ultra-wideband (UWB) frequency range. Each layer of the lens system is engineered to concentrate beams within specific frequency bands, enabling consistent performance across a broad spectrum. In addition, we introduce a composite feeding structure characterized by exponential gradient slots that enhance UWB performance while minimizing cross-polarization effects. The sensor demonstrates exceptional capabilities, achieving over 8 dB of radiation gain and maintaining a voltage standing wave ratio (VSWR) under 2.5 within a 2–22 GHz frequency band. Its compact size (95 mm in diameter and 75 mm in height) does not compromise its ability to emit a Gaussian beam with a narrow radiation pattern of less than ±30°, simulating plane-wave characteristics essential for near-field on-site measuring. Accompanying the sensor is a novel on-site reflectivity measuring method that leverages time-domain gating technology to mitigate interference signal impact. Measurement results validate the sensor and method’s precision, with an average error margin of 0.78 dB. This breakthrough in sensor design and methodology marks a significant advancement in the on-site evaluation of absorbing materials and coatings.