A novel ultra-miniaturized 2.5-D frequency selective surface temperature sensor with linear frequency–temperature behavior

Abstract

This paper presents the design and proposal of a new ultra-miniaturized Frequency Selective Surface (FSS) sensor configuration for applications in temperature sensing systems. A 2.5-D (2.5 Dimensions) FSS is proposed with elements inspired by convoluted metal lines with metal vias inserted and printed on an FR4 dielectric substrate. The vias contribute to the capacitive and inductive effects in the structure, providing ultraminiaturization of the unit cell dimensions. The size of the unit cells of the proposed FSS is equal to 4.42 % of the wavelength in free space for the frequency of 2.21 GHz (resonance frequencies). The principle of operation of the FSS is evaluated using the equivalent circuit model proposed in this work. A new application for ultra-miniaturized FSS based on convoluted geometries as non-contact temperature sensors is analyzed in this work. According to the literature, the electrical permittivity of the dielectric material used as the FSS substrate changes as the temperature of the environment in which the circuit is inserted changes, thus altering its frequency response. Numerical analyses were carried out and found that the FSS has an almost linear relationship between the resonance frequency and the electrical permittivity. The numerical results simulated for the prototype were obtained using the ANSYS HFSS software and the equivalent circuit model. The prototype sensor was built and the transmission coefficient and resonance frequency were experimentally characterized. The proposed sensor was experimentally analyzed from a temperature of 20° to 120°C and showed a sensitivity of 0.640 MHz/°C. The values obtained in the experiments were compared and discussed with the results of the simulations, which showed good agreement.