Capacitively-Loaded Slow-Wave Transmission Lines for Sensitivity Improvement in Phase-Variation Permittivity Sensors

Planar Phase-Variation Microwave Sensors for Material Characterization: A Review and Comparison of Various Approaches

“…Nevertheless, by implementing the sensing line by means of a slow-wave structure based on shunt capacitors, the impedance of the host line increases (and the width decreases), thereby allowing meandering of the line, as depicted in Figure 5a, in a relatively small space. Therefore, the implementation of an ordinary line with the same impedance (and consequently with wider strip width) in the same sensing area does not easily allow the necessary length to achieve the same sensitivity (this has been experimentally corroborated in [61], to be discussed next).…”

“…In all these cases of reactive loading, the effective capacitance and/or the effective inductance of the line are increased, and the result is a decrease in the phase velocity. To illustrate the approach, a slow-wave transmission line-based sensor implemented by means of a capacitively-loaded line, first reported in [61], is presented. The topology and circuit schematic (unit cell) of these periodically loaded lines consists of a host line loaded with a shunt capacitor C at its central position (Figure 5).…”

“…Naturally, this impedance will be modified by the presence of the MUT in the sensing region, but, with such value, mismatching reflections will be minimized (especially for small perturbations), and the phase of the structure, φ, the output variable, will be similar to the phase of the transmission coefficient, an easily measurable quantity. Indeed, in [61], the effects of the variation in the dielectric constant of the MUT on the impedance of the host line, Z 0 , were neglected, but this only provides a rough estimate of the sensitivity. Indeed, by including such term, if the impedance of the structure is set to Z B = 50 Ω, it is not possible to infer a value of the sensitivity better than the one of an ordinary line with identical physical length and characteristic impedance, for reasonable values of the slow-wave factor.…”

“…The slow-wave ratio of the structure was set to swr = 1/2 in [61]. This represents an appreciable reduction in sensor size and phase velocity, yet keeping an implementable value of the impedance of the host line, Z 0 .…”

“…The main objective of this review paper is to point out, discuss and compare different sensing strategies devoted to performance optimization (specifically sensitivity enhancement, the typical challenging aspect) in phase-variation microwave sensors [36][37][38]45,46,[61][62][63][64].…”