Passive Matched Mushroom Structure for a High Sensitivity Low Profile Antenna-Based Material Detection System

A passive multi-layered structure based on a mushroom high-impedance surface is proposed as a part of a high sensitivity low profile antenna-based material detection system. The system consists of an interrogating antenna placed outside the near field of a three-layered planar sensor. The sensor is a periodic structure containing 10 × 10 unit cells with overall dimensions of 180 mm × 180 mm × 10.2 mm. Achieving high sensitivity at a large monitoring distance (150 MHz per unit relative permittivity at 430 mm), this sensor provides sensing capability in hazardous or inaccessible environments while also maintaining independence of the sensor and the antenna for greater system flexibility. Material detection is based on wireless tracking of frequency shifts in the absorption peak of the sensor. These shifts arise from changes in the permittivity of a sensing layer sandwiched between a bottom resonant layer (a mushroom high-impedance surface) and a top inductive matching layer. Two independent mechanisms contribute to the resonant shifts for heightened sensitivity at a low profile. Two sets of experiments were performed to verify the operation of the sensor over a wide range of solid and liquid material permittivities. First, solid samples of Teflon, Plexiglass, and Rogers microwave laminates (TMM06 and TMM10) were detected with frequency shifts up to 1233 MHz. Second, liquid samples of isopropyl alcohol, ethanol, methanol, and de-ionized water were detected with corresponding frequency shifts up to 130 MHz. Measurements are performed using a single reader antenna to reduce the cost and complexity for in-field applications.