This study proposes a high-sensitivity microwave sensor based on an interdigital-capacitor-shaped defected ground structure (IDCS-DGS) in a microstrip transmission line for the dielectric characterization of planar materials. The proposed IDCS-DGS was designed by modifying the straight ridge structure of an H-shaped aperture. The proposed sensor was compared with conventional sensors based on a double-ring complementary split ring resonator (CSRR), a single-ring CSRR, and a rotated single-ring CSRR. All the sensors were designed and fabricated on 0.76-mm-thick RF-35 substrate and operated at 1.5 GHz under unloaded conditions. Five different standard dielectric samples with dielectric constants ranging from 2.17 to 10.2 were tested for the sensitivity comparison. The sensitivity of the proposed sensor was measured by the shift in the resonant frequency of the transmission coefficient, and compared with conventional sensors. The experiment results show that the sensitivity of the proposed sensor was two times higher for a low permittivity of 2.17 and it was 1.42 times higher for a high permittivity of 10.2 when compared with the double-ring CSRR-based sensor.

A slot-loaded microstrip patch sensor antenna is proposed to enhance sensitivity in measuring the permittivity of planar materials. A thin rectangular slot was etched along the radiating edge of a rectangular patch antenna fed by a microstrip transmission line. Two resonant frequencies were created at a lower frequency compared to the single resonant frequency of a conventional ordinary patch antenna. The sensitivity of the proposed slot-loaded patch antenna was measured by the shift in the resonant frequency of the input reflection coefficient when the planar dielectric superstrate was placed above the patch, and was compared with that of a conventional patch antenna without the slot. The two antennas were designed and fabricated on a 0.76 mm-thick RF-35 substrate for the first resonant frequency to resonate at 2.5 GHz under unloaded conditions. Five different standard dielectric samples with dielectric constants ranging from 2.17 to 10.2 were tested for sensitivity comparison. The experiment results showed that the measured sensitivity of the proposed patch antenna were 3.54 to 4.53 times higher, compared to a conventional patch antenna, for the five samples.

An eight-bit chipless radio frequency identification tag providing humidity sensing and identification information is proposed. A compact, enhanced-sensitivity resonator based on an interdigital capacitor (IDC) structure is designed for humidity sensing, whereas seven electric-field-coupled inductor capacitor (ELC) resonators are used for identification information. These eight resonators are placed in a two-by-four array arrangement. A step-by-step investigation for the effect of varying the number of elements and array configuration on the resonant frequency and radar cross-section (RCS) magnitude of the IDC resonator is conducted. The RCS value of the resonant peak frequency for the IDC resonator increases as the number of array elements placed nearby increases due to the mutual coupling among the elements, and the increase in the RCS value becomes larger as the number of arrays increases in the vertical direction. Polyvinyl alcohol (PVA) is coated on the IDC-based resonator at a thickness of 0.02 mm. A non-reflective temperature and humidity chamber is fabricated using Styrofoam, and the relative humidity (RH) is varied from 50% to 80% in 10% intervals at 25 °C in order to measure a bistatic RCS of the proposed tag. The humidity sensing performance of the IDC resonator in the proposed tag is measured by the shift in the resonant peak frequency and the RCS value, and is compared with a single ELC resonator. Experiment results show that when RH increased from 50% to 80%, the sensitivities of both the resonant peak frequency and the RCS value of the IDC resonator were better than those of the ELC resonator. The variation in the RCS value is much larger compared to the resonant peak frequency for both IDC and ELC resonators. In addition, the resonant peak frequency and RCS value of the PVA-coated IDC-based resonator change, whereas those of the other seven resonators without a PVA coating do not change.

A high-sensitivity microstrip patch sensor antenna (MPSA) loaded with a meander-line slot (MLS) is proposed for the measurement of relative permittivity. The proposed MPSA was designed by etching the MLS along the radiating edge of the patch antenna, and it enhanced the relative permittivity sensitivity with an additional effect of miniaturization in the patch size by increasing the slot length. The sensitivity of the proposed MPSA was compared with that of a conventional rectangular patch antenna and a rectangular slit (RS)-loaded MPSA, by measuring the shift in the resonant frequency of the input reflection coefficient. Three MPSAs were designed and fabricated on a 0.76 mm-thick RF-35 substrate to resonate at 2.5 GHz under unloaded conditions. Sensitivity comparison was performed by using five different standard dielectric samples with dielectric constants ranging from 2.17 to 10.2. The experiment results showed that the sensitivity of the proposed MPSA is 6.84 times higher for a low relative permittivity of 2.17, and 4.57 times higher for a high relative permittivity of 10.2, when compared with the conventional MPSA. In addition, the extracted relative permittivity values of the five materials under tests showed good agreement with the reference data.

A resonant transmission of an electrically small aperture with a ridge in a conducting plane is analyzed. It is an example of resonant transmission and the ridge inserted at the center line of the electrically small aperture resonates it by increasing an equivalent capacitance of the aperture. At resonance condition, the transmitted power through the aperture can be significantly enhanced compared to that at off-resonance. It is found that the transmitted power at the resonance condition converges to the transmission cross section of the aperture regardless of its size or shape. In addition, the electric and the magnetic fields distributions on the aperture with and without the ridge are compared to facilitate the understanding of the resonant behavior.

A comparative study to determine the most highly sensitive resonant frequency among the first four resonant frequencies of a conventional patch antenna and defected ground structure (DGS)-loaded patch antennas using commonly used DGS geometries in the literature, such as a rectangular slit, single-ring complementary split ring resonators (CSRRs) with different split positions, and double-ring CSRRs (DR-CSRRs) with different locations below the patch, for relative permittivity measurement of planar materials was conducted. The sensitivity performance for placing the DGS on two different locations, a center and a radiating edge of the patch, was also compared. Finally, the effect of scaling down the patch size of the DGS-loaded patch antenna was investigated in order to enhance the sensitivities of the higher order resonant frequencies. It was found that the second resonant frequency of the DR-CSRR DGS-loaded patch antenna aligned on a radiating edge with a half scaled-down patch size shows the highest sensitivity when varying the relative permittivity of the material under test from 1 to 10. In order to validate the simulated performance of the proposed antenna, the conventional and the proposed patch antennas were fabricated on 0.76-mm-thick RF-35 substrate, and they were used to measure their sensitivity when several standard dielectric substrate samples with dielectric constants ranging from 2.17 to 10.2 were loaded. The measured sensitivity of the second resonant frequency for the proposed DGS-loaded patch antenna was 4.91 to 7.72 times higher than the first resonant frequency of the conventional patch antenna, and the measured performance is also slightly better compared to the patch antenna loaded with a meander-line slot on the patch.