Search

This paper proposes a metamaterial inspired radio frequency touchpad sensor system based on two split ring resonators (SRRs) realized on a Rogers/Duroid 5870 with 2.33 dielectric constant = 2.33, loss tangent = 0.0012, and thickness = 0.8 mm. The touchpad consists of two SRRs resonating at $f_{1} = 2.74$ GHz and $f_{2} = 3.86$ GHz. When a finger is placed on SRR 1, $f_{1}$ changes from 2.74 to 2.57 GHz due to electromagnetic coupling between the SRR and the finger, and similarly, $f_{2}$ changes from 3.86 to 3.76 GHz. The proposed touchpad system can detect a finger wearing a glove, in contrast to capacitive touchpads utilized in contemporary smart phones. We demonstrated 56- and 49-MHz frequency shifts for $f_{1}$ and $f_{2}$ , respectively, with 1.66-mm total thickness (0.69-mm-thick photo paper and 0.97-mm-thick glass slide).

Functional metasurfaces help wireless communication to reach beyond current electromagnetic control device limitations. However, current reconfigurable functional metasurfaces require separate systems for function control. In particular, it is difficult to realize millimeter-wavelength regimes due to the increasing number of active elements with the reduction in unit cell size. This paper proposes a four-dimensional printed memory metasurface to memorize absorption and reflection function in millimeter-wavelength regimes. Thus, metasurfaces with electromagnetic absorption and reflection functions can be realized through mechanical shape memory by memorizing electromagnetic properties using four-dimensional printed structures. The desired electromagnetic performance was experimentally demonstrated and deformation time to memorize the initial structure was measured. The results confirmed that the proposed four-dimensional printed metasurface has potential for considerable contribution to multifunctional wireless devices such as smart electromagnetic wave control systems in reconfigurable intelligent surface, stealth, and wireless sensing systems.

Liquid materials&rsquo, characterization using commercial probes and radio frequency techniques is expensive and complex. This study proposes a compact and cost-effective radio frequency sensor system to measure the dielectric constant using a three-material calibration. The simplified approach measures reflection coefficient magnitudes for all four materials rather than the complex values in conventional permittivity detection systems. We employ a sensor module based on a circular substrate-integrated waveguide with measured unloaded quality factor = 910 to ensure measurement reliability. Miniaturized quarter-mode substrate-integrated waveguide resonators are integrated with four microfluidic channels containing three known materials and one unknown analyte. Step-wise measurement and linearity ensures maximum 4% error for the dielectric constant compared with results obtained using a high-performance commercial product.

This paper proposes a gain-enhanced metamaterial (MM) absorber-loaded monopole antenna that reduces both radar cross-section and back radiation. To demonstrate the proposed idea, we designed a wire monopole antenna and an MM absorber. The MM absorber comprised lumped elements of subwavelength unit cells and achieved 90% absorbance bandwidth from 2.42&ndash, 2.65 GHz. For low-profile configurations, the MM absorber was loaded parallel to and 10 mm from the monopole antenna, corresponding to 0.09 &lambda, 0 at 2.7 GHz. The monopole antenna resonated at 2.7 GHz with a 3.71 dBi peak gain and 2.65 GHz and 6.46 dBi peak gain, before and after loading the MM absorber, respectively. Therefore, including the MM absorber increased peak gain by 2.7 dB and reduced back radiation by 15 dB. The proposed antenna radar cross-section was reduced by 2 dB compared with a monopole antenna with an artificial magnetic conductor.

This study focused on the radome and proposed an absorber with a meta-dome structure for ultra-wideband radar absorption using an FR-4 dielectric material on the metasurface absorber for protection. In addition to protecting the absorber, the metasurface absorber exhibited ultra-wideband frequency absorptivity from radar signals, with an absorptivity band from 4.6–12 GHz, including the C and X frequency bands of radar signals. A wide incidence angle should also be considered in addition to the absorption frequency band. Experimental results were obtained for all polarization angles at normal incidence for 5–14 GHz. Sensitivity to incident angle from 0° to 40° in the transverse electric mode and 0° to 60° in the transverse magnetic mode were observed. The proposed concept was demonstrated using full-wave simulation and experimental measurements.

In this paper, we propose a subwavelength metamaterial unit cell for low-frequency electromagnetic absorber applications. To realize a periodic array for a metamaterial absorber, the footprint size and thickness of a unit cell must be miniaturized to a subwavelength. We achieved the electrical size of the unit cell as 0.027λ × 0.027λ × 0.043λ at 2.4 GHz by introducing the inductive lump elements to a symmetric square-loop resonator. The performance of the proposed absorber was demonstrated by full-wave simulations and measurements. An inductance tolerance of 2% yielded errors of 1.2% and 1.25% in the absorptivity and absorption frequency, respectively. A prototype with 13 × 27 unit cells was fabricated and its absorptivity was measured to be 99.6% at 2.4 GHz.

In this study, we propose a broadband frequency-reconfigurable metamaterial absorber using a novel switchable ground plane (SGP). A double resistive square-ring resonator is introduced for broadband absorption. The distance between the top resonator pattern and the ground plane determines the resonant frequency; the proposed SGP is thus capable of switching the absorption frequency band. The SGP can be either ground or reactive, by switching the PIN diodes on and off, respectively. The SGP is placed as the middle layer, between the top pattern and the bottom ground plane. In the low frequency band, the SGP becomes reactive and the bottom ground plane works as the ground plane of the absorber. In the high frequency band, the SGP works as the ground plane and the bottom ground plane does not affect the absorber. The proposed idea is demonstrated via full-wave simulations and measurements. The absorption of the fabricated sample with 27 × 27 unit cells is measured under normal incidence. When the PIN diodes of the SGP are turned on, an absorption higher than 90% is achieved between 3.5–11 GHz. When the PIN diodes of the SGP are turned off, an absorption higher than 90% is achieved between 1.7–5.2 GHz.

An optically transparent metamaterial absorber that can be obtained using inkjet printing technology is proposed. In order to make the metamaterial absorber optically transparent, an inkjet printer was used to fabricate a thin conductive loop pattern. The loop pattern had a width of 0.2 mm and was located on the top surface of the metamaterial absorber, and polyethylene terephthalate films were used for fabricating the substrate. An optically transparent conductive indium tin oxide film was introduced in the bottom ground plane. Therefore, the proposed metamaterial absorber was optically transparent. The metamaterial absorber was demonstrated by performing a full-wave electromagnetic simulation and measured in free space. In the simulation, the 90% absorption bandwidth ranged from 26.6 to 28.8 GHz, while the measured 90% absorption bandwidth was 26.8&ndash, 28.2 GHz. Therefore, it is successfully demonstrated by electromagnetic simulation and measurement results.

A novel high-efficiency, compact metamaterial-inspired rectifier is proposed for 900-MHz wireless power transmission (WPT) applications. The open-ended composite right/left-handed (CRLH) transmission line (TL) stub, which is introduced to realize metamaterials, replaces the band-pass filter (BPF) of a conventional rectifier (and is smaller than the traditional BPF). The open-ended CRLH TL stub is designed to suppress second- and third-order harmonics and to match the fundamental frequency to a 50-Ω impedance. The device is implemented using lumped elements and a microstrip line. The maximum RF-to-DC conversion efficiency of the proposed rectifier is 69.9% at an input power of 50 mW and a load of 1.8 kΩ. A design procedure that is based on minimizing the phase response of a CRLH TL is presented and validated by simulated and measured results. Because the single CRLH stub is used for harmonic suppression, miniaturization is achieved while retaining efficiency similar to that of previous rectifiers.

Metasurfaces have been studied intensively over the past two decades as they offer the advantages of extraordinary electromagnetic characteristics and highly dispersive responses. Furthermore, electrically or optically reconfigurable metasurfaces can dynamically switch their operation or provide diverse functions via control of their electromagnetic responses. Consequently, mechanical transformations of metasurfaces have paved the way for fabrication of physical sensors. Specifically, in this paper, we propose a stretchable metasurface for wireless strain sensor applications. Because of its dispersive characteristic, the proposed metasurface can reject a specific frequency while transmitting all other frequencies. When the metasurface is stretched, its rejection frequency is shifted. Therefore, the proposed metasurface can wirelessly sense the strain level. The stretchable metasurface is implemented by screen printing a stretchable conductor on a stretchable dielectric material. Further, the frequency response of the fabricated metasurface is measured by loading between the transmitting and receiving antennas. The measured modulus of the fabricated sample is 2.07 MPa for a maximum strain of 15%, and its measured sensitivity is 0.085 × 109 Hz/%.

This paper proposed a hybrid printed electromagnetic pressure sensor using a metamaterial absorber. We used a three-dimensional printed flexible resin for the transformable substrate with inkjet printed conductive patterns stacked on the printed substrate to realize a hybrid printed electromagnetic pressure sensor. A square ring pattern was implemented for the metamaterial absorber unit cell. Since absorption frequency varied with substrate thickness, the device could be used as an electromagnetic pressure sensor with mechanically transformable substrate. Performance was numerically and experimentally measured, and absorption frequency increased from 5.2 to 5.66 GHz by applying 0 and 20 N pressure, respectively; device sensitivity = 7.75 × 108 Hz/mm (0.2 × 108 Hz/N); and repeatability was retained up to 100 cycles.

In this study, we propose a thermal frequency reconfigurable electromagnetic absorber using germanium telluride (GeTe) phase change material. Thermally-induced phase transition of GeTe from an amorphous high-resistive state to a crystalline low-resistive state by heating is used to change the resonant frequency of the absorber. For full-wave simulation, the electromagnetic properties of GeTe at 25 &deg, C and 250 &deg, C are characterized at 10 GHz under normal incidence for electromagnetic waves. The proposed absorber is designed based on the characterized electromagnetic parameters of GeTe. A circular unit cell is designed and GeTe is placed at a gap in the circle to maximize the switching range. The performance of the proposed electromagnetic absorber is numerically and experimentally demonstrated. Measurement results indicate that the absorption frequency changes from 10.23 GHz to 9.6 GHz when the GeTe film is altered from an amorphous state at room temperature to a crystalline state by heating the sample to 250 &deg, C. The absorptivity in these states is determined to be 91% and 92%, respectively.

In this study, the novel electronically switchable broadband metamaterial absorber, using a PIN diode, is proposed. The unit cell of the absorber was designed with a Jerusalem-cross resonator and an additive ring structure, based on the FR-4 dielectric substrate. Chip resistors and PIN diodes were used to provide both a broadband characteristic and a switching capability. To satisfy the polarization insensitivity, the unit cell was designed as a symmetrical structure, including the DC bias network, electronic devices, and conductor patterns. The performance of the proposed absorber was verified using full-wave simulation and measurements. When the PIN diode was in the ON state, the proposed absorber had a 90% absorption bandwidth from 8.45–9.3 GHz. Moreover, when the PIN diode was in the OFF state, the 90% absorption bandwidth was 9.2–10.45 GHz. Therefore, the absorption band was successfully switched between the low-frequency band and the high-frequency band as the PIN diode was switched between the ON and OFF states. Furthermore, the unit cell of the proposed absorber was designed as a symmetrical structure, and its performance showed insensitivity with respect to the polarization angle.

In this paper, a mechanically actuated frequency reconfigurable metamaterial electromagnetic absorber is proposed. The absorber’s metamaterial unit cell is designed to exploit LC resonance from inductive and capacitive coupling. Because this inductance and capacitance determine the absorber’s resonant frequency, we propose a mechanical tuning method that changes the resonant frequency by changing the overall thickness of the metamaterial unit cell. The proposed unit cell consists of an FR4 dielectric substrate with fixed thickness and an air substrate with tunable thickness. When the air substrate thickness is varied over the range of 17 mm to 26 mm, the absorber’s resonant frequency changes from 6.96 GHz to 5.79 GHz in EM simulation. In order to verify the proposed idea, a metamaterial absorber was fabricated as a 17 × 17 array of unit cells and a linear actuator was used to control the thickness of the air substrate. We experimentally demonstrated that the absorption frequency changes from 6.96 GHz to 5.78 GHz with 0.12 (GHz/mm) sensitivity when the air substrate thickness is mechanically changed from 17 mm to 26 mm.