Optically transparent and single‐band metamaterial absorber based on indium‐tin‐oxide

This article proposes and experimentally demonstrates an optically transparent and polarization‐insensitive metamaterial absorber in the terahertz (THz) frequencies. The absorber is formed by indium‐tin‐oxide (ITO) resistive films, providing efficient absorption with absorptivity of 94.1% at the peak absorption frequency of 120.8 GHz. We systematically investigate the surface current distribution and the power loss analysis, and explain the architecture of the absorber. Moreover, the absorber exhibits unique absorption properties at resonant frequencies, that is, featuring single‐band or dual‐band operation by changing the surface resistance of the ITO patterns. In addition, the experimental demonstration and measurement results are in good agreement with the simulated results. Most importantly, the fabricated absorber exhibits an optical transparency above 70% over the entire visible waveband, thereby enabling a wide range of applications such as optically transparent THz absorbers and detectors.     

A wideband switchable absorber/reflector based on active frequency selective surface

A wide‐band absorber and reflector using PIN diodes based on active frequency selective surface (AFSS) is presented, which the AFSS is performed as a wide‐band absorber and reflector with OFF and ON state diodes. By changing the states of the PIN diodes, the measured reflectivity of the structure can dynamically switch from reflection to less than ?10 dB absorptivity ranging from 7.5 to 18 GHz under normal incidence. The unique characteristic of the proposed structure lies in its capability to switch between two working states. In addition, the bandwidth of the designed structure covers a wider band compared with earlier switchable absorber/reflector structures. The fabricated structure shows good agreement with the simulated results.     

Triple‐band polarization‐insensitive metamaterial absorber with low profile

In this article, a triple‐band metamaterial low‐profile absorber with polarization independence is proposed. The proposed metamaterial unit cell is composed of two modified rings with square patch at corners. In addition, the proposed absorber is consists of 10 × 10 periodic unit cells with size of 100 mm × 100 mm. To explain the mechanism, the electric field, the surface current distribution, and equivalent circuit model are present. The structure exhibiting three absorption peaks of 99.01%, 97.18%, and 99.53% under normal incidence at 8.92‐9.11 GHz, 13.78‐14.05 GHz and 14.92‐15.21 GHz which cover X and Ku‐band, respectively. In addition, the proposed structure is insensitive for the transverse magnetic/transverse electric field polarization of incident waves and also the angle of incidence. Furthermore, the three operating bands of the absorber can be adjusted independently and offers low profile which makes the design suitable for different curved surface applications. The proposed structure is fabricated and experiments are carried out to validate the design principle. Good agreements are observed between the measured and the corresponding simulated results.     

Ultrathin dual‐layer triple‐band flexible microwave metamaterial absorber for energy harvesting applications

An ultrathin dual‐layer flexible metamaterial absorber with triple‐band for RF energy harvesting applications has been reported in this article. The sub‐wavelength unit cell of the proposed absorber is composed of six distinct concentric annular having outer circumference of ring and octagonal inner circumference. The metallic resonators are constructed from copper foil self‐adhesive tape which are affixed on flexible neoprene rubber sheet terminated by metal ground plate. The proposed absorber prototype is ultrathin and compact with the thickness less than 0.037λ₀ and cell size less than 0.2λ₀ at the lower absorption frequency of 1.75 GHz. Flexible dual‐layer absorber exhibits triple absorption peaks of 96.91%, 96.41% and 90.12% at 1.75 GHz, 2.17 GHz and 2.6 GHz with full width at half maximum (FWHM) bandwidth of about ~6.5%. The RF performance of proposed absorber is numerically computed for different polarization and incidence angle variations. The absorption value is above 76% for the oblique incidence angle up to 45° in TE mode operation, whereas the absorption value is 94% for oblique incidence angle up to 60° in TM mode operation. The measured outcomes are in agreement with the numerically calculated results. The energy harvesting potential of the proposed absorber structure is numerically confirmed by the resulting improved RF absorption value in dependence to different resistive loading of the polarization insensitive unit cells.     

Compact ultrathin conformal metamaterial dual‐band absorber for curved surfaces

A compact, ultrathin conformal metamaterial dual‐band absorber for curved surfaces has been presented in this article. The absorber unit cell composed of circular and split ring resonators which are connected with plus‐shaped structure. The proposed absorber unit cell is compact in size (0.22λ_o × 0.22λ_o) and as well as ultrathin thickness (0.006λ_o), where λ_o is the wavelength at 5.8 GHz. The designed absorber gives two absorption tips at 5.8 and 7.7 GHz with more than 90% absorptivity. The full width at half maximum bandwidths are 220 MHz (5.67‐5.89 GHz) and 250 MHz (7.58‐7.83 GHz). The proposed conformal absorber is sensitive to the polarization angle and has a stable absorptivity over a wide range of incident electromagnetic wave. The parametric analysis and equivalent transmission line model have been investigated. The surface current and electric field distribution also discussed for understanding the absorption mechanism. To analyze the performance of proposed absorber on the curved surfaces, it is wrapped on the different radius of cylindrical surface and measured the absorptivity. Simulated and measured results have good agreement between them.     

Design of switchable band‐notched frequency selective absorber

An active band‐notched frequency selective absorber (BNFSA) with switchable notch band is proposed in this article. The BNFSA is a two‐layer structure composed of a lossy layer at the top and a ground plane at the bottom, separated by an air spacer. The element of the lossy layer is a lumped‐resistor‐loaded metallic dipole with a parallel LC resonance structure, which is realized by complementary n‐shaped resonator (CnR) inserted in the center, and PIN diode is welded at two arms of CnR. The bias circuit printed on the back of the substrate of the lossy layer connects to anode and cathode of the diode by via hole and isolates by the inductor. Simulation results show that the notch bands are located at 4.50 and 6.81 GHz when the diode sets to ON and OFF, respectively. To validate the performance of switchable BNFSA, the prototypes are fabricated and measured, reasonable agreement between simulated and measured results is obtained.     

A novel dual‐band ring coupler based on dual‐band phase inverter

A novel dual‐band ring coupler based on dual‐band phase inverter is proposed. And two types of dual‐band phase inverters (Type I and Type II) are designed in this article. The design method of dual‐band ring coupler is simpler than the traditional ways like replace the single‐band λ/4 transmission line with dual‐band λ/4 transmission line. Its main idea is replacing the wide‐band phase inverter with dual‐band phase inverter. Two dual‐band ring couplers (0.9/2.88 and 0.9/2.43 GHz) using the two types of dual‐band phase inverter, respectively, are simulated and measured. The measured results validate the proposed method.     

Wide band fractal‐based perfect energy absorber and power harvester

In this article, a novel wide band polarization and incident angle independent metamaterial absorber (MA) and energy harvesting applications which operates at C (4GHz‐8 GHz) and X (8GHz‐12 GHz) is proposed. The unit‐cell of the proposed structure based on fractal circle loop. Four lumped resistors are mounted the structure to obtain a broad band absorption characteristics. Resistors increase the absorption characteristic of proposed MA significantly at mentioned frequency ranges. In addition, under favor of the resistors proposed MA can convert absorbed energy from incident wave to appearing power.     

Volterra series‐based model for concurrent dual‐band power amplifier using dynamic memory depth

In this article, a dynamic dual‐band baseband equivalent Volterra (DDBE) model is proposed to compensate the nonlinear distortions of the concurrent dual‐band power amplifier (PA). The DDBE model is obtained by improving the discretized dual‐band baseband equivalent Volterra model which can describe the output characteristics of concurrent dual‐band PA completely in theory but is lack of practicability. Three simplification rules are proposed in the article, and the relevance between in‐band intermodulation and in‐band cross modulation is employed to simplify the establishment complexity of the model. Then the dynamic kernels are categorized into three groups, and based on this, DDBE models with different level dynamical kernels are derived. In addition, draw on the experience of single‐band PA behavioral model, even‐order kernels are introduced into DDBE models. Digital predistortion performances of a wideband PA, which works in concurrent dual‐band mode, are evaluated to verify the feasibility and advantages of the proposed model.     

Compact dual‐band single‐ended‐to‐balanced power dividers with open/short‐ended stubs

In this article, two novel topologies of compact‐size dual‐band single‐ended‐to‐balanced power dividers that are loaded with open‐ and short‐ended stubs are presented. Quarter‐wavelength open‐ended stubs and half‐wavelength short‐ended stubs are respectively exploited in the proposed dual‐band power‐divider configurations to incorporate the dual‐band functionality into them for flexibly‐adjustable dual‐frequency‐ratio specifications. Each engineered five‐port power‐divider circuit features high in‐band input/output power‐matching levels, high in‐band power‐isolation levels between the two differential‐mode outputs, and high common‐mode‐rejection levels in a broad spectral range. Two microstrip prototypes designed at 0.9/1.8 GHz (GSM bands) and 1.57/2.45 GHz (GPS and WLAN bands) are constructed and characterized for experimental‐demonstration purposes.     

Behavioral modeling for concurrent dual‐band power amplifiers using 2D hammerstein/wiener models

Behavioral modeling for the concurrent dual‐band power amplifier (PA) is a critical problem in practical applications. The nonlinear distortion in the concurrent dual‐band PA is quite different from that in the conventional single‐band PA. This article analyzes the nonlinearities in the concurrent dual‐band PA and reveals that both input signals in the dual bands are important for the behavioral modeling. The 2D Hammerstein model and 2D Wiener model are proposed for the first time for the concurrent dual‐band PA. They are extended versions of conventional Hammerstein and Wiener structures used in the single‐band PA by including the cross‐band intermodulation in the static nonlinearity block. The proposed 2D models require much less coefficients than the original work of the 2D‐DPD model. Experiments were carried out for an 880 MHz/1960 MHz concurrent dual‐band Doherty PA to demonstrate the effectiveness of the proposed models. The results clearly show that less than ?40 dB normalized mean square errors (NMSEs) are obtained in the dual bands in the behavioral modeling. © 2012 Wiley Periodicals, Inc. Int J RF and Microwave CAE 23: 646–654, 2013.     

Dual‐band filtering power divider with unequal power division ratio and low‐loss characteristic

In this article, a dual‐band filtering power divider with unequal power‐division ability is proposed. Different from conventional equal power dividers constructed by filters or coupled resonators, noncoupled structures are employed in this design. As a result, low‐loss characteristic is realized for the proposed power divider. In this proposed structure, the dual‐band unequal power allocation is realized by replacing conventional single‐band λ/4 transformers with dual‐band ones (T‐junction structures). Three identical λ/4 stepped impedance resonators are properly attached to all the three ports of the proposed power divider to generate an extra transmission zero between two operational bands. Therefore, a filter‐like shaping in its S‐parameter results is obtained. A resistor is located between two outputs for output isolation. Mathematical derivations of the overall design procedure are also provided based on the circuit models and transmission line theory. Meanwhile, a resistor for output isolation is also included between two outputs, whose value can be calculated using given equations. For validation, a prototype operating at 0.9 and 2.1 GHz are designed, fabricated, and measured. The isolations between two outputs are 30 and 26 dB while the phase differences are only 2.5°and 4.9° at 0.9 and 2.1 GHz in the measurement, indicating good consistence of outputs. Measured |S21| and |S31| are ?(1.76 + 0.3) dB, ?(4.77 + 0.2) dB at 0.9 GHz and ?(1.76 + 0.6) dB, ?(4.77 + 0.5) dB at 2.1 GHz.     

Analytical method for optimal design of RF dual‐band rat‐race couplers for arbitrary frequency ratios

This article presents an analytical method to design a hybrid structure dual‐band rat‐race coupler at microwave frequencies. The proposed structure uses six identical cells of which each is engineered to work as a quarter wavelength transmission line with proper characteristic impedance at two distinct frequencies having arbitrary frequency ratio. The performances of the π‐ and T‐cells are studied to assess their ability to provide the required electrical parameters for dual‐band operation. It is demonstrated that the single‐section π‐topology can only lead to a suboptimal design for a dual‐band rat‐race cell at two nonharmonic frequencies. In contrast, the proposed double‐section π‐cell structure allows achieving an optimal dual‐band cell design. A dual‐band rat‐race coupler designed at 2.14 and 3.6 GHz has been simulated and fabricated in hybrid microstrip technology. Measurement results agree well with analytically based simulation results, which demonstrate the effectiveness of the proposed structure for dual‐band operation. © 2012 Wiley Periodicals, Inc. Int J RF and Microwave CAE 22: 690–700, 2012.     

Frequency reconfigurable RF circuits using photoconducting switches

Designs for a frequency switchable dual‐band branch‐line coupler and a reconfigurable S‐band power amplifier input matching network with photoconducting switches are presented. Frequency switching is achieved by increasing the power of the laser applied to the highly resistive silicon wafer and changing the properties of silicon under optical illumination. The advantages of this approach are high‐speed switching, electromagnetic transparency (no interference), and thermal and electrical isolation between the device and the control circuit. A branch‐line coupler frequency shift of 35% and 10% has been achieved from all switches off to all switches on in lower (900 MHz) and upper (1800 MHz) frequency bands, respectively. Frequency switchable class AB power amplifier with silicon switch in the input matching circuit has obtained the frequency tuning range of 2.5–3.5 GHz with no significant loss in efficiency and linearity. © 2009 Wiley Periodicals, Inc. Int J RF and Microwave CAE, 2010.     

A compact dual‐band filtering antenna for wireless local area network applications

A printed dual‐band filtering antenna with decent frequency selectivity at 2.45 and 5.2 GHz for wireless local area network (WLAN) applications is developed. The filtering antenna is compact, which comprises a tapped feed line, two dual‐band stub‐loaded open‐loop resonators, and a dual‐band bended monopole. It can be easily printed on a single layer PCB substrate with low profile and low cost. The entire structure is very simple compared with the previously reported dual‐band filtering antennas that requiring multi‐layer structures. The monopole functions as not only a radiator, but also the last resonator of a dual‐band filter. The developed antenna exhibits good frequency selectivity and out‐of‐band suppression. In addition, the two operation bands can be adjusted relatively individually. The proposed antenna is optimized and fabricated. The experimental results show it has good frequency selectivity at both 2.45 and 5.2 GHz, wide bandwidth 11.8% and 7.8%, and excellent out‐of‐band suppression.     

Dual‐band band pass filter using stub‐loaded open‐loop resonators with wide controllable bandwidths

Two dual‐band band pass filters (BPF) using stub‐loaded open‐loop (SLOL) resonator are presented in this article. A novel coupling tuning method by changing the relative coupling position of the resonators is proposed to control the bandwidth of each passband in a wide range. Transmission zeros are created to improve the selectivity by source‐load coupling. Because of the large ratio of two bandwidths, a novel dual‐band matching method is proposed to match the different load impedances at two passband frequencies to the same source impedance. Hence, relax the fabrication requirement of gap. The proposed dual‐band band pass filter is designed and fabricated. The measured 3 dB fractional bandwidths (FBWs) of two 2.45/5.25 GHz dual‐band BPFs are 6.5%/14.5% and 9.8%/5.5%, respectively. The results are in good agreement with the simulation. © 2013 Wiley Periodicals, Inc. Int J RF and Microwave CAE 24:367–374, 2014.     

Dual‐band and enhanced‐isolation MIMO antenna with L‐shaped meta‐rim extended ground stubs for 5G mobile handsets

A novel compact planar dual‐band multiple input multiple output (MIMO) antenna with four radiating elements for 5G mobile communication is proposed. Each radiating element has a planar folded monopole, which is surrounded by L‐shaped meta‐rim extended ground stubs. The compact folded arms act as the main radiating elements, while combined with the L‐shaped meta‐rim stubs, the proposed antenna forms multiple resonances so as to achieve dual‐band coverage. The simulated and measured results show that the proposed antenna has two wide bands of ?6 dB return loss, consisting of 1.6 to 3.6 and 4.1 to 6.1 GHz, respectively. Without any additional isolation structure between the elements, the isolation for the proposed 2 × 2 MIMO antenna in both desired bands can be achieved better than 12 dB. The measured results show that the proposed MIMO antenna with good performance, that is, stable radiation patterns, high efficiencies, low specific absorption ratio (SAR) to human tissues, is suitable for WLAN/LTE, 4G and future 5G mobile phone applications.     

A dual‐band eight‐antenna multi‐input multi‐output array for 5G metal‐framed smartphones

A dual‐band eight‐antenna array operating in the long‐term evolution (LTE) band 41 (2.496‐2.69 GHz) and 3.5‐GHz band (3.3‐3.7 GHz) for fifth‐generation (5G) metal‐framed smartphone is presented. The proposed dual‐band antenna array is composed of four identical dual‐antenna building blocks (DABBs). Each DABB consists of two identical antenna elements with a neutralization line between them. The antenna array is simulated, fabricated, and measured. The isolations are better than 10.5 dB and 11.0 dB in the low band (LB; LTE band 41) and high band (HB; 3.5‐GHz band). The total efficiencies are 41% to 54% and 46% to 64% in the two operation bands, respectively. In addition, the measured envelope correlation coefficients are less than 0.11 and 0.06, the calculated channel capacities are better than 34.5 and 36.3 bps/Hz in the LB and HB, respectively. Furthermore, four hand‐grip scenarios are investigated, and results show that proposed antenna array can maintain excellent multiple‐input multiple‐output performances in all scenarios.     

A dual‐band dual‐mode microstrip Yagi antenna with quasi‐end‐fire radiation

A dual‐band dual‐mode microstrip Yagi antenna with quasi‐end‐fire radiation patterns is proposed in this paper. It consists of five radiating patches driven by a single slot‐loaded patch placed in the middle. Meanwhile, two slot‐loaded parasitic patches are symmetrically located on two sides of the driven patch, respectively. In the lower band, the five patches involved resonate at TM₀₁ mode. While in the upper band, all the patches resonate at TM₀₂ mode. In order to ensure quasi‐end‐fire radiations in the both bands, four slots are symmetrically etched around the strongest surface currents of each patch resonating at TM₀₂ mode. As a result, the resonant frequency of TM₀₂ mode is decreased dramatically, while the resonant frequency of TM₀₁ mode almost remains unchanged. With these arrangements, the separations between any two of the adjacent patches at their centers satisfy the requirements in design of the microstrip Yagi antenna in both bands, so as to realize the dual‐band dual‐mode microstrip Yagi antenna on a single‐layer substrate. Finally, an antenna prototype is fabricated and tested. The measured results reveal that the dual operating bands of 2.76~2.88 and 4.88~5.03 GHz for |S₁₁| < ?10 dB are satisfactorily achieved. Most importantly, the proposed antenna can indeed realize the quasi‐end‐fire radiation patterns in dual operating bands.     

A W‐band broadband power divider/combiner using two parallel antisymmetric tapered probes

In this article, a broadband microstrip‐to‐waveguide transition with antisymmetric tapered probe as well as a W‐band power divider/combiner using dual proposed antisymmetric tapered probes is presented. Because of tapered microstrip shapes and metallic steps, the proposed transition is proved to be broadband, efficient, and compact. The insertion loss of the transition sample is less than 0.56 dB between 75 GHz and 100 GHz. Under the assistance of the gradually changed waveguide and dual parallel tapered probes, the operating band of the power divider/combiner has been significantly improved, which is adequate to work in the whole W‐band. A back‐to‐back prototype of the divider/combiner is fabricated and measured. The measured insertion loss of the single divider/combiner is less than 0.29 dB between 90 GHz and 100 GHz, and agrees well with the simulations. Because the circuit size is smaller than 8.0 mm × 2.2 mm (Thanks to the excellent performance and compact size), the proposed design can find wide applications in miniaturized MCM/MMIC systems.