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In this paper, a frequency selective surface (FSS) based shared-aperture antenna is designed for 5G/Wi-Fi applications. The 5G and Wi-Fi channel are separated to achieve different polarizations and simultaneous work. The proposed antenna is of ±45°-polarization in the 5G N79 band and vertical polarization in the Wi-Fi5.8 GHz band. The antenna is vertically stacked with the N79 band antenna located above the Wi-Fi band antenna. The N79 antenna is composed of FSS units with transmissive characteristics in the Wi-Fi band, and serves as an electromagnetically transparent surface to avoid blocking the Wi-Fi antenna. A prototype of our design is fabricated, assembled and tested, and measured results show that the prototype is able to cover the entire N79 band (4.1–5.2 GHz, 23.7%) and Wi-Fi5.8 GHz band (5.73–5.89 GHz, 2.8%). Measured average gain is 8.2 and 7.8 dBi in the N79 and Wi-Fi band, respectively, and radiation efficiency is over 86 and 80%. The proposed design exhibits separated channels, tri-polarizations, high gain and compact size, which is sufficient for regular 5G/Wi-Fi applications. The antenna also achieves a relatively wide and highly consistent signal coverage in the two bands, making it suitable for 5G/Wi-Fi multi-function communication and indoor 5G blind compensation.

As reconfigurable intelligent surface (RIS) is expected to be implemented with a large aperture in terms of wavelengths, using a RIS to perform near-field rather than conventional far-field communications might gain some benefits. The electric fields in the near-field region from the near-field focusing and conventional far-field RISs are simulated and compared to assess and validate the benefits. Two metrics of benefit distance and near-field gain are defined to characterize the electric fields in the near-field region. The cumulative distribution function (CDF) related to the near-field gain is calculated to describe the benefits in the area of interest where the total scan patterns from the near-field focusing and conventional far-field RISs are plotted. For demonstration, the benefit distance, near-field gain, and CDFs are presented and calculated for near-field focusing RISs with assigned focal distances of 80 and 300 mm. It is concluded that the benefit distance is a critical factor affecting the CDF directly. The effects of the size of a RIS and phase quantization of elements on the benefit distance, near-field gain, and CDF are also discussed. Moreover, the maximum focal distance of a near-field focusing RIS with a certain size is derived and specified. Finally, the electric fields of three prototypes (e.g., conventional far-field RIS, near-field focusing RISs with assigned focal distances of 500 and 1500 mm) at 26 GHz are measured and compared, where the benefit distances and near-field gains are experimentally observed.

A new efficient synthesis method of angular selectivesurfaces (ASSs) based on the accurate equivalent circuit modelis proposed. The angular dispersion of different componentsunder oblique incidence is modeled by virtual lumped elementsin the angular domain. The processes of obtaining the equivalentcircuit for substrates, electrically small grids and patch arraysare presented in detail. The synthesized circuits are simple andaccurate. Full-wave simulations are carried out to obtain theangular dispersion of different structures, and the calculatedresults are in good agreement with those of the equivalent circuit.A third-order ASS with bandpass response and a second-orderASS with a tunable bandwidth are given as design examples. Theimplemented second-order ASS shows the 0.33 to 0.52 tunablebandwidth in the angular domain. The transmission loss is betterthan 1.2 dB when the incidence angle is 65 to 77 degrees. Thesimulation results and calculated results are in good agreementwith the measured results.

A low-profile wideband tri-polarized MIMO antenna is proposed in this paper. Based on characteristic mode analysis of a circular patch, four slots are etched symmetrically on the patch to transform the high-order monopole-like modes into broadside modes with lower resonant frequencies. Four sets of shorting pins are also applied to simultaneously adjust the resonant frequencies of the monopole-like modes and broadside fundamental modes of the patch. The effects of the slots and the shorting pins on these modes are analyzed in detail. Three dual-resonance ports with high isolation are achieved by exciting three pairs of orthogonal modes of the patch. The proposed antenna is wideband with respect to its profile (bandwidth/height = 216), as facilitated by the dual-resonance property of each port. Experimental results show that the proposed antenna features high port isolation (> 35 dB) and high gain (> 5.58 dB). The proposed low-profile wideband tri-port antenna can be applied in highly integrated MIMO systems.

In this letter, a broadband, wide-scanning phased array with radar cross section (RCS) reduction performance is presented. The proposed phased array is composed of two types of antenna elements: Ant-0 and Ant-1. Ant-0 and Ant-1 are y-polarized dipole antennas that have almost the same radiation performance over the frequency range of 6.5-12 GHz. Ant-1 is obtained by co-designing Ant-0 with a polarization selective metamaterial surface (PSMS), and the PSMS has little effect on the y-polarized radiation performance. As of x-polarized incident waves, the phase difference of the reflected waves on Ant-0 and Ant-1 is around 180 over the frequency range of 3.74-15.1 GHz, thus, a broadband scattering cancellation is achieved. A 516 finite array is fabricated and measured, and the measured results show great agreement with the simulated results. The fabricated phased array antenna has the ability to scan to 60 with active VSWR < 3 over the frequency range of 6.5-12 GHz, and over the frequency range of 3.9-15.1 GHz, ultrabroadband monostatic RCS reduction is achieved.

In this letter, the reflection elements based on artificial dielectric layers (ADLs) are proposed for broadband wide-angle bistatic radar cross section (RCS) reduction. ADLs are introduced to improve the interference cancellation bandwidth of the reflection elements, and avoid the dilemma of mixing multiple dielectric mediums. The reflection elements achieve an approximate 180° phase difference over the frequency range of 4.8–17.5 GHz (113.9%) under normal incidence, and the bandwidth under TE 40° incidence and TM 55° is 113.9% and 100%, respectively. To validate this design, a coding scatterer surface is simulated and fabricated. Measured results show that the proposed coding scatterer surface achieves a 10 dB bistatic RCS reduction from 5 to 17.5 GHz under normal incidence for both polarizations. Bistatic RCS reduction under oblique incidence is analyzed and presented as well.

In this article, we present a new microwave system with the tunable ability of the heating region and heating effect. The heating region is changed by choosing different antennas above different heating region, which leads to larger heating range. And this system can be utilized in precise temperature control in ammonia synthesis phase. The frequency-space domain modulation technology is ingeniously suggested in our heating system. The different antennas above different heating region is then selected by changing the input working frequency. Based on the frequency-space domain modulation, we further add time domain modulation to increase the tunability of heating effect at different heating regions. By adjusting the frequency distribution of the input microwave source in time domain, where we named as time-frequency-space domain synthesis modulation method, the tunable heating region and effect is finally achieved. A simple microwave system is fabricated, and different heating effects are measured. The proposed heating method and system has benefits as following, simple structure, low cost, uniform heating, and controllable heating effect, which results in great improvement in microwave heating.

This letter presents a low-cost third-harmonic mixer suitable for W-band retrodirective systems. Measured results show that when fixing the local oscillator (LO) frequency at 35 GHz, the conversion loss (CL) below 11 dB is achieved over the radio frequency (RF) range of 100-110 GHz with intermediate frequency (IF) from dc to 5 GHz, while for the RF range of 108 GHz, the minimum CL of 6.8 dB is obtained. Therefore, the presented mixer exhibits an excellent performance with low CL in comparison with that of a normal subharmonic mixer (SHM). The isolations of RF/LO and RF/IF are improved to the level of 35 and 38 dB, respectively. The simulated and measured isolations of LO/RF are 41 and 37 dB, respectively. This mixer has properties of low LO frequencies, high isolations, and low CL; therefore, it provides a high-performance and low-cost alternative solution for the applications of retrodirective systems.

This article develops a broadband absorber by fully using the radiation features of monopole antennas for ultrawide-angle absorption application. A unit cell (UC) based on monopole antennas is proposed, and its working polarization is analyzed with the antenna reciprocity method and image theory. A wide-angle impedance matching layer is also introduced to further enhance the absorption at large angles. The optimized monopole-based UC reveals 90% absorptivity from 5.5-14.76 GHz at the incident angle of 80°. To the best of our knowledge, this is the first time to report an absorber that can work perfectly under such ultrawide angles in a broadband manner. An 11 × 36 array is configured and simulated to verify the effectiveness of the monopole-based UC. The monostatic and bistatic scattering are both reduced significantly as well as other diffuse scattering directions from the simulated results. A prototype of 110 mm × 360 mm is fabricated and measured. Time gating methods are adopted to diminish the direct coupling effect. The measurements are in good agreement with the full-wave simulation results.

A high-gain transmit–reflect-array (TRA) antenna with asymmetric bidirectional beams using sparse-array method is presented in this communication. The proposed element consists of three-layer hexagonal-ring-patch (HRP) structure, able to achieve both amplitude (0 or 1) and phase modulation. Based on this kind of element structure, a 125 mm $\times \,\, 125$ mm square-shaped TRA is designed and fabricated. A 45.3% of the radiation units operate in transmission mode while others in reflection mode. Spatially fed by a conical horn antenna, two well-defined beams are obtained on both sides of the TRA, pointing to the direction of $\theta = 0^{\circ }$ and 170° ( $\varphi =0^{\circ }$ ), respectively. The measured results show a maximum gain of 21.4 dBi with a 1 dB gain bandwidth of 6.7% for the transmitted beam, and a maximum gain of 24.4 dBi with a 1 dB gain bandwidth of 9.3% for the reflected beam. To the authors’ best knowledge, it is the first time that the sparse-array method is employed in spatially fed antenna design. The proposed method can conveniently manipulate the pointing directions and beampattern characteristics of the bidirectional beams, providing a promising candidate for bidirectional wireless communication applications.

A millimeter-wave planar resonator sensor for the detection of deformation and crack in metal device with high precision and sensitivity is presented in this article. The deformation and crack directly imply great damage in the industrial metal structures, which made the detection crucial in structural health monitoring. The main structure of the proposed sensor is based on the concept of multiband electromagnetic induced transparency (EIT) effect. EIT effect is an interference quantum phenomenon, which can be realized in microwave band by electromagnetic resonate structures. Multiband EIT is actualized using a series of split-ring resonators (SRR) coupling with each other to produce one wide stopband and four narrow passbands. The resonance frequencies of the SRRs are affected by the variation of coupling amount between the SRRs and the part of metal device under test that places close to the specific SRR. The orientation of metal deformation and crack is achieved by different frequency shifts of the four independent group delay peaks induced by multiband EIT. Thus, the proposed sensor can orientate the deformation on the metal device with the precision of 1.3 MHz/μm.

This communication proposes a dual-wideband antenna that is composed of a shared structure with self-diplexing capability. The antenna structure is a combination of a tapered slot antenna (TSA) operating in the range of $S$ - to $X$ -bands and a horn antenna array that operates in $Ka$ -band. The structure-shared antenna has high design freedom for each band with controllable properties for a dual-wideband application. The back-lobe level is greatly reduced at both bands due to the implementation of shared structure. The isolation between two ports is enhanced without additional decoupling structures that develops the self-diplexing characteristic of the antenna. Moreover, the electric field distributions of the proposed antenna are analyzed through simulation, which proves its potential in high-power microwave (HPM) application.

This letter presents a planar, low-cost microwave ablation antenna operating at 2.45 GHz with the minimal cross-sectional area. The proposed antenna can produce local spherical ablation zones by double dipole structure, which is fed through substrate-integrated coaxial line (SICL) without a balun. The input impedance of the double dipole structure, which is in fact extensions of the outer and inner conductors of the SICL, is very low. SICL is mismatched with the double dipole structure when designed as the smallest cross-sectional area. Impedance matching is achieved through a microstrip line cascaded with SICL. A prototype of the antenna is fabricated and the measured results show good agreement with the simulations. An ablation experiment in the porcine liver tissue confirms the capability of the antenna with the minimal cross-sectional area in generating localized ablation zones. The proposed antenna can be manufactured through mature and cheap printed circuit board technology, serving as a promising commercial candidate for minimally invasive ablation.

A probe‐fed dipole antenna with parasitic patches is proposed in this study as the element for wideband and wide‐scanning planar‐phased arrays. Parasitic patches are used to realize wideband performance by significantly reducing the active input impendence. The printed dipole antenna with parasitic patches is directly matched with the subminiature version A (SMA) connector through the modified probe structure. The cell size is 0.5 × 0.5 × 0.145 λhigh3. The planar and low‐profile characteristics make it suitable for modular phased array applications. It is shown that the infinite array simulation results achieve 45.2% bandwidth (7.86–12.45 GHz) with active voltage standing wave ratio (VSWR) < 2 at broadside, 60° scans in E‐, D‐planes and 50° scan in H‐plane. An 8 × 8 prototype array has been manufactured and measured to validate the proposed design and the active element patterns agree well with simulation results.

In this letter, a novel method is proposed to design a Ka-band series-fed antenna that is applied to generate the accelerating and self-bending beam. The leaky-wave feature of the series-fed array is also investigated by the simulation of the unit patch. Fundamental relationship between width and length of the resonant patch is acquired based on the five-unit model. The fabricated prototype is composed of 40 different-size units that control the aperture magnitude. To adjust the phase difference between the two patches, the length of the transmission line is adjusted. Two symmetrically designed series-fed arrays reduce the cross-polarization level. The shape of the beam varies with the frequency from 33 to 36 GHz, and | S 11| is below −15 dB in this operating band, which also illustrates the property of frequency scanning. This model can be easily fabricated, and it suggests the realization of the self-bending beam in the microwave band. Meanwhile, the fabricated Ka -band antenna can be easily fed, and it has the advantages of low profile and low cost.

In this communication, a novel planar structure-shared diverse two-port ultrawideband (UWB) antenna with high isolation and common ground is proposed based on a new slot-loaded method. A tapered slot with an end-fire radiation pattern is etched in the top middle section of a coplanar waveguide (CPW)-fed printed monopole antenna to reduce its effects on the monopole mode. The two-port UWB antenna features diverse patterns which can fully cover the upper space. A nonuniformly symmetric-coupled coplanar waveguide (NSC-CPW) is introduced to enable the two sets of the feeding structures sharing a ground plane with minimum interaction. Due to the modal orthogonality, high isolation between the two ports is obtained without additional decoupling structures. The experimental results of the proposed multiple-input multiple-output (MIMO) antenna indicate that the isolation is higher than 31.4 dB and the diversity gain is approximately 10 dB within a wide frequency range from 2.98 to 10 GHz. Meanwhile, high realized peak gain (>2.5 dB) and good efficiency (>80%) are also obtained for both the ports.

A dual-band structure-shared antenna with large frequency ratio is proposed in this letter by integrating two substrate integrated waveguide slot antennas (SIWAs) on the branches of dipole. In the proposed structure, dipole is fed by coaxial connector while SIWAs are fed by two microstrip lines. To improve the stability of the dipole and the isolation among three antennas, microstrip high-pass filter is introduced. Measured results show that the proposed antenna works at 27.3−28.8 GHz with directional radiation pattern and 3.02−5.03 GHz with omnidirectional radiation pattern.

A broadband wide-scanning planar phased array is studied in this letter. An accurate equivalent circuit model is used and studied to provide more insights into the working principle of the proposed antenna element. Surface wave suppression is achieved by introducing an air gap, which also improves the active voltage standing wave ratio (VSWR) in H -plane as a wide-angle matching layer. The infinite array cell in simulation achieves nearly 65.2% bandwidth (6.3–12.4 GHz) with active VSWR E - and H -plane. Scanning patterns are measured and show good agreements with simulated results.

A broadband short-length lens antenna operating in W band is presented in this study. The lens is composed of a dielectric host material with numerous variable square holes drilling on it, which is fabricated using the three-dimensional printing method with high accuracy and low cost. Then, the lens is mounted directly on the end of a length-shortened horn antenna, obtaining a compact lens antenna. Phase variations of the dielectric unit-cell under different oblique incidences are taken into consideration while constructing the lens. Simulation results reveal that gain enhancement of about 1 dB is achieved compared with the designs using the conventional method. Effects of lens height on antenna radiation performances are also studied and the lens profile is reduced to half with little performances deterioration. For the total lens antenna, a maximum gain of 23.2 dBi is measured at 94 GHz, which reveals 7.4 dB gain enhancement compared with the feed source. The measured 1 and 3 dB gain bandwidths are 13.8 and 37.8%, respectively. The proposed lens antenna shows a profile reduction of 42.3% with respect to the optimum horn while exhibiting comparable gain performance within a broad frequency band.

A dual-band filter of a large frequency ratio is proposed, studied, and developed in this article, which is based on mode composite coplanar waveguide (MCCPW). Both low and high bands of the dual-band filter can be designed independently with a high degree of design freedom. The filter can achieve more than ten times the frequency difference between the two bands, which is usually hard to accomplish by conventional dual-band filter design techniques. All harmonics between the two bands can be suppressed by a cascaded low-pass filter and impedance matching structure to form a qualified dual-band frequency response. The characteristic of the proposed MCCPW dual-band filter makes it a promising candidate for multiband systems, especially in 5G communication systems. The demonstrated MCCPW dual-band filter is fabricated with a single-layer substrate. Measured results are in good agreement with simulated results, which validate the feasibility and superiority of the proposed filtering technique.

In this article, a class of mode composite transmission line called mode composite sandwich slot line (MCSSL) is proposed, studied, and demonstrated. The MCSSL is composed of two substrate integrated waveguides (SIWs) and a sandwiched quasi-slot line. Structural parts of the two SIWs inherently serve as the two conductors of the quasi-slot line. The SIW supports the TE10 fundamental mode propagation, whereas the quasi-TEM mode is supported by the quasi-slot line. Two types of transition structures are proposed in this article, which enable an easier integration of the studied MCSSL with other transmission lines. An MCSSL-based antenna is designed and fabricated to theoretically and experimentally demonstrate its advantages and feasibility. It has been shown that the MCSSL features the physical and electrical characteristics of a joint structure (or structure sharing) in connection with a mode composite with planar structures, and it can find applications in broadband and multiband circuits and antenna developments.

We propose a self-quadruplexing four-element multiple-input-multiple-output (MIMO) antenna system with frequency agility. Four separately-fed orthogonal U-slots are cut from the same cavity to locate the radiating patches. The frequency tuning is realized by inserting four varactors into the radiating structures. Metalized vias are used for antenna miniaturization and to reduce the coupling between the radiating elements. Also, the antenna exploits the slits realized in its ground plane to further increase the isolation. Truly, the isolation between the elements is enhanced from 8 to 16 dB when they are tuned concurrently. The antenna system has the added merit of tuning each element separately to excite four different operating frequencies exhibiting the self-quadruplexing function. Measurements reveal that the frequency bands can be tuned together or individually from 4.67 to 5.84 GHz without affecting each other. The measured peak gain and efficiency are 3.73 dBi and 65%, respectively. The proposed MIMO antenna is a promising candidate for communication systems as it is printed on one substrate layer, has a miniaturized size, exhibits good diversity performance with a maximum envelope correlation coefficient of 0.11, and the four elements can be reconfigured separately without the additional quadruplexer circuit.

A simple method to achieve multi-focus and pattern manipulation in antenna near field (NF) is proposed in this study, whose effectiveness is then validated by measurements based on different transmit-array antennas. The radiating aperture is divided into several regions and each region corresponds to one assigned focal point, thus achieving multi-focusing performance. Then, the superposition principle is employed and constrains the multiple foci into a certain area to get a contoured pattern. Three examples are investigated. Ant-A presents four balanced focal spots simultaneously, while Ant-B presents two unbalanced foci. A square flat-top field pattern is obtained for Ant-C. Three-dimensional printing technology is employed to fabricate these lenses, resulting in a low cost and light weight. Measured results coincide well with the numerical design and simulation. The proposed method provides a simple and effective alternative for NF power redistribution and owns great value in such applications as radio-frequency identification, wireless power transmission, millimetre-wave imaging etc.

Future communications will provide higher transmission rates and higher operating frequencies. In addition, agile beam tracking will be an inevitable trend in technology development. The terahertz retrodirective antenna array proposed and discussed in this paper can be a better solution for agile beam tracking. The array receives a 40-GHz navigation signal and accurately retransmits a 120-GHz beam in the direction of the arrival wave. Simulation results indicate that the proposed array with a stacked sandwich structure has realized the tracking of the received wave. The scanning radiation pattern shows that the array gain is 23.87 dB at 19.9° when the incident angle is 20° with a relative error of only 0.5%, meaning that there is a lateral error of only 8.7 m at a transmission distance of 5 km.

A multilayer ultrawideband circuit analog absorber is proposed in this letter. The absorber comprises three layers: two absorbing layers and a metal ground layer. With the whole profile unchanged, the effects of the air spaces between absorbing layers on bandwidth enhancement are mainly addressed. The equivalent circuit is employed to analyze the appearance of reflection maximum poles, which limit the absorption at high frequencies. Based on the optimal equivalent circuit, two single-polarization perforated absorbing layers consisting of interdigital capacitors, meander lines, and lumped resistors are designed to achieve an ultrawide absorption band. For demonstration, a multilayer absorber is manufactured to validate the approach, where a bandwidth ratio of 1: 12.88 with a total profile of $0.101\lambda _{L}$ at the lowest operating frequency is achieved under normal incidence. The simulated results are experimentally verified by the measured results.

This communication develops a triple-band shared-Aperture antenna operating at X/K/Ka-band with high figures of merit such as high aperture reuse efficiency and simple configuration to achieve high gain. The proposed antenna is implemented by integrating a Fabry-Perot (FP) cavity antenna operating at X-band into a dual-band reflectarray antenna enabled by frequency selective surfaces (FSSs). The unit cells (UCs) to form the phase-shifting surfaces for the dual-band reflectarray antenna can offer partial reflectance/transmission abilities at X-band simultaneously, thereby enabling the implementation of an FP cavity antenna. The frequency responses of the proposed antenna are highly independent at X/K/Ka-band, which facilitates the design and optimization. For demonstration, a prototype has been described, fabricated, and measured. The simulated results present 3 dB gain bandwidths of 11.2% (8.4-9.4 GHz), 10.0% (24-26.6 GHz), and 13.3% (35-40 GHz), which are all experimentally verified. The measured results also present a peak gain of 16.8 dBi at 9.0 GHz, 23.8 dBi at 26 GHz, and 26.7 dBi at 38 GHz.

In this letter, we present a systematic approach to design a compact dual-mode monopole-like patch antenna using characteristic mode analysis (CMA). The modal analysis of a slotted circular patch structure incorporating a new shorting pin loading technique is presented. To achieve a compact monopole-like antenna with the wideband operation, it is demonstrated that the first two significant modes with monopole-like patterns are the most suitable ones for dual-mode excitation. Based on the analysis of the modal currents and electric fields, four groups of shorting pins and four slots are introduced to individually tune the two modes, which facilitates the optimization. The effects of these slots and shorting pins on the resonant frequencies of the two modes are analyzed in detail. Finally, a CPW T-junction power divider is applied to simultaneously excite these two modes and suppress the undesired modes. Apart from a more compact form factor and higher gain than existing work, it also features a competitive gain-bandwidth per volume ratio.

A novel coaperture antenna based on the concept of mode composite transmission line and named mode composite antenna (MCA) is proposed in this communication. The MCA combines the structure of a monopole antenna and a substrate integrated waveguide (SIW) slot antenna to form a new coaperture antenna. Because of the structure sharing characteristic in MCA, it can reduce the size of the antenna. The MCA serves as the monopole antenna supporting the surface current on the antenna patch in its low-band operation. In its high-band operation, the SIW will support TE10 to excite the slot array. The two-mode operation has high isolation which makes the MCA a promising candidate for multiband application. The measured and simulated results agreed well and validated the feasibility of the MCA.

A two-port structure-shared planar ultrawideband (UWB) antenna with high isolation is presented in this communication. The proposed antenna consists of a planar monopole and two back-to-back tapered slots etched on the monopole. The monopole and the tapered slots are fed by the microstrip line and the differential feeding network, respectively. The differential feeding network of the tapered slots excites the modes, which are orthogonal to those of the monopole. Therefore, high isolation between the two ports can be achieved without additional decoupling structures. The measured results show that the proposed antenna can operate from 3 to 10 GHz with the reflection coefficient of the two ports lower than −10 dB and the isolation higher than 38 dB. The measured realized peak gains are above 2.6 dB, and the measured envelope correlation coefficients are less than $9 \times 10^{-5}$ . The proposed UWB multiple-input-multiple-output (MIMO) antenna can be applied in the MIMO communication systems, especially those requiring high isolation.

This letter presents multiband absorbers with wide absorption bandwidths based on an impedance matching theory. A wideband absorber with a good absorption performance is served as a matching load for multiband bandpass frequency selective surface (FSS) filters. As a result, the reflection coefficients of the proposed absorbers are in high accordance with the counterparts of the original bandpass FSS filters. For demonstration, a triple-band absorber with wide absorption bandwidths is designed and fabricated by cascading a triple-band bandpass FSS filter and a wideband absorber together. The simulated results reveal that the proposed absorber has wide fractional bandwidths of 5.2%, 8.0%, and 6.4%, which is experimentally validated by the measured results.

This paper presents a low-profile coplanar dual-polarized dual-band base station antenna array. A slotted patch antenna operating from 0.82 to 0.99 GHz with four shorting strips is proposed and analyzed. The four shorting strips are employed not only to improve the impedance matching in the lower band but also to remove the center part of the patch, thus making room for higher band dipoles operating from 1.68 to 2.86 GHz. The height of the proposed array is nearly half of the conventional base station array while other performances are guaranteed. The measured results show that the proposed antenna array has wide impedance bandwidths of 18.7% (0.82–0.99 GHz) and 57.9% (1.68–2.86 GHz) (VSWR $0.109\lambda _{0}$ at 820 MHz). The measured gains are 13.8±0.5 dB and 16.7 ± 1 dB in lower and upper bands. Large front-to-back ratio (>24.6 dB) and high isolations (>30 and >25 dB in lower and upper bands, respectively) are achieved. The proposed antenna can be easily built with printed circuit board fabrication technique and applied in low profile multiband base stations for 2G/3G/4G and the next-generation systems.

A high-sensitive microwave sensor based on the variation of group delay is presented in this paper. The sensor is designed by mimicking the concept of electromagnetically induced transparency (EIT). The EIT-like effect that applied in the microwave circuit can provide a narrow transmission window in the stop band, which also brings ultraslow group velocity. Combined with the utilization of active resonator, the quality factor and group delay of the sensor are significantly enhanced. The active resonator is actualized by appending an active feedback loop that couples with the split-ring resonator of the EIT-like structure. The group delay of the active EIT-like sensor is particularly sensitive to the tiny phase variation of the active feedback loop, which is adopted to the high-performance sensing functions. The active EIT-like sensor with double sensing applications in this paper was fabricated and tested. The sensitivity of the proposed sensor is 0.012 pF/ns and 0.03 mV/ns, respectively.

A low-profile transmitarray (TA) antenna consisted of three different types of frequency-selective surface (FSS) unit cells and a “short” feed horn is proposed. The FSS unit cells, which have different magnitude and phase responses, are properly arranged on one layer of a substrate to enhance the phase-shift range. The proposed TA with a thickness of 0.127 mm ( $0.042~\lambda _{{0}}$ at 100 GHz) is placed near the feed aperture to achieve a low-profile TA antenna. The full TA prototype is fabricated and measured. The maximum gain measured at 100 GHz is 23.8 dBi, which has an 8 dB improvement compared to the feed without increasing the radiation aperture size. A relative bandwidth of 15% is obtained within 3 dB gain variations. The cross-polarization level is −43.4 and −47.9 dB for E- and H-planes, respectively. A maximum gain of 24.5 dBi with an aperture efficiency of 46.1% is achieved at 97.7 GHz. This method considerably reduces the complexity of TA and achieves a much more compact structure.

We propose a simple and compact design of a patch antenna with the frequency-reconfigurability function. Six p-i-n diodes are positioned symmetrically along the nonradiating edges to allow the selection of one frequency band among 36 different states. One patch edge is shorted to the ground plane to reduce the antenna footprint. The antenna measured operating frequency ranges from 2.35 to 3.43 GHz. The measured peak gain and the measured peak efficiency are 4.3 dBi and 73%, respectively. The antenna has a compact size, preserves stable and unidirectional radiation patterns, uses a simple dc bias circuit, and is printed on a single-layer substrate. Hence, it is suitable for wireless applications requiring miniaturized size and acceptable performances.

In this study, wideband bandpass power divider with good out-of-band performance is proposed. Two bandpass filters (BPFs) are utilized to substitute the quarter-wavelength transmission line in conventional Wilkinson power divider. A resistor is specially arranged between two BPFs for a good isolation. Four transmission zeros (TZs) are found to be distributed in the lower and upper stopband of the power divider. Moreover, the locations of two TZs can be shifted by tuning the impedance ratio of the center-loaded open stub, which is propitious to improve the frequency selectivity. Even- and odd-mode methods are applied to analyze the proposed power divider and closed-form design formulas are obtained. Finally, two prototype power dividers with measured rejection level in the upper stopband larger than 29.1 and 32 dB till to 2.7f0 and 2.69f0, respectively, are designed and fabricated to testify the proposed design concept. Good agreement between the simulated and measured results is observed, validating the validity of the proposed design principle.

The authors propose the design of a compact mechanically frequency and pattern reconfigurable patch antenna. The frequency tuning is achieved by changing the positions of two adjustable shorting screws which are used to modify the length of the antenna radiating patch. Simulated and measured results are carried out to validate the proposed approach. When the screws are symmetrically positioned, the measured operating frequency can be tuned from 2.95 to 5.16 GHz without degrading the radiation characteristics over the entire comparatively wide measured tuning range of 54.5%. The antenna main radiating beam can be tilted from θ = 0° to θ = 30 ° when the screws are asymmetrically positioned. The measured gains and efficiencies range from 3.01 to 5.53 dBi and 68 to 74%, respectively. The overall antenna size is only 0.39 λ 0 × 0.39 λ 0 × 0.02 λ 0 .

A low radar cross section (RCS) antenna designed with band-notched absorber is described. First, a dual-polarization absorber with relative bandwidth more than 80% is designed using loaded resistors. Pairs of circular slot resonators and metal strip array resonators are introduced in the absorber to realize a notch band with full reflectance in the vertical polarization, while a wide absorption band in the horizontal polarization is maintained. The proposed band-notched absorber is thus realized. Within the notch band, the absorber can be served as a metal ground for antenna; while a great RCS reduction is obtained out of the notch band and in the horizontal polarization band. Then, a dipole antenna rigorously designed is mounted above the band-notched absorber, whose operating frequency is exactly in accordance with that of the notch band. The proposed low RCS antenna is established based on assembling the dipole antenna and the band-notched absorber together. The measured results demonstrate that the proposed antenna has fairly good radiation patterns. Added to that, more than 10 dB RCS reduction in two polarizations is realized simultaneously compared with that one of a conventional dipole antenna.

We propose a miniaturized design of a four-element dual-band multiple-input–multiple-output (MIMO) antenna system. The dual-band function is obtained by etching an “L” slot in the radiating patch. The edge-to-edge elements spacing is only 0.023 λ0 . A decoupling structure consisting of cross-shaped metal strips shorted to the ground plane using vias is first introduced. Then, metalized walls are used to enhance the isolation level by realizing the pattern diversity. The measured resonant frequencies are centered at 3.5 and 5.7 GHz with bandwidths of 58 and 43 MHz, peak gains of 2.7 and 2.85 dBi, and peak efficiencies of 63% and 65%, respectively. A considerable measured mutual coupling reduction of 10.8 dB in the lower band and 15.6 dB in the upper band is realized. The overall size of the design is 0.41λ0 × 0.41λ0 × 0.04λ0 with minimum isolation levels of 18.4 and 22.7 dB in the lower and upper bands, respectively. The envelope correlation coefficient is lower than 0.08 across both operating bandwidths. The measured results validate the good diversity performance of the MIMO system.

A low radar cross section (RCS) and low profile antenna co-designed with absorbent frequency selective radome (AFSR) is investigated. A pair of circular slot resonators is embedded on surface of the AFSR to realize a transmission window in the vertical polarization, while the wide absorption band is still maintained in the horizontal polarization. When a patch antenna is etched within the AFSR, where the metal grounds of the patch antenna and AFSR are co-used, the co-designed antenna with low RCS and low profile is thus realized. For demonstration, an AFSR is designed with its transmission window has a minimal insertion loss of 0.45 dB at 8.9 GHz, and two separate absorption bands (a lower absorption band from 4.8 to 7.5 GHz and an upper absorption band from 10 to 13 GHz) in the vertical polarization, a wide absorption band (from 4.5 to 12.5 GHz) in the horizontal polarization. A patch antenna etched within the AFSR is optimized to operate at 8.9 GHz, then it is simulated and fabricated. The measured results demonstrate that the proposed antenna not only has good radiation patterns, but also obtains significant RCS reduction.

A compact dual-band patch antenna with independent tuning of both frequencies is introduced. The frequency agility is achieved by inserting two varactor diodes into the radiating patch, while the antenna miniaturisation is realised by shorting one patch edge using metallised vias. Simulated and measured results are in good agreement. Measurements reveal that the two operating bands can be tuned separately without any frequency shifting and with high stability and similar unidirectional radiation patterns during the tuning process. The measured tunable ranges of the lower and upper bands are 27.95% (from 2.4 to 3.18 GHz) and 28.27% (from 4.1 to 5.45 GHz), with measured peak gains and efficiencies of [3.23 dBi, 65%] and [3.42 dBi, 62%], respectively. The antenna presents comparatively wide tuning ranges with similar unidirectional radiation patterns, has a miniaturised size, and uses a simple direct current bias circuit. Thus, it can be easily integrated and used in different wireless systems.

In this research, bandpass filters (BPFs) consisting of short-ended quarter-wavelength (λ/4) resonators with wide-stopband and compact size are presented. Two feed lines with loaded open stubs are coupled to the λ/4 resonators. Since the spurious passbands near even times of the fundamental frequency f 0 have been naturally suppressed by the short-ended λ/4 transmission line, only the harmonics around odd times of f 0 are remained. Transmission zeros are generated and controlled by the electrical lengths of coupled lines and loaded open stubs. They can be shifted to the locations of (2n + 1)f 0 by properly allocating the electrical length of each part. Hence, in theory all the harmonics are suppressed and broad stopband is achieved. For verification, two BPFs with different orders are designed. Measured results of the second-order BPF centred at 1.97 GHz indicate that the rejection level in the upper stopband, up to 7.1f 0 and 9.09f 0, is better than 41.8 and 27.1 dB, respectively. Simulated results of the third-order BPF reveal that the rejection level of the upper stopband up to 17f 0 is better than 32 dB. Excellent results demonstrate that the proposed BPFs are good candidates for wireless communication systems.

In this paper, a band-notched absorber with high notch-band-edge selectivity is proposed and investigated. It is formed by introducing a narrower and full reflectance band into a wider absorption band. For each absorbing unit cell, two independently magnetic and electric resonances with adjacent resonant frequencies are employed to develop the narrower and full reflectance band, while a dipole-shaped metal strip with a resister printed on one side of the supporting substrate and a ground plane is used to realize the wider absorption band. By retrieving relative equivalent circuits of each cell, the performance of the proposed absorber can be qualitatively analyzed. Simply through tuning the frequencies of the independently magnetic and electric resonances, the performance of the notched band can be adjusted with high notch-band-edge selectivity. A sample of an absorber with the notched band frequency from 8.2 to 9.8 GHz, good absorption band from 4.8 to 8.0 GHz and 10.2 to 16 GHz has been designed, fabricated, and measured. Good agreement among circuit analysis, simulation results, and measurement results is finally obtained.

We propose a simple and compact design of a tunable self-diplexing patch antenna. Two U-shapes are etched on the same platform to locate the radiating patches which are excited separately using two feed ports. Metalized vias acting like metal walls are applied to reduce the antenna footprint, and to avoid coupling effects when the frequency ratio ( fr ) approaches 2. The tuning process is achieved by using two varactors properly inserted into the radiating structure. The operating frequencies can be tuned independently with high isolation level. Simulation and measurement results are quite consistent. Measured results show that the lower frequency band ranges from 3.9 to 4.37 GHz with measured peak gain and efficiency of 3.82 dBi–65%, while the upper band ranges from 4.63 to 5.34 GHz with peak gain of 4.48 dBi and peak efficiency of 64%. The self-diplexing antenna achieves an isolation level better than 22 and 32 dB for the lowest fr of 1.05, and the highest fr of 1.37, respectively. The radiation pattern is unidirectional and extremely stable during the frequency tuning of both bands.

In this paper, a wideband bandpass filter using multi-mode resonator and mixed electromagnetic (EM) coupling is presented. To increase the bandwidth while not enlarge the circuit size, multi-short-stub multi-mode resonator, which produces multiple resonances, is utilized. Furthermore, the capacitive gap between open ends of the stubs is utilized to realize electric coupling between the multi-mode resonator and quarter-wavelength stepped impedance resonator. Meanwhile, a high-impedance transmission line between them is used to implement magnetic coupling. This mixed EM coupling can produce transmission zero near the passband, consequently improving the frequency selectivity of proposed filter. Measured results of the filter prototype are in good agreement with the simulated ones. The filter is with an insertion loss at central frequency of 15.86 GHz about 2.5 dB, a 3 dB bandwidth about 5.72 GHz (the fractional bandwidth about 36%), and the variation of group delay over the whole passband 2, where λg is the wavelength of 15.86 GHz. All these results have shown that proposed filter is promising for future high-precision imaging system or high-speed communication application.