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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 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.

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.

International audience; Reconfigurable intelligent surface (RIS), one of the key enablers for the sixth-generation (6G) mobile communication networks, is considered by designers to smartly reconfigure the wireless propagation environment in a controllable and programmable manner. Specifically, an RIS consists of a large number of low-cost and passive reflective elements (REs) without radio frequency chains. The system gain of RIS wireless systems can be achieved by adjusting the phase shifts and amplitudes of the REs so that the desired signals can be added constructively at the receiver. However, an RIS typically has limited signal processing capability and cannot perform active transmitting/receiving in general, which leads to new challenges in the physical layer design of RIS wireless systems. In this paper, we provide an overview of the RIS-aided wireless systems, including the reflection principle, channel estimation, and system design. In particular, two types of emerging RIS systems are considered: RIS-aided wireless communications (RAWC) and RIS-based information transmission (RBIT), where the RIS plays the role of the reflector and the transmitter, respectively. We also envision the potential applications of RIS in 6G networks.

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.

In this paper, a mode composite coplanar waveguide (MCCPW) is proposed and studied. The MCCPW can be regarded as a combination of a quasi-coplanar waveguide and a substrate integrated waveguide (SIW). In this composite structure, the center conductor of the quasi-coplanar waveguide serves as the SIW supporting TE10 mode, while the TEM mode operates in the quasi-coplanar waveguide part. The characteristics and design method of the MCCPW are analyzed and illustrated in this work. The MCCPW is fabricated together with two transition structures to demonstrate its integration capability further with other planar circuits. The MCCPW is a single layer structure, and it has the advantages of joint structure and mode composition, which make it a possible solution for multifunction and multiband application systems. The measured and the simulated results agree well with each other, and good transition performances are achieved.

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 microwave split-ring resonator (SRR) is presented for leaf moisture detection in this paper. The proposed sensor is based on a split-ring resonator with a single port. The sensor operates at a resonant frequency of 2.8-3.4GHz in order to detect varying degrees of moisture. The dielectric characteristic of the leaf is detected by shifting resonant peaks. Relationship between the resonance characteristics and the moisture content of the leaf is established by experimental data. A change in leaf moisture content from 0% to 45% corresponds to a decrease in resonant frequency from 3.4 to 3.0GHz. This work presents a miniaturized, environmentally friendly, and non-destructive microwave device and provides a practical method of leaf moisture sensing.

In this paper, one method to synthesize the large depth-of nondiffracting beam is proposed by applying the ring focus reflectarray antenna (RA) which operates at 35 GHz. The synthesis method and operation principle of this RA is specified to design the required phase distribution of the aperture. The phase of the single unit can be adjusted by controlling the patch size of the single element. This system can be utilized in many applications like near-field (NF) Wireless Power Transmission (WPT), NF detection and microwave imagine. The presented antenna owns the merits of low profile, easy to fabricate and low cost.

This paper presents a high-isolation, low crosspolarization, differential-feed, dual-polarized microstrip patch antenna array. By using differential feeding and coupled structure, the array realizes high port isolation, which is suited for the full duplex operating mode with two orthogonal linear polarizations of transmitting and receiving, respectively. Simulations results show isolation is greater than 40 dB at 2–5.2 GHz and greater than 60 dB at 2.42–2.47 GHz. The crosspolarization level is less than –40 dB on both the E and H planes. The proposed array has a simple structure to fabricate and practical for use in the retrodirective system.

This paper describes the design of a helical dipole antenna for microwave ablation (MWA) based on substrate integrated coaxial line (SICL) technology. The dipole arm extending from the outer conductor is nine quarters of the wavelength, and it rotates around the other dipole arm extending from the inner conductor, the length of the dipole arm extending from the inner conductor is one quarter of the wavelength. The center frequency of this antenna is 2.45 GHz, and the shape of localized heating patterns is like a spherical which is the shape of many tumors. Measurement results showed good agreement with the simulations.

the mode composite ridged waveguide is proposed in this paper, it is a composition of substrate integrated waveguide and substrate integrated ridged waveguide with the mode composite and structure-shared characteristic. A Ka-band dual-polarized leaky wave antenna is designed based on the mode composite ridged waveguide, it utilizes the SIW TE10 mode and SIRW TE20 mode to achieve different polarizations within a compact size, the cross-polarization level will be reduced by using this two modes combination.

In this paper, a probe-fed stacked patch antenna as dual-band wide-angle phased array element is designed. Antenna element and 1×11 finite array are studied in simulation, respectively. The proposed antenna element operates at 5.44 GHz and 6.65 GHz, and the 3dB-beamwidth of the element in finite array is wider than 160° in E-plane at both frequencies. The proposed array enables the ability to scan up to 70° at both frequencies. The proposed phased array antenna shows the advantages of dual-band, wide-scanning angle and good polarization purity.

An improved 1-4 power divider is presented in this paper, operating from 210 to 225GHz. The simulated results of the model from the software Ansoft HFSS show that it can achieve the intended goal of phase shift at the required frequency with isolation more than 20 dB. The power divider is constructed by three couplers rather than the traditional T-juncture to improve the isolation between the output ports. The matching-loads in isolation ports of the couplers are replaced with E-plane square horn antennas. Meanwhile, the lengths of the connecting waveguides in the power divider can be adjusted to achieve an arbitrary phase shift.

In this paper, a novel compact diplexer with high isolation based on the mode composite ridged waveguide is pro-posed and designed. The mode composite ridged waveguide is a composition of substrate integrated waveguide and substrate integrated ridged waveguide with the mode composite and structure-shared characteristic. This diplexer has several advantages in terms of high isolation, compact size, and easy integration. It operates at 20 GHz and 28 GHz, and the simulation results show that the insertion loss is less than 2dB at both channels. The isolation is greater than -55dB.

This letter presents a compact dual-band shared-aperture linear antenna array with a large frequency ratio for broadband antenna system applications. The tapered slot antenna (TSA) and dipoles are chosen and combined to achieve shared aperture. And a 12-element antenna array is designed to obtain dual-broadband properties. Base on VSWR < 2 criteria, broad bandwidth of 123.5% (1.3 – 5.5 GHz) and 20.5% (30.2 – 37.1 GHz) are obtained for TSA and dipoles respectively. The dual-band antenna has a high design freedom for each band, and a large frequency ratio can be achieved easily with a compact size.

A wideband dual-polarization antenna with high isolation for the phased array applications has been proposed in this article. In order to meet the wideband and high isolation requirement, the array element uses dipole pairs with narrow-slot structure. The dual-polarization characteristic is achieved by exciting two orthogonal dipole patches. The working band of the antenna array is from 8 to 11 GHz with a relative bandwidth of 40%. The simulated results show that the active VSWR is less than 1.5 and 2.5 in the band 8 ~ 12 GHz while the range of scan angle is 0° and ±45°. The port isolation is below -30dB and the cross-polarization is below -30 dB across the whole band.

This letter presents a terahertz (THz) orthomode transducer (OMT) based on a simple structure and the equivalent circuits of these two modes are given. As one equivalent inverter is introduced, the bandwidth of one mode is broadened. The measured results of a back-to-back setup shows a relative good performance in the frequency of 210 GHz to 220 GHz. For both polarization, the reflection coefficients are all below -10 dB, and the cross polarization transmission coefficients are below -30 dB. At the center frequency of 216 GHz, the insertion loss is less than -3 dB and the isolation is below -55 dB.

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.

A compact, microstip patch antenna is proposed in this paper to sense and identify water content. The proposed antenna sensor is excited through a 3.2-mm-wide microstrip line. The microstrip antenna is designed to resonate at 5.8 GHz. Subsequently, parametric analysis of the proposed antenna sensor is carried out to show the true behaviour of the system. The proposed patch antenna acts as a sensor due to the fact that a sample under test has different dielectric permittivity depending on the water content and, hence, will be having different resonance frequencies. The designed sensor has been experimentally validated with four different levels of water (Empty, 25%, 50% and 75%).

A broadband Bessel lens antenna with stable quasi-nondiffraction range and considerable broadband characteristics is designed and fabricated. The theoretical basis and operating principles for the broadband near-field quasi-nondiffraction antenna are introduced. Then, design procedure for the structure of the Bessel lens is discussed in detail. Considering the influence of oblique incidence waves, phase shifts of the lens elements with different incident angles are simulated, and interpolation analysis method is applied for the lens modeling to minimize phase error. The proposed lens antenna can be used as a Bessel beam launcher operating at 26–32 GHz. The lens structure is fabricated using 3D-printing and fed by a dual-mode conical horn. Measured results agree well with the simulation.

Demonstration of group delay modulation based on electromagnetically induced transparency (EIT) is presented by using LC circuits. The EIT effect is realized by trapping one ‘dark’ mode with high Q-factor into one ‘bright’ mode with low Q-factor. The ‘bright’ mode can be realized by using an LC circuit, which consists of a capacitor and an inductor, and the ‘dark’ mode can be realized by coupling with another LC circuit. The circuit model is simulated, and three pass bands are obtained. Based on this circuit model, the group delay modulation is realized by changing the value of the parameters. The proposed circuit model has the advantage of simple structure and practical for use, which makes it easy to design the novel group-delay sensor.

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.

In this paper, a novel design of an E-plane waveguide filter, called electromagnetically induced transparency (EMIT) filter, is proposed. This EMIT filter is implemented by a rectangular ring (RR) on one side of the substrate and two strip lines resonators (SLRs) on the opposite side, which brings in electrically induced transparency and magnetically induced transparency by the electrical and magnetic couplings between the RR and SLRs, respectively. An equivalent circuit is developed further. Properties of high selectivity, low insertion loss, flat group delay, and small size of only about 0.2 λg are realized by this design method. A sample has been fabricated and tested. The measured central frequency is 33.77 GHz with 3-dB fractional bandwidth of 9.4% and the minimum insertion loss in the passband is 0.5 dB. Using the proposed miniature EMIT filter, a diplexer is further designed with compact sizes and low insertion losses. The measured minimum insertion losses are 0.9 and 0.5 dB in the lower and upper passbands centered at 31.79 and 34.74 GHz with 3-dB relative bandwidths of 7.2% and 8.6%, respectively. Good agreements between the simulated and measured results are obtained both in the EMIT filter and the EMIT diplexer.

This communication introduces an approach for designing miniaturized single-feed multiband patch antennas. The size reduction is obtained by loading shorting metalized vias on one edge of the radiating patch, while multiband is obtained by etching inverted multiple U-shapes. A dual-band antenna with a large measured frequency ratio of 2.74 is first discussed. Then triple-band, and further quad-band antennas are, respectively, designed. The antennas have sufficient electrical small size when considering the radius of the minimum enclosing sphere to the radiating patch. A radius value of 0.09λ 0 is obtained for the quad-band antenna with respect to the lowest operating frequency centered at 3.04 GHz. Simulated and measured results of the antennas' reflection coefficient and radiation patterns are provided. Good agreement between the simulation results and measurement results is achieved. The antennas' measured peak gains and efficiencies vary from 1.43 to 3.06 dBi and 42% to 74%, respectively. Compared with other designs, the proposed antennas exhibit multiband performance by using a simple single-feed structure, realize different combinations of highly isolated frequency bands with wide tunability of frequency ratios, and have directional stable radiation patterns that are compact and of electrically small size. Indeed, they have the potential to meet the practical requirements for wireless applications.

A highly integrated tunable triple-band patch antenna is designed and fabricated. Two U-slots are cut in the same radiating patch to achieve triple-band performance. While the antenna miniaturization is obtained by loading one patch edge with metalized vias, theory of characteristic modes is adopted to explain the U-slots scaling. The antenna radiating patch size is only 0.1λ 0 × 0.17λ 0 . The operating frequencies can be independently tuned after properly inserting three varactor diodes into the radiating structure. The radiation patterns have similar unidirectional shapes with high stability of the resonant frequencies during the entire tuning of each band. Good agreement between simulation and measurement results is obtained. The antenna covers the following bands: 2.07-2.6 GHz, 2.86-3.57 GHz, and 4.59-5.39 GHz. The measured peak gains and efficiencies are 2.72 dBi and 62.6%, 2.76 dBi and 63.3%, and 2.87 dBi and 66.8%, respectively. Size reduction, multiband function, and independent tuning of three operating bands using only one antenna layer and one RF feedline are combined together to realize a compact and practical antenna.

In this paper, a 2.45-GHz wideband harmonic rejection rectenna for wireless power transfer is proposed. The rectenna comprises a microstrip-fed circular ring slot antenna (CRSA) and a series-parallel rectifier (SPR). A compact micro strip resonant cell is inserted into the CRSA so that the harmonic suppression over a wide bandwidth (3–8 GHz) can be obtained. The radio-frequency (RF)–DC conversion efficiency of the SPR is improved effectively by loading a proper compensating inductance, especially under the low input power levels. Furthermore, the proposed rectenna can easily achieve large-scale rectenna arrays using its simple structure. The adopted rectenna fabricated on a low cost Taconic RF-35 substrate has been measured. By up to 3rd-order harmonic rejection, the efficiency of the rectenna can achieve 70.2% with the optimum load resistance 1 kΩ. Good agreement among the calculated, simulated, and measured rectenna is observed.