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Series feeds are commonly used in antenna arrays because of their compact size and low loss compared to corporate feeds but typically feature beam squint over wide bandwidths. This work proposes a series-fed waveguide horn array with reduced beam squint by introducing synthesized time delay sections between the horns and the feed line. The array, implemented in a WR42 waveguide, achieves 1.53° beam squint over a 4 GHz operating band, compared to 10.52° for a reference design without time delay compensation. [ABSTRACT FROM AUTHOR]

This paper presents a novel 3-D-printed circularly polarized (CP) open-ended waveguide (OEW) antenna array. It aims to provide a lightweight scheme for OEW array design and concurrently address the high fabrication complexity problem of conventional OEW arrays. To realize these, the proposed design adopts dielectric 3-D printing technology to build the main body of the array and employs electroplating to form the waveguide walls. To attain CP radiation, an in-built antipodal exponential groove polarizer is proposed and applied to a square waveguide. With the perturbation of the grooves, degenerate modes T E 10 and T E 01 can be excited simultaneously, and their differential phase can be 90 degrees, resulting in CP radiation. Moreover, a novel matching cube based on effective medium theory (EMT) is proposed for the antenna to mitigate the reflections at the waveguide aperture and therefore realize good impedance matching. To validate the design idea, prototypes for a single element and a 2 × 2 array were fabricated and measured. Experimental results show that their weights are 15 and 76 grams, and their axial ratio bandwidths (ARBWs) are 16% (5.45-6.4 GHz) and 11.1% (5.52-6.17 GHz), respectively. [ABSTRACT FROM AUTHOR]

In this paper, a convenient process for the fabrication of flexible liquid metal mesh films (LMMF) is proposed first. Then, the light transmittance and square resistance characteristics of LMMF are studied theoretically and experimentally. The light transmittance of the LMMF can reach 85% when the line width and spacing are 50 μm and 1000 μm, respectively. Furthermore, as an example of LMMF, a coplanar waveguide loop antenna is designed and fabricated that contains an LMMF with a line width of 50 μm and a line spacing of 500 μm. The measured square resistance and transmittance for the LMMF are 0.0456 Ω/sq and 72%, respectively. The measured peak gain of the antenna is 3.38 dBi while the average efficiency is 61%. The antenna's working frequency covers most of the S-band, C-band, and X-band, as well as multiple channels of fifth generation (5G) communication. Therefore, the antenna can be used in fields such as radar and mobile communication. Uniquely, the fabricated antenna performs well in terms of light transmission, conductivity, and flexibility. In particular, it remains stable in stretching and bending deformation. As a highly light-transmissive stretchable flexible antenna, the antenna is equipped with various functions such as concealability, conformability, and reconfigurability. This LMMF-based antenna has good prospects for applications in the fields of flexible electronics and transparent electronics. [ABSTRACT FROM AUTHOR]

This paper proposes two flexible ultra‐wideband (UWB) multiple‐input multiple‐output (MIMO) slot antennas with connected ground and high isolation. Two MIMO antennas consist of two parallel slot antenna elements with coplanar waveguide feeds and ground. The antennas are etched on the liquid crystal polymer substrate and have the consistent sizes of 30 × 56 × 0.1 mm3. The first UWB‐MIMO antenna can cover 3–13 GHz. By adding a U‐shaped slot on the first antenna, a second UWB‐MIMO antenna with a notch‐band is designed, which can work in 3.2–13 GHz and filter the wireless fidelity frequency band (5–6 GHz). High isolation better than 20 dB is obtained by adding fence‐shaped branches. In addition, the performance of the flexible MIMO antenna is analyzed, including diversity performance parameters such as envelope correlation coefficient, diversity gain, gain, and total active reflection coefficient. The proposed UWB‐MIMO antenna can be applied in internet of things and wireless body area network frequency bands. [ABSTRACT FROM AUTHOR]

A circularly polarized pyramidal horn antenna based on substrate integrated waveguide (SIW) technology is proposed and developed for Ku‐band applications. The proposed horn antenna is excited by the upper and lower SICL differential feed networks placed vertically to each other, so that two orthogonal waveguide modes with the equal amplitude are generated at the horn aperture. A 90° coupling network based on SICL technology is proposed to achieve a 90° phase difference between the two orthogonal waveguide modes, thus realizing the function of left‐hand circular polarization (LHCP) and right‐hand circular polarization (RHCP) of the horn antenna. In addition, in order to make the phase distribution of the horn aperture more uniform, a cross metal strip structure at the horn aperture is proposed, thereby improving the radiation efficiency. The proposed horn antenna is manufactured by using low‐cost printed circuit boards (PCBs) process. The measured results show that the proposed circularly polarized pyramidal horn operates at center frequency 12.5 GHz, the measured gain is 8.4 dBi, the relative standing wave and axial ratio (AR) bandwidth are approximately 24% and 5%, respectively. And the radiation aperture efficiency is as high as 78.5%. The developed circularly polarized pyramidal horn antenna can be applied in the field of satellite communications. [ABSTRACT FROM AUTHOR]

In this article, a transmissive metasurface has been designed using five different types of unit cells with linearly varying phase for 5G communication system operating at 28 GHz. The phase gradient profile is responsible for alteration of transmission phase of electromagnetic waves at each unit cell, which causes tilting of the radiated beam when vertically placed above a microstrip patch antenna. Passive beam‐steering from −30 to +30° is achieved with the change in orientation of the metasurface which reverses the sequence of unit cells. This novel technique is an efficient means of beam‐steering, which does not require the employment of active elements or bulky multilayered structures. The proposed antenna operating at the same frequency as that of the phase shifting surface exhibits 2 dB gain enhancement when integrated with it. A prototype has been fabricated and measured as a proof of concept. The measured results are consistent with the full–wave simulation results. [ABSTRACT FROM AUTHOR]

In this article, a millimeter‐wave frequency‐reconfigurable metasurface antenna by electrically controlling vanadium dioxide (VO2) is proposed and studied. As a phase change material, VO2 is an insulator at room temperature but can turn into high conductivity metallic state when activated by continuous direct current (DC) voltage. By integrating the VO2 film between two simple rectangle metasurface cells, a frequency‐reconfigurable paired metasurface structure (FRPMS) is achieved. The FRPMS can be considered as one unit when VO2 film is activated to metallic state while as two independent cells when VO2 film is in insulating state. In this way, the FRPMS can operate, respectively at 28 and 38 GHz with independent tunability. Moreover, a coplanar and staggered DC bias circuit with little effect on radiation is designed to switch the operating modes of VO2 easily. The antenna is fed by a stepped waveguide with ridges through a loop slot, which is suitable for dual‐band feeding with large frequency ratio. One prototype is fabricated and measured for demonstration. When the VO2 film is in metallic state, the measured working band covers from 28.5 to 29.6 GHz with the maximum gain of about 7.1 dBi. When cooled down to insulating state, the antenna can work in a higher band from 36.7 to 38.3 GHz with the maximum gain of about 7.9 dBi. Due to the advantages of simple structure, high gain and easy tuning, the antenna can be expected to use in mmW wireless communication. [ABSTRACT FROM AUTHOR]

An efficient technique to realize dual‐band multi‐polarized shared‐aperture slot antenna array is proposed in this paper, featuring in all‐metal, low‐profile, light‐weight and high radiation efficiency. The cavity‐backed longitudinal slots are used at L‐band for vertical polarization, while horizontal and vertical polarizations are both achieved at C‐band, by utilizing interleaved ridged waveguides with two different kinds of slots. A coaxial‐feed end‐grounded metallic bridge is developed to excite the L‐band cavity‐backed slots, so the volume and weight of the L‐band cavity can be obviously reduced. To further make the whole antenna structure compact and tight, the reduced L‐band cavity is embedded into the serration, surrounding by two adjacent C‐band ridged waveguides for vertical polarization and one for horizontal polarization. An array prototype, which consists of two L‐band single‐polarized antennas and four pairs of C‐band dual‐polarized antennas, is fabricated and measured. Experimental results are well consistent with the simulated ones, showing a satisfying impedance bandwidth of 12% in L‐band and 5.5% in C‐band, and besides high radiation efficiency above 85% for both bands. The all‐metallic structure of this proposed array provides the thermal and mechanical advantages for space‐borne synthetic aperture radar (SAR) application. [ABSTRACT FROM AUTHOR]

A coplanar waveguide (CPW) fed polarization reconfigurable antenna for WLAN 2.4 GHz operation is studied. To achieve reconfiguration between left‐hand circular polarization (LHCP), right‐hand circular polarization (RHCP), and linear polarization (LP) with broad CP and LP operating bandwidths, the technique of switching only two PIN diodes embedded between the CPW feeding line and an open square‐ring radiating element is introduced. As the radio frequency (RF) and direct current (DC) signals must be fed from the same feeding line (input port), during the experiment, an additional Bias‐Tee element is introduced to the input port for combining the two signals. [ABSTRACT FROM AUTHOR]

A leaky wave antenna (LWA) based on half‐mode substrate integrated waveguide (HMSIW) is presented. Integrated phase inverters are used to eliminate the far‐field cancelation from the adjacent radiating element with half‐wave spacing. The ground plane is extended to reflect the quasi‐magnetic‐dipole radiation pattern from the backfire to the boresight for enhanced antenna gain. Two linear arrays with differential feeding are deployed back‐to‐back along their longitude, by which the far‐field radiation adds constructively that leads to further increased antenna gain. The proposed antenna features shorter electric length compared with counterparts of the same gain. Agreement is achieved between simulation and measurement. The proposed antenna has a measured impedance bandwidth of 11–12.8 GHz (15.6%), the measured maximum gain of 16.2 dBi @ 11.6 GHz. The antenna's beam scans from 20° to 50° in the boresight with the input signal's the frequency. By using the phase inverter, the extended ground plane and the back‐to‐back deployment, the proposed antenna produces equal gain with shorter antenna's electric length. [ABSTRACT FROM AUTHOR]

This article presents a polarization‐insensitive 1‐bit coding metasurface (CM) for dual‐band orbital angular momentum (OAM) beam generation. The metasurface unit cell is composed of two symmetric metal layers, which are separated by a dielectric substrate. Each metal layer consists of an outer square ring, a middle square ring, and an inner rhombus patch. By rotating the direction of the inner rhombus patch to 0° and 90°, two states with out‐of‐phase transmitting responses for both bands are achieved. An equivalent circuit is proposed to analyze the design. The OAM beams are then realized by properly distributing the two states on the metasurface aperture. A metasurface with 30 × 30 elements is designed, simulated, and experimentally demonstrated. The experimental results agree well with the simulating ones, showing that 3‐dB transmission bandwidths of 13.6–14.8 GHz and 26.7–27.7 GHz are realized in both Ku and Ka bands, with the mode purities 0.89 and 0.73, respectively. [ABSTRACT FROM AUTHOR]

In this article, double‐layered spoof surface plasmon waveguide (DLSSPW) which consists of double‐layered metal gratings is used as a main transmission line for realizing a broadband bandpass filter with a center frequency of 1.5GHz and a relative bandwidth of 80%. Four equally spaced short‐circuited double‐stripped lines are used as resonators and mounted on the main transmission line. Efficient conversion from the quasi‐transverse electromagnetic (TEM) waves in the coplanar waveguide to spoof surface plasmon polaritons (SSPPs) in the DLSSPW is achieved by using gradient double‐layered metal gratings and sector‐structure impedance matching circuit. This filter has lower insertion loss than using single‐layered spoof surface plasmon polaritons, as the DLSSPW has tighter field confinement and lower radiation loss. Moreover, due to the slow‐wave effect of DLSSPW, the main‐part length of the filter is shorter than microstrip‐form counterpart by 30%. The measured S‐parameters of the plasmonic bandpass filter agree well with the simulated ones, validating the good features. [ABSTRACT FROM AUTHOR]

In this article, the design and development of an artificial magnetic conductor (AMC)‐backed compact wideband rectangular ring monopole antenna with high gain is proposed for millimeter‐wave (mmWave) applications. It is constructed through stacking a coplanar waveguide (CPW)‐fed rectangular ring patch monopole antenna over a square modified AMC reflector surface of 5 × 5 periodic unit cells. Primarily, the ring monopole antenna of size 1.21λ0 × 1.21λ0 × 0.023λ0 (at 28 GHz) is designed and exhibits wide impedance bandwidth of 30.64% (25.98–35.38 GHz, 9.4 GHz), a peak gain of 4.86 dBi and front‐to‐back ratio (FBR) up to 5.2 dB. To enhance the radiation performance of the monopole antenna, a periodic lattice of 5 × 5 unit cells modified square AMC surface with a gap of 0.84λ0 is proposed and it functioned as reflector which enhances the peak gain of about ~77% and it contributes FBR up to 15 dB. Measured results of the modified square AMC‐backed monopole antenna reveal a wide impedance bandwidth of 31.27% (25.90–35.50 GHz, 9.6 GHz), a peak gain of 8.59 dBi, and stable radiation characteristics. [ABSTRACT FROM AUTHOR]

In this article, we propose the designs of planar pattern manipulation surfaces (PPMSs) applying patch resonating elements loading with dual‐polarized slots for multi‐user communication systems in future networks. Firstly, the amplitude and phase of the element reflection coefficient are discussed with resorting to the varied length of the slot. Secondly, the dual‐polarized slot‐loaded patch is consequently utilized for obtaining the proposed PPMS. By suitably selecting the reflection coefficient distribution of the entire PPMS, the reflected wave can be manipulated to gain different dual‐beam reflection properties with the response of dual‐polarization. In the final, two prototypes with 10 × 10 elements are fabricated and tested to prove the presented design concept. Measurements agree well with calculated results, indicating that dual‐beam reflections with the controllable beam pointing ability are well achieved so as to provide effective coverage for multiple users simultaneously. [ABSTRACT FROM AUTHOR]

A circularly polarized Yagi array with a compact structure, small size, and symmetric pattern is reported in this paper. It consists of four 3‐segment counter‐wound helices, one driven element, one director, and two reflectors, for a moderate gain and front‐to‐back ratio. Similar to conventional Yagi with linear polarization, all helical elements have different but close resonant frequencies. The driven element has a resonant frequency higher than that of the reflectors but lower than that of the director. Owing to the uniform pattern of the counter‐wound helical element, a very symmetric pattern with circular polarization at the endfire direction is achieved. A prototype of the 4‐element array is fabricated and tested. Measured results show that an impedance bandwidth with |S11| < −10 dB from 1.345 to 1.42 GHz (5.42%) and a 3‐dB AR band from 1.34 to 1.43 GHz (6.5%) are obtained, showing an overlapped bandwidth of 5.42%. A realized gain up to 7.03 dBi and an offset angle within 3° are also realized. The overall size of the array is as small as 0.66λ0 × 0.107λ0 × 0.40λ0, showing possibility of further being formed to large scanning or multi‐beam arrays. [ABSTRACT FROM AUTHOR]

In this letter a novel notched multi‐input‐multi‐output (MIMO) flexible slot antenna with a C‐slot is proposed and designed for wireless body area network (WBAN) and Internet of Things (IoT) applications. The proposed antenna is comprised of four similar elements with the feed line of coplanar waveguide (CPW). Antenna is printed on flexible liquid crystal polymer (LCP) substrate with compact dimensions. The proposed UWB‐MIMO slot antenna can operate in range of 2.85–10.85 GHz with notched band from 4.9–6.425 GHz. The achieved notched band can filter the interference of WLAN (5.15–5.825 GHz), by adding an inverted orthogonal branch, great isolation is achieved, which is better than 24 dB over working bands. What's more, the parameters of diversity performance like gain, envelope correlation coefficient (ECC) and diversity gain (DG) are also analyzed. The actual measurement of bending test and the effect of human body indicates that the presented antenna is suitable for WBAN and IoT applications. [ABSTRACT FROM AUTHOR]

Slot antenna based on microstrip line and waveguide is a popular design for future 5G antennas. To improve performance of slot antenna, a high‐gain wideband slot antenna based on integrated substrate gap waveguide (ISGW) is presented, in which a stepped impedance feeding line is adopted to broaden bandwidth, and ISGW is used to increase gain. First, the transmission characteristics and the effective relative permittivity of ISGW is discussed and calculated, with operation band from 25 to 45 GHz. The antenna has a passband of 24.9–38.1 GHz, that is, almost Ka band, and an average gain of ~9 dB. Further, bandstop filtering (BSF) characteristic is introduced in the above wideband antenna to achieve filtering antennas for 5G millimeter‐wave (mm‐wave) bands of 24.75–27.5 GHz for China, and 26.5–29.5, 27.5–28.5, and 37–40 GHz for n257, n258, and n260 of 5G new‐radio (NR). The BSF is obtained by etching U‐shaped CSRR on feeding patch of the antenna. First‐ and second‐order BSF are achieved by etching first and second U‐shaped CSRR on the antenna feeding patch, respectively, thus correspondingly one bandstop resonance at 34.6 GHz, and two bandstop resonances at both 32.5 and 34 GHz are obtained. It is found that the passband performance is unchanged, but the stopband radiation drops sharply at the boresight direction. The three antennas (wideband, and first‐ and second‐order BSF antennas) proposed are all fabricated and verified. [ABSTRACT FROM AUTHOR]

Aiming at the problems of large size and small gain of log periodic dipole antenna (LPDA) in high frequency (HF), this article introduces miniaturization technology to design a miniaturized HF broadband LPDA unit antenna with high gain. In this article, a dovetail structure is designed for LPDA to greatly reduces its transverse size, and adopts a larger scale factor and a smaller spacing factor to reduce its longitudinal size. The effects of scale factor, spacing factor, oscillator angle, parallel oscillator and erection height on the impedance characteristics and radiation performance of dovetail LPDA are studied. Finally, the feasibility of dovetail LPDA is verified by simulation and experiment, which shows that the proposed antenna size is only 7.92 × 15.52 m2, voltage standing wave ratio is less than 1.8, and gain is more than 11 dBi. The proposed dovetail LPDA has high gain, wide bandwidth, and small size, which can work independently and can also be used as the unit antenna of HF broadband antenna array. [ABSTRACT FROM AUTHOR]

In this paper, an ultra‐wideband low scattering metamaterial based on the combination of water absorber and polarization rotation metasurface (PRM) is proposed. The water absorber and PRM realize low backscattering in higher and lower frequency bands, respectively. The mutual influence between the water absorber and PRM is insignificant due to their different design strategies of absorbing and diffusion respectively. Therefore, a continuous low‐scattering bandwidth is obtained by designing the water absorber and PRM operating in two adjacent frequency bands. The simulated results show that the radar cross section (RCS) of the proposed metamaterial can be reduced by more than 10 dB in the ultra‐wideband band of 2.8–80 GHz at room temperature of 25°C. Moreover, excellent thermal and angle stabilities are obtained for temperatures from 10 to 90°C and incident angles from 0 to 40°, respectively. A prototype of the proposed metamaterial with 12 × 12 unit cells was fabricated and measured. The measured and simulated results were in good agreement, which validated the correctness of the design concept. The proposed metamaterial has promising potential applications for low‐observable platforms and antennas. [ABSTRACT FROM AUTHOR]

This article proposes an active frequency selective surface (FSS) with application to satellite multi‐band antennas in X and Ka bands. The FSS offers two reflective bands at 20 and 30 GHz (downlink and uplink frequencies of broadband communication satellites in Ka‐band) under one biasing state, whereas the frequency characteristics are switched in X‐band (between 9 and 12 GHz, frequencies used for broadcast satellite services and space exploration) under the second biasing state. The unit cell of the FSS consists of three metallization layers, including four PIN diodes on the bottom layer that provide reconfigurability in X‐band. The FSS performance has been analyzed using an equivalent circuit model diagram, which provides a better understanding of the operation principle. The proposed FSS exhibits a robust transmission response for incidence angles up to 40°. Several samples of the FSS formed by a 2 × 4 cell array have been fabricated and measured in a waveguide simulator to confirm the results of the electromagnetic simulations at the lower operating frequencies. [ABSTRACT FROM AUTHOR]

An optically transparent circularly polarized (CP) radio frequency identification (RFID) antenna is presented in this paper. The RFID antenna consists of a coplanar‐fed waveguide inverted L‐shaped monopole with a dual feeding structure. The CP wave with a bidirectional radiation pattern is obtained by feeding the radiator at both ends. The transparency of the antenna is obtained by using fluorine tin oxide (FTO) coated soda‐lime glass substrate. The proposed antenna operates at a Universal RFID frequency (800–1100 MHz) with circular polarization. The antenna has an impedance bandwidth of 300 MHz and an overlapped 3‐dB axial ratio (AR) bandwidth of 110 MHz (860–970 MHz, 12% centered at 915 MHz). It has a peak realized gain of 1.4 dBic. The antenna with the size of 100 × 100 mm2 is designed for ultrahigh‐frequency RFID reader applications. [ABSTRACT FROM AUTHOR]

In this paper, an enhanced upper band 5G multiple‐input multiple‐output (MIMO) array is presented. The proposed array is based on a wide band LPDA antenna loaded with epsilon near zero (ENZ) index metamaterial. The metamaterial cells result in more than 3 dBi gain enhancement. In addition, multiples metamaterial cells arrangements are studied including straight, concave, and convex rows. The array covers a wideband of MMW frequencies extending from 26 up to 39 GHz. The LPDA antenna element design consists of alternating five arms printed on RO5880 substrate. All of the dipole arms are uniformly placed along parallel microstrip feeding transmission lines on both sides of the substrate. The main feeding line is placed over a partial ground plane at the backside of the substrate. The MIMO‐LPDA array exhibits stable radiation characteristics over the operating band. A maximum realized gain of 11 dBi is achieved at 36 GHz for a single element radiator. In addition, essential MIMO performance indicators such as Envelope Correlation Coefficient (ECC), Diversity Gain (DG), and Channel Capacity Loss (CCL) are examined and determined to fulfill the needed standard of ECC, DG, and CCL. The fabricated prototypes of both the single antenna element and the MIMO array show very good agreement with simulated results. [ABSTRACT FROM AUTHOR]

A novel dual circularly polarized slot antenna with wideband characteristics is designed in this article. The quarter‐wavelength patch mode and half‐wavelength slot mode are combined in this antenna, which is excited by double perpendicular coplanar waveguide (CPW)‐fed ports. The good circular polarization and isolation are achieved by the L‐shaped ground plane and two open slots, respectively. The protruded arrow‐shaped strip introduces a new monopole mode, which broadens the impedance bandwidth (IBW) and axial ratio bandwidth (ARBW). In addition, it simultaneously improves isolation as well. The proposed prototype provides an impedance bandwidth (|S11| < −10 dB) 1.12–3.8 GHz (108.9%) and the ARBW is about 1.6–4.36 GHz (92.6%). Dual‐port isolation is better than 10 dB within ARBW. Compared with others, the dimension of this prototype is more compact with wider IBW, wider ARBW, wider isolation bandwidth and better gain performance. [ABSTRACT FROM AUTHOR]

A 1‐bit ultra‐thin (~λ/12) reflective metasurface is proposed to generate vortex beams, whose mechanism of realizing 1‐bit phase quantization is based on wideband polarization converter (PC). A diamond‐shaped patch is studied as the unit cell of PC, which has the properties of high polarization conversion transmission efficiency, simple structure, wideband, and ultra‐thin. By employing the proposed PC and its mirror image, a stable 180° phase shift is realized over the bandwidth of 18–36 GHz. Phase distributions of vortex beams with different modes and the corresponding radiation patterns are calculated and simulated. Reflective metasurface employing the proposed unit cells are arranged according to the calculated phase distribution, and the microstrip quasi‐Yagi antenna is used as the feed antenna to illuminate the metasurface to generate vortex electromagnetic waves. A prototype is fabricated and measured, verifying that vortex beams can be generated flexibly by the proposed design. The measured gain of the vortex beams reaches to 19.5 dBi with the narrow divergence angle of ±4°. The proposed 1‐bit measurface is compact, ultra‐thin and offers a new possibilities for vortex beams generation. [ABSTRACT FROM AUTHOR]

This article reports a novel compact branch‐line coupler (BLC) in coplanar waveguide technology for X‐band monolithic microwave integrated circuit (MMIC) applications. This article consists of theoretical basis, design, and measurement results of the developed miniaturized BLC. In this study, the theoretical basis of the miniaturized BLCs were analyzed and the T‐ and π‐type equivalent transmission line approaches and meandering technique were utilized to significantly miniaturize the conventional BLC. The proposed BLC was manufactured with gallium nitride MMIC technology. The manufactured BLC has a return loss of better than −12 dB over 8.5–11.1 GHz which yields a 26.8% bandwidth. In addition, the insertion losses |S21| and |S31| were measured as −3.5 dB and −3.6 dB at 9.6 GHz. Moreover, the phase difference between the output ports were 90° ± 5° over the frequency bandwidth of 9.2–11.6 GHz. Furthermore, the proposed BLC has a size of 3.9 mm2 (0.152λ × 0.156λ) and this achieves 62% size reduction with respect to the conventional BLC design. The developed novel miniaturized BLC can be utilized in the X‐band applications. [ABSTRACT FROM AUTHOR]

A compact, high‐gain 32 × 32‐horn array antenna with wideband and low side‐lobes for millimeter‐wave applications in the E‐band (71–86 GHz) is presented. The antenna is composed of four metallic layer structures under the waveguide‐feeding. The top layer contains the horn elements and the bottom layer is a corporate‐fed network. From top to bottom, the 2 × 2 cavity‐backed sub‐arrays and the T‐shaped waveguide junctions are loaded in the middle layers. The RF signal from the planar 1–256 ways divider is fed to the horn elements. Due to the uniform excitation, the antenna acquires the high‐gain pencil beam. To verify this design concept, the antenna has been fabricated, measured, and evaluated. Experimental results show that the antenna array has achieved an improved bandwidth (|S11| < −10 dB) in the range of 71–86 GHz (19.1%). More importantly, the peak gain of the proposed antenna keeps a high value of around 38 dBi with the ETSI (European Telecommunications Standards Institute) class II radiation pattern envelope (RPE). [ABSTRACT FROM AUTHOR]

A dual metasurface‐based wideband antenna is proposed herein. The antenna is fed through the stepped aperture of a coplanar waveguide. One layer of the metasurface is printed directly above the aperture, where the other layer is printed on the dielectric plate and placed above the antenna. The antenna bandwidth is significantly improved by the high‐order modes excited by the double‐layer metasurface. The proposed antenna is simple and compact. The − 10 dB relative impedance bandwidth of the proposed antenna is approximately 95.6% (4.8–13.6 GHz), and the main radiation direction deviates from the normal direction by 30°. The proposed antenna has an ultra‐wideband (UWB), which facilitates short‐range indoor UWB positioning. [ABSTRACT FROM AUTHOR]

In this paper, an array antenna fed by a novel double layer 3 × 3 Butler matrix for Ku‐band application was presented. An antipodal Vivaldi antenna (AVA) with dielectric lens was suggested as radiating elements. AVA has a broadband specification along with directive stable patterns at operational bandwidth. Combinational structure of substrate integrated waveguide topology by modified radiating element (AVAs) results to miniaturized size and acceptable radiation efficiency in whole of Ku‐Band. Presented array covers frequency range of 13–17.5 GHz (30%) with average gain of 14.2 dBi. The proposed beam‐steering antenna with compact size and good performance is capable to cover an angle range from −39 to 38° in whole operation frequency. This structure with unique configuration and specification can be a good candidate for beam‐forming application for wireless devices. [ABSTRACT FROM AUTHOR]

This article presents a circularly polarized (CP) endfire multibeam antenna with wide axial ratio (AR) beamwidth coverage. It is based on a dielectric loaded waveguide structure and consists of a 4 × 4 Butler matrix and four rhombic waveguide elements. Regarding to each element, it decomposes the incoming linearly polarized waves into two orthogonal modes with equal amplitude and obtains 90° phase difference between the two modes via their different dispersion characteristics to realize CP radiation. Study found that the proposed element has low cross‐polarization performance within a wide beamwidth, which contributes to realize wide 3‐dB AR beamwidth coverage in the multibeam array. In fabrication process, injection modeling and electroplating techniques are employed to construct the proposed antenna. By using this way, the structure splitting and misalignment problems, which usually existed in conventional designs, can be avoided, and the antenna weight can be maintained to a moderate level. Through the experiment, it is found that the proposed antenna can achieve an overlapped 3‐dB AR bandwidth of 10.7% from 9.7 to 10.8 GHz and a wide 3‐dB AR beamwidth coverage of 91°. [ABSTRACT FROM AUTHOR]

A broadband coplanar‐to‐rectangular waveguide transition using contactless electromagnetic bandgap (EBG) structure in the form of interdigital‐pin bed of nails has been introduced in this paper. The electromagnetic (EM) wave propagates from the CPW, and then radiates from the probe into the WR4. The rectangular patch is adopted as the probe to accomplish a broadband performance without an impedance transformer. At the same time, CPW to rectangular waveguide (RW) directly transition without EBG unit also has been designed and the simulated results show the transmission performance has low tolerance to air gap. The interdigital‐pin EBG structure is added to the machining splitter of the RW which is no need for strict electrical contact and is not harmful for transition performance. The greatest advantage of the interdigital‐pin lattice has a significant pin gap increase and is easy to fabricate for millimeter‐wave (mm‐Wave) and terahertz (THz) applications. The back‐to‐back prototype is designed, fabricated and measured. The measured |S11| is better than −10 dB over 170–260 GHz, covering a bandwidth of 41.8%. [ABSTRACT FROM AUTHOR]

A compact ultra‐wideband (UWB) multiple‐input–multiple‐output (MIMO) slot antenna with dual band‐notched performance is presented. Unlike the traditional slot and Vivaldi antennas, the novel slot‐line to coplanar waveguide (S‐CPW) transmission structure improves the impedance of the antenna matching and realizes the wideband characteristics. By utilizing the odd mode operation of the S‐CPW structure, the in‐phase superposition radiation of four‐way open slots is realized. The isolation and the band‐notched characteristics are achieved by etching the T‐slot structure and loading the split‐ring resonators (SRRs), respectively. The actual design with the overall size of 0.25 × 0.25 × 0.006 λ03 (λ0 is the free space wavelength at the lowest operating frequency) has been fabricated and measured. The measured results demonstrate that the impedance bandwidth of the antenna is 2.5‑10.8 GHz for |S11| < −10 dB, and the notched band is 5.2–5.9 GHz and 6.6–7.3 GHz, respectively. Besides, the isolation of the proposed antenna is higher than 20 dB, and the antenna maintains relatively stable radiation characteristics and gain, as well as a lower envelope correlation coefficient (ECC < 0.004). [ABSTRACT FROM AUTHOR]

This article presented a broadband low‐profile monopole antenna that was equipped with coplanar waveguide (CPW), substrate integrated waveguide (SIW), and slots based on characteristic mode theory (CMT). Compared with the traditional antenna design method, the antenna design method based on CMT can design the antenna more effectively. A prototype of the proposed antenna with an overall size of 0.98 λ0 × 0.98 λ0 × 0.027 λ0 (λ0 is the free‐space wavelength at the X‐band of 8.2 GHz center frequency) was fabricated and measured. The measurement results showed that the prototype had a impedance bandwidth of 3.2 GHz (6.8‑10 GHz), a relative bandwidth of 38.1%, the maximum gain of 4.1 dBi at 9.5 GHz, and the maximum radiation efficiency of 92%. Besides, it was found that the prototype almost achieved symmetrical radiation in E‐plane and omnidirectional radiation in H‐plane. Its operating frequency band covered that of broadcast band (7.1–7.3 GHz), the water mobile band (8.1–8.815 GHz), and the aviation mobile band (8.815 ‑9.04 GHz). [ABSTRACT FROM AUTHOR]

Wearable electronic devices are the basic components of wireless human body local area network. Due to their close contact with the human body, it is necessary to improve the flexibility and isolation of the antenna to maintain high signal transmission performance. In this paper, a flexible wearable antenna based on highly conductive graphene assembled film (GAF) and polydimethylsiloxane dielectric material is proposed. The planar combination of the powerful novel materials and supple polymer maximizes the flexibility and chemical stability of the antenna. In addition, the coplanar waveguide feeding and artificial magnetic conductor (AMC) structure enhance the efficiency and isolation with an easier preparation process. In the wearing measurement, the GAF antenna operates at 3.5 GHz for 5G communication frequency band and 5.8 GHz for ISM (Industrial, Scientific, Medical) frequency band. The GAF antenna has a minimizes SAR level, which in accordance with IEEE C95.1 International standard, by taking up a small space with 50.5 × 48.5 × 2.08 mm3. Furthermore, the GAF antenna has good radiation performance both under several different bending conditions and human body conditions. [ABSTRACT FROM AUTHOR]

A millimeter‐wave (mm‐wave) dual‐polarized antenna achieved by stacking single patch and 2 × 2 patches with square holes is proposed to achieve low profile and wide bandwidth. The single patch and the 2 × 2 patches operate at two pairs of orthogonal modes, TM10/TM01 modes and antiphase TM20/TM02 modes, respectively. The square holes are introduced to adjust the coupling between the two layers of patches with small spacing. The appropriate coupling between two layers of patches and the low quality factor of the upper 2 × 2 patches make the proposed antenna achieve wide bandwidth with relatively low profile. A pair of perturbation slots is introduced to adjust the port‐to‐port isolation. The characteristic mode analysis (CMA) method is utilized to analyze the antenna element to reveal the modal behavior of the proposed antenna. For demonstration, a prototype of 2 × 2 array with a profile of 0.05λ0 (λ0 is the wavelength in free space at the center frequency) is designed and fabricated, which achieves the measured 10‐dB bandwidth of 24.5%, the maximum gain of 11.6 dBi, and the port‐to‐port isolation of 23.5 dB. [ABSTRACT FROM AUTHOR]

A filtering antenna composed of a monopole and two stacked circular substrates integrated waveguide (SIW) cavity for 5.8 GHz WiFi application is proposed in this letter. Cross‐coupling between the feeding probe and the second SIW cavity resonator is utilized to generate a radiation null at the frequency above the working band. By properly designing the location of the feeding probe and the size of the coupling slot between the two SIW resonators, the frequency of the radiation null can be adjusted. Experimental results of the fabricated filtering antenna show good agreement with the simulated performance. The measured filtering antenna response has a return loss greater than 10 dB with a fractional bandwidth of 16.4% centered at 5.48 GHz while there is a radiation null at 6.85 GHz. Omnidirectional patterns are achieved in horizontal plane with a maximum gain of 1.1 dBi. The proposed filtering antenna has the merits of compact size and good stopband radiation suppression. [ABSTRACT FROM AUTHOR]

In this article, a co‐planar waveguide‐fed ultra‐wideband (UWB) multiple‐input multiple‐output flexible antenna with dual band‐notched characteristics on liquid crystal polymer substrate with high isolation for wireless body area network application is designed and fabricated. Two fan‐shaped monopole UWB antenna elements are designed and placed vertically, with tampered feeder, asymmetrical ground plane and two C‐shaped slots on the radiation patch. The measured working bandwidth can reach 2.63–11.6 GHz and the rejection bands are 3.25–4.36 and 5.41–6 GHz. The port isolation (S21) is better than −22 dB. When the bending radius is 35 and 45 mm, the antenna is measured for flexibility. In addition, the antenna is placed on human tissue to test its performance. All the results show that it has good radiation, high isolation and flexibility, and has broad application prospects in the wearable field. [ABSTRACT FROM AUTHOR]

In this paper, an approach to designing filtering rat‐race coupler (FRRC) with hybrid circular substrate integrated waveguide (CSIW) and coplanar waveguide (CPW) resonators is proposed. The FRRC is developed in a single CSIW cavity embedded with four CPW resonators and four feeding ports, featuring miniaturized size. The desired 0° and 180° phase characteristics of the RRC are realized by the inherent in‐phase and out‐of‐phase electromagnetic fields of the dual‐mode CSIW cavity. By properly arranging the sum, difference and other two output ports, the FRRC is exploited to achieve the desired properties of a RRC with bandpass filtering responses. In addition, transmission zeros are generated in the vicinity of the upper stopband. For validation and demonstration, a miniaturized FRRC utilizing hybrid CSIW and CPW resonators is synthesized, fabricated and experimented in a single‐layered process. Comparisons with previously state‐of‐the‐art research, the proposed FRRC prototype indicates the superiorities of miniaturized footprint, high‐quality‐factor, high‐order filtering response and enhanced out‐of‐band rejection. [ABSTRACT FROM AUTHOR]

This article presented a novel compact multi‐input‐multi‐output (MIMO) hexagonal monopole antenna with a uniplanar compact frequency‐selective‐surface (FSS) array for microwave imaging (MWI). The ultra‐wideband (UWB) dual‐element linear MIMO antenna was designed on the FR4 substrate with 50 Ω coplanar waveguide feed, T‐strip isolation, novel numerical calculation and equivalent circuit analyses. The main issues of realising high‐resolution images based on planer UWB antenna for MWI are the low gain, omnidirectional pattern, design size and mutual coupling of MIMO design. A novel technique was proposed to solve a hybrid issue (mutual coupling) of the MIMO reflected‐waves from the FSS array and direct‐wave. The uniplanar UWB‐FSS unit cell was compacted by combining a square‐loop and cross‐dipole with a size of 0.095λ × 0.095λ. The novel‐isolated UWB‐MIMO antenna and UWB‐FSS array (IMAF) were integrated, after investigating the distance between the antenna and FSS. The fabricated IMAF with a stable gain improvement of 4.5 dBi higher than the antenna without FSS, directional radiation pattern, size of 30 × 73.8 × 21.6 mm3 observed that a low mutual coupling of −27 dB, and operation bandwidth of 3.0–11.7 GHz. Moreover, a handbag was scanned experimentally via the bi‐static approach to detect a small concealed object. The MWI system based on the MIMO antenna with FSS was displayed image resolution of 55% higher than that of MIMO antenna without FSS. The new baggage‐scanner approach confirmed that the proposed MIMO antenna with FSS array can lead the humanity for healthy MWI applications. [ABSTRACT FROM AUTHOR]

This article presents a miniaturized reconfigurable tri‐polarization metantenna with compact size of 0.58 λ0× 0.58 λ0× 0.04 λ0 (λ0 is the free space wavelength referring to the antenna center frequency). Metantenna means the metasurface is directly used as the antenna radiator aperture instead of an accessory. The metantenna is formed by miniaturized windmill like units, which is evolved from square patch units based on characteristic mode analysis, and 46% size reduction is achieved. Two PIN diodes are introduced into the feeding microstrip for switching among tri‐polarization. The measured −10 dB impedance bandwidths are 24.6% (3.24–4.15 GHz) and 24.1% (3.26–4.05 GHz) for circular polarization (CP) and linear polarization (LP), respectively. The measured overlapped 3 dB axial ratio (AR) bandwidth is 15% (3.4–3.95 GHz). The measured peak gains are 5.15 dBi, 5.21 dBic, and 5.25 dBic for LP, left hand (LH) CP and right hand (RH) CP, respectively. The aperture efficiency is 77.3%, 78.5%, and 79.1% for LP, LHCP, and RHCP, respectively. The measured AR beamwidth (BW) of LHCP in XZ‐plane and YZ‐plane is 192° and 174°, respectively, the measured ARBW of RHCP in XZ‐plane and YZ‐plane is 201° and 166°, respectively. The developed metantenna has the following advantages: broadband 3 dB AR bandwidth, wide ARBW > 166°, least PIN number (tri‐polarization with 2 PIN), and high‐aperture efficiency. [ABSTRACT FROM AUTHOR]

Fifth‐generation millimeter‐wave devices with multiple antennas require minimal mutual interference for better overall performance. This article presents a simple but novel technique to improve the isolation between closely packed slot antenna arrays. A printed ridge gap waveguide (PRGW) structure is used to realize a low loss millimeter‐wave array's feeding mechanism. By optimally introducing a defective upper plate within the PRGW structure, the measured port isolation of the antenna elements is improved by 17 dB at 61.3 GHz. The antenna array is impedance matched to the feeding PRGW structure over a wide frequency range of 54–67 GHz. Moreover, the radiation patterns are consistent showing broadside radiation before and after introducing the defect in the PRGW structure. The fabricated prototype of the PRGW fed slot antenna array validated the predicted isolation and return‐loss bandwidths of the proposed radiating system. [ABSTRACT FROM AUTHOR]

By introducing a V‐shaped air slot in antenna substrate, the peak gain can be improved significantly for traditional Vivaldi antennas without size enlargement and additional components. The air slot is conformal to the radiating conductors of the target Vivaldi antenna, and can force the electromagnetic wave to propagate within the central dielectric part between the antenna's opening slot, by decreasing the effective dielectric permittivity of the regions along the radiating conductors. As a result, the wave distributes more evenly within the radiating aperture comparing with the traditional designs, especially at high frequencies, hence the peak gain can be enhanced. In addition, the removal of antenna back substrate is evaluated on radiation characteristics, which can further improve antenna gain and front‐to‐back ratio. For verification, three antennas within operating frequency band from 0.8 to 6.0 GHz are designed with the same dimensions (220 mm × 256 mm) but different configurations. The simulations show that the peak gain increases from 2.0 to 12.0 dBi gradually for the air slot loaded antenna. The gain increment is about 4–6 dB from 4.0 to 6.0 GHz comparing with the one without air slot. A sample antenna with both air slot and removal of back substrate is fabricated and measured. The measured reflection is better than −10 dB from 0.8 to 6.0 GHz, and the measured gains at (0°,0°) direction are from 2.3 to 11.0 dBi. The antenna patterns at high‐band shows that the main lobe in E‐Plane is narrowed successfully after introducing the V‐shaped air slot. The experimental data agree well with the prediction with in‐band radiation efficiency better than 65%. [ABSTRACT FROM AUTHOR]

A compact dual‐band inverted microstrip ridge gap waveguide (IMRGW) diplexer composed of four dual‐mode IMRGW resonators and an IMRGW T‐junction matching network is proposed in this letter. The dual‐band diplexer is based on a dual‐mode IMRGW resonator which can produce both the inverted microstrip resonant mode and the cavity resonant mode. Two resonant modes of the IMRGW resonator are highly independent, so the passbands of dual‐band IMRGW diplexer can be easily tuned to the desired frequencies. A second‐order dual‐band IMRGW diplexer is designed and simulated. The simulated passbands of the second‐order dual‐band IMRGW diplexer are 13.13‐13.38, 14.85‐15.26, 18.57‐19.42, and 20.85‐21.35 GHz with fractional bandwidth of 2%, 2.7%, 4.8%, and 2.4%. A prototype is fabricated and measured to verify the feasibility of the proposed IMRGW diplexer. The measured passbands of the second‐order dual‐band IMRGW diplexer are 13.07‐13.35, 14.88‐15.35, 18.45‐19.37, and 20.82‐21.28 GHz with fractional bandwidth of 2.1%, 3.1%, 4.9%, and 2.2%. The measured isolation is better than 23 dB over 12‐22 GHz. The measured results agree well with the simulations. [ABSTRACT FROM AUTHOR]

This article presents a compact slot array antenna combined with quad‐polarization for mm‐wave applications. A novel modified short‐circuit termination with inclined slot is proposed in this paper and a dual‐layer structure has been adopted to achieve miniaturization and also to design a circular polarization feeding network. Two linear polarization feeding networks are designed independently so that four polarization states can be realized. Four polarization states, namely vertical linear polarization, horizontal linear polarization, left hand circular polarization (LHCP), and right hand circular polarization (RHCP) can be switched by feeding corresponding ports. Judging from impedance matching and axial ratio (AR), the antenna exhibits wide bandwidth. The −10 dB impedance bandwidth and 3 dB AR bandwidth are better than 14.2% (26‐30 GHz) with gain about 5‐8 dBic for both LHCP and RHCP. The linear polarization states have |S11| < −10 dB and a gain of 5‐7 dBi over 26.5‐29.5 GHz (10.7%). The size of the slot array antenna is 65 × 65 × 1.156 mm and measured results are consistent with simulations. [ABSTRACT FROM AUTHOR]

This communication reports the design of highly compact cavity‐backed self‐diplexing Y‐shaped slot antenna for high‐isolation employing quarter‐mode substrate integrated waveguide (QMSIW). The proposed antenna is built by using QMSIW, Y‐shaped slot and two 50 Ω feed lines. Two unequal radiating apertures are developed by inserting an Y‐shaped slot on the top side of the rectangular SIW for radiation at 3.9 and 4.9 GHz for long term evolution (LTE) and public safety band (PSB) applications, respectively. By keeping another band unaltered, the proposed antenna allows any frequency band to be tuned independently. The size of antenna is highly compact due to the use of QMSIW cavity. The isolation between two ports of the antenna is greater than 34 dB. The antenna achieves 5.43 and 5.85 dBi peak gains at 3.9 and 4.9 GHz, respectively. On both the frequency bands, the antenna efficiency is greater than 85%. The antenna has a front‐to‐back‐ratio of more than 20.6 dB and a separation from co‐to‐cross polarization level of more than 21 dB. Through manufacture and measurement, the designed antenna is validated. [ABSTRACT FROM AUTHOR]

This article presents two ground radiation antennas with simple feeding structures for mobile applications. Characteristic mode theory is exploited to design the proposed antennas. For antenna 1, a small loop without any lumped elements is used as the feeding structure. And the total length of the feeding loop is only 20.2 mm (0.06 λ0.9GHz). The operating band of the proposed antenna 1 is 834‐1020 MHz. Across the whole operating band, the measured efficiencies of antenna 1 are higher than 72%. In addition, a new method to generate an extra resonant frequency is proposed, which has little effect on the existed resonant modes at the lower band. The dual‐mode antenna 2 without loading any lumped elements is obtained by implementing this method, which covers 812‐940 MHz and 1543‐1576 MHz bands. Furthermore, the new resonant frequency can be controlled independently. Namely, the higher frequency band of antenna 2 can be adjusted freely without changing the lower band. To validate the design strategy, two ground radiation antennas are fabricated and measured. The measured results indicate that the proposed antennas are suitable for mobile terminals. [ABSTRACT FROM AUTHOR]

A tunable microwave amplitude equalizer is proposed to realize arbitrarily prescribed transmission responses, which is mainly composed of a spurline and some tunable resonant cavities. Compared with traditional resonant‐cavity equalizers, the introduction of the spurline can significantly reduce the total number of the tunable cavities, and can improve the attenuation level. The operation mechanism of the equalizer is detailed analyzed, and the experimental results demonstrate that the amplitude equalizer can realize an attenuation control of about 25 dB at the frequency band of 5.0–6.0 GHz, and the return loss is better than 17 dB. [ABSTRACT FROM AUTHOR]

A new class of multi‐layer microstrip patch antenna (MPA) array fed by substrate integrated waveguide (SIW) cavities is presented for Ku‐band applications. An efficient compact multi‐layer SIW feeding network is designed, which has lower losses than the conventional corporate feeding network based on microstrip lines or coplanar waveguides. A 4 × 4 array consists of four stacked layers, including three based on the SIW technology as a feeding network and one layer for the radiating antenna array. Each layer can easily be fabricated from a dielectric substrate and processed using the printed circuit board (PCB) technology. Afterward, all the PCB laminates are stacked together and tied by screws. The measured antenna operates from 11.2 to 13.6 GHz achieving 19.35% bandwidth. The measured peak gain at 12.5 GHz is 17.1 dBi, equivalent to 71.47% aperture efficiency. The fabricated array exhibits several advantages: broadband, low cost, compactness, high radiation efficiency, high aperture efficiency, and lightweight, which makes it a candidate for Ku‐band satellite applications. [ABSTRACT FROM AUTHOR]

A novel wideband, triple‐resonant sectorial dipole antenna with simple configuration is advanced. The antenna is evolved from a 1.5‐wavelength prototype electric dipole, which is backed by a circular metallic planar reflector. The flared angle of the sectorial radiator and the corresponding usable resonant modes for wideband radiation is determined by mode synthesis chart. As numerically and experimentally validated, the flared angles of the principal dipole should be 300°, with its radius approximately equal to 0.23‐wavelength centered at 2.4 GHz. By introducing a pair of tuning stubs and a pair of slits along circumference, three resonant modes simultaneously excited in a single sectorial dipole to generate a wideband, triple‐resonant radiation characteristic. Prototype antennas are then fabricated to experimentally validate the design approach. It is shown that the proposed antenna exhibits a unidirectional pattern with its impedance bandwidth over 73.3%. Meanwhile, stable radiation patterns with gain fluctuation less than 2.4 dB, low cross polarization level of −27 dB, and maximum gain of 6.9 dBi are realized within the same frequency range. [ABSTRACT FROM AUTHOR]

In this article, a new subwavelength element is introduced into the broadband folded reflectarray antenna (FRA). The element comprises a rectangular split‐ring resonator with two slits and a square patch located in the center of the rectangular ring. It can achieve superior cross polarization conversion (CPC) performance from 16.2 to 26.4 GHz with 1 dB bandwidth of 45.4%, and high orthogonal polarization isolation. Furthermore, changing the positions of two splits of the element can achieve two discrete reflection phases (0° and 180°) with the high CPC efficiency and angular stability. To verify the feasibility, an FRA using the digitally quantized phase distribution method is designed and measured at 22 GHz. The measured results show that the 1 dB gain bandwidth of the proposed FRA is 30.8%, and the maximum aperture efficiency is 15.4%. This superior performance is comparable with the existing FRAs. [ABSTRACT FROM AUTHOR]

Compact and low‐loss U‐band shielded asymmetrical coplanar stripline (SACPS) to unilateral and antipodal finline transitions are proposed in this article for millimeter‐wave circuits and systems applications. The principles of mode conversion and impedance matching are applied to the proposed transitions. In the SACPS to unilateral finline transition, the SACPS is firstly transformed into the asymmetrical unilateral finline to convert the quasi‐TEM mode to the quasi‐TE10 mode and then a tapered unilateral finline section is applied to realize impedance matching. For the U‐band back‐to‐back SACPS to unilateral finline transition, the measured return loss is larger than 15 dB and the insertion loss is less than 1.5 dB with a minimum of 0.5 dB. In the SACPS to antipodal finline transition, the SACPS is firstly transformed into the defected ground microstrip (DGM) without changing the characteristic impedance. Then, the DGM is transformed into the asymmetrical antipodal finline to make mode conversion. At last, a tapered antipodal finline section is used for impedance matching. For the U‐band back‐to‐back SACPS to antipodal finline transition, the measured return loss is larger than 12 dB and the insertion loss is less than 1.5 dB with a minimum of 0.3 dB. [ABSTRACT FROM AUTHOR]