Guest editorial: wideband/multiband millimetre‐wave antennas for 5G/6G and radar applications

Antennas are essential components for wireless systems, which are extensively deployed in various devices such as mobile phones, base stations, vehicles, radars etc. With the increasing demand of transmission data rates and system capacity, the operating frequency of antennas moves towards millimetre-wave (mm-wave) bands. Compared to their low-frequency counterparts, the sharply reduced wavelength and increased loss at mm-wave bands yield stringent requirements for mm-wave antennas, such as high antenna gain, high efficiency, wide bandwidth, multiple beams, versatile polarizations etc. On the other hand, mm-wave antennas for 5G/6G and radar applications are undergoing comprehensive research and innovative designs from academia to industry. The design paradigm of mm-wave antennas is far different from that of low-frequency antennas. For these reasons, this Special Issue aims to present state-of-the-art research works on antennas at mm-Wave, sub-mm-Wave, and microwave bands for 5G/6G and radar applications. There are in total 16 papers accepted for publication in this Special Issue through strict peer reviews and revisions. The theme of the Special Issue is broadly divided into four topics, with the summary of every topic given below. You are, however, strongly encouraged to read the full paper if interested. Wang et al., in their paper, ‘A Multi-Port Interconnected Magneto-Electric Dipole Antenna Array for 5G Applications’, present a 28-GHz multi-port magneto-electric (ME) dipole array for 5G applications. The proposed antenna array can be configured as a balanced antenna, a dual polarized antenna array, or as circularly polarized antenna array through changing the phase of the input ports. Co-planar waveguide (CPW) lines are used to connect ME dipole radiators to achieve a high gain with simple feeding structure. The proposed multiport interconnect ME dipole antenna array not only has the advantage of the conventional ME dipole antenna arrays, such as low profile, light weight and high gain, but also enlarges the polarization diversity and design flexibility of the system. Wu, et al., in their paper, ‘Multi-Resonance Dual-Polarised Symmetrically Cross-Slotted Square Patch Antenna for 5G Millimetre-Wave Broadband Applications’, present a Ka-band dual-polarized multimode resonance antenna element for 5G broadband applications. The proposed antenna element is enabled by cutting a symmetrical cross-slot in the radiating patch, exciting multiple resonance. Two substrate-integrated coaxial lines (SICL) are used to feed the patch radiators, achieving a broadband impedance matching. The proposed dual-polarized patch antenna has a low profile, a broad operational bandwidth of more than 50%, and a high isolation of greater than 30 dB. Wang et al., in their paper, ‘Highly-Efficient Optically Transparent Planarized Transmit-Arrays Using Heterogeneous Unit Cells’, present an optically transparent transmit-array operating in the 5G millimetre-wave band around 26 GHz. The transmit array is based on four layers of low-loss and transparent COC substrates and cascaded metallic patterns in the form of meshed wires printed on PET films. A number of eight unit cells are designed to provide a 3-bit phase resolution. Based on the cells, two transmit-array prototypes are demonstrated—one generating a broadband pencil beam and the other radiating shaped flat-top beam. Li et al., in their paper, ‘6G Energy-Efficient Systems Based on Arrays Combined with Dielectric Lenses’, propose a solution of combining phased array antennas with dielectric lenses to increase the energy efficiency of 6G system. A dielectric lens is designed to enhance the gain of an array antenna operating in the 3GPP n257 band. It is demonstrated that the dielectric lens increases the gain of the phased array antenna in a large scanning range, meaning that the output RF power can be decreased for a same equivalent isotropic radiated power required from the base station. Alshammari et al., in their paper, ‘Compact Folded-Shorted Patch Array Offering Dual-Band Operation and Dual-Circularly Polarised Radiation for Picosatellites and Other Small Satellites’, propose a dual-band folded shorted patch (FSP) antenna that offers dual-circularly polarised (DCP) radiation. The proposed 2 × 2 FSP array antenna operates at 1.1 and 2.4 GHz and offers good radiation performance in terms of minimal reflection coefficient losses, broad beamwidths, and low cross-polarisation levels. With these merits, the proposed antenna is suitable for picosatellites and other small satellites. Jiang et al., in their paper, ‘A 3-D Metal Printed Folded-Shorted Patch Array with an Integrated Feeding Circuit Offering Dual-Band Circularly Polarised Radiation for CubeSat Applications’, present a 3-D printed metallic folded-shorted patch element and its array for CubeSat applications. The characteristic mode analysis method was used to design the folded-shorted patch radiator working in the first and third modes, which is fed by a five-port feeding network. A metal 3-D printing technique was then employed to fabricate the antenna element, based on which a sequential rotation array was built. The array can support CP radiation at 1.2 and 2.45 GHz simultaneously, with high total efficiencies of 84% and 95%, respectively. Liang et al., in their paper, ‘Low-profile Multi-Polarisation and Pattern Reconfigurable Antenna Based on Metasurface’, present a low-profile metasurface antenna with multi-polarisation and pattern reconfigurability. By controlling the PIN diodes and rotating the metasurface, the antenna can operate in horizontal-polarisation, vertical-polarisation, right-hand circular-polarisation, and left-hand circular-polarisation modes and achieve pattern reconfigurability within a very low profile. Jiang et al., in their paper, ‘Low-Profile Broadband Circularly Polarized Short-Circuited Dipole Antenna with Backed Cavity’, present a broadband circularly polarized (CP) dipole antenna with a backed cavity. A single Γ-shaped feed structure is adopted to couple-feed a pair of rotational symmetric branches designed to be axe-shaped, achieving a wide VSWR bandwidth and a broad 3 dB axial ratio (AR) bandwidth. The shorting-to-the-ground technique miniaturizes the lateral dimensions of the antenna. The back cavity effectively enhances the boresight gain and improves the isolation level, making the antenna suitable for use as an element in massive antenna arrays. Xue et al., in their paper, ‘A Wideband Tri-Polarized Water Antenna’, present a wideband tri-polarized water antenna. Two orthogonal horizontal polarizations are realized by exciting two orthogonal broadside modes of a water patch while one vertical polarization is produced by exciting a water dielectric resonator antenna (DRA). Two pairs of L-shaped probes with differential feeding technique are introduced to achieve wide impedance bandwidth and high isolation of the water patch. It is shown that the antenna has a wide overlapped bandwidth between the three polarizations. Since the proposed antenna is mainly realized by water of high optical transparency, it holds tremendous promise for future communication systems. Shang et al., in their paper, ‘Phase Retrieval method for Near-Field Antenna Measurement Based on differential evolution algorithm in Millimetre band’, present a phaseless planar near-field measurement scheme for phase retrieval. The differential evolution algorithm is used to improve the initial guess of the phase distribution. Then, the standard iterative Fourier transform algorithm can be used to reconstruct the complex-valued radiation pattern of the antenna under test. Such phaseless near-field measurement methods are especially useful for millimetre-wave antenna measurements where accurate phase measurement becomes increasingly difficult due to the probe positioning errors. Mbugua et al., in their paper, ‘Experimental Characterization of Scattering from the Trihedral Corner Reflector in the E-Band’, investigate the monostatistic scattering characteristics of three metallic trihedral corner reflectors (TCRs) with varying geometrical sizes in the E-band from 60 to 90 GHz. The measured radar cross section (RCS) highlights the importance of modelling diffraction when simulating the RCS of the TCR using asymptotic methods in the mm-wave frequency band. Shafiq et al., in their paper, ‘Self-Calibrating Circuit for Phase Correction to Support Phased Array Antenna Systems’, propose a self-calibrating circuit system for the application within active phased array antennas. The proposed circuit system allows for the consecutive phase shift between RF channels to be inspected and self-corrected within a feedback loop managed by a MATLAB-Arduino program. By using the proposed system, the active S-band feed system can self-calibrate in about 25 s. Mbugua et al., in their paper, ‘Experimental Investigation of the Scatterer-array-based Decorrelation Technique Applied to a Real MIMO Base Station’, propose a scatterer-array-based decorrelation method for improving the multiple-input multiple-output (MIMO) performance. It is shown that the antenna correlation can be reduced by loading a specially designed scatterer array without distorting the antennas’ radiation patterns. The effectiveness of the method has been demonstrated by real-life throughput measurements using a real base station (BS). This is the first time that the scatterer-array-based decorrelation technique has been successfully applied to a real BS in real-life scenarios. Ge et al., in their paper, ‘Self-decoupling coupled-fed patch antennas for 5G MIMO applications’, propose a self-decoupling coupled-fed patch antenna based on the weak-field region formed by the T-shaped probe feed and the radiating patch itself. Both the two-element and four-element arrays were explored to demonstrate the effectiveness of the method with more than 30-dB isolation. It does not require any additional decoupling circuitry or structures, paving a way for the future 5G MIMO antenna systems. Ouattara et al., in their paper, ‘120 GHz 2-bit reflection-type phase shifter (RTPS) based on PIN diodes switched-lines’, present a 2-bit digital reflection-type phase shifter operating at 120 GHz. PIN diodes were employed to obtain 90°, 180°, and 270° phase shifter relative to the reference (0°), which has validated the concept of RTPS loaded by switched lines for the passive phase shifters at mm-waves beyond 100 GHz. Xue et al., in their paper, ‘A polarization-intensive ultra-wideband absorber based on hybrid structure’, design a polarization-intensive ultra-wideband absorber (PIUWA) based on hybrid structure with more than 90% absorptivity from 4 to 24.53 GHz, aiming to solve the problems of narrow bandwidth or complex structures. Theoretical analysis with equivalent circuit models, experimental measurements, and design procedures are all introduced to validate the approach. All of the papers select for this Special Issue show that millimetre-wave antennas and related techniques are widely investigated and deployed. However, there are still many challenges in this field that requires future research attentions, such as wide-angle beam-scanning capability, low-cost multi-beam realization, and integration into millimetre-wave systems. The future research work can extend the application scenarios of millimetre-wave antennas. Long Zhang received the B.S. and M.S. degrees in electrical engineering from the Huazhong University of Science and Technology (HUST), Wuhan, China, in 2009 and 2012, respectively, and the Ph.D. degree in electronic engineering from the University of Kent, Canterbury, UK, in 2017. From January 2018 to April 2018, he was a Research Fellow with the Poly-Grames Research Center, Polytechnique Montreal, Canada. He is currently an Assistant Professor with the College of Electronics and Information Engineering, Shenzhen University, Shenzhen, China. His current research interests include mm-wave antennas and arrays, reflectarrays and tranmitarrays, phased arrays, and characteristic mode theory. Dr. Zhang has authored or co-authored more than 100 papers in peer-reviewed journals and conference proceedings. He served as the TPC Member and Session Chair for several international conferences. He also serves as a reviewer for several technique journals, including the IEEE Transactions on Antennas and Propagation, IEEE Antennas and Wireless Propagation Letters etc. Xiaoming Chen received the B.Sc. degree in electrical engineering from Northwestern Polytechnical University, Xi'an, China, in 2006, and the M.Sc. and Ph.D. degrees in electrical engineering from Chalmers University of Technology, Gothenburg, Sweden, in 2007 and 2012, respectively. From 2013 to 2014, he was a postdoctoral researcher at the same University. From 2014 to 2017, he was with Qamcom Research & Technology AB, Gothenburg, Sweden. Since 2017, he has been a professor at Xi'an Jiaotong University, Xi'an, China. His research areas include MIMO antennas, over-the-air testing, reverberation chambers. He has published more than 150 journal articles on these topics. Prof. Chen currently serves as a Senior Associate Editor for IEEE Antennas and Wireless Propagation Letters, and an Associate Editor for IEEE Transactions on Antennas and Propagation. He was the general chair of the IEEE International Conference on Electronic Information and Communication Technology (ICEICT) in 2021. He won the first prize of universities’ scientific research results in Shaanxi province, China, 2022. He received the IEEE outstanding Associate Editor awards 2018, 2019, 2020, 2021, and 2022, and URSI (International Union of Radio Science) Young Scientist Award 2017 and 2018. Zhi Hao Jiang received the B.S. degree in radio engineering from Southeast University, Nanjing, in 2008, and the Ph.D. degree in electrical engineering from The Pennsylvania State University, University Park, USA, in 2013. From 2013 to 2016, he was a Post-Doctoral Fellow with the Computational Electromagnetics and Antennas Research Laboratory, Department of Electrical Engineering, The Pennsylvania State University. He is currently a Professor with the State Key Laboratory of Millimeter Waves, School of Information Science and Engineering, Southeast University. His research interests include microwave/millimetre-wave antennas and circuits, metasurfaces, and analytical methods. Dr. Jiang has authored or co-authored more than 100 papers in peer-reviewed journals, over 80 papers in conference proceedings, as well as nine book chapters. He has also co-edited two books: Electromagnetic Vortices: Wave Phenomena and Engineering Applications (Wiley/IEEE Press, 2021), Electromagnetics of Body-Area Networks: Antennas, Propagation, and RF Systems (Wiley/IEEE Press, 2016). He holds seven granted US patents and 18 granted Chinese patents. He is serving as a committee member of the IEEE AP-S New Technology Direction Committee (NTDC) and has served as the TPC Co-Chair or a TPC Member for multiple international conferences. He was a recipient of the ZTE Outstanding Collaboration Program Award in 2022, the Outstanding Youth Scholar of National Science Foundation of China in 2021, the IEEE Microwave Prize in 2021, the Young Scientist Award at the URSI-GASS in 2020, the Young Scientist Award at the 2019 ACES-China Conference, the High-Level Innovative and Entrepreneurial Talent presented by Jiangsu Province, China, in 2017, the Thousands of Young Talents presented by China government in 2016, the 2012 A. J. Ferraro Outstanding Doctoral Research Award in Electromagnetics, and several best (student) paper awards at international conferences. He is a Senior Member of CIE, serves as the Associate Editor of IET Communications, was a Guest Editor of International Journal of RF and Microwave Computer-Aided Engineering. Yujian Li was born in Hunan, China, in 1987. He received the B.S. and M.S. degrees in communications engineering from Beijing Jiaotong University, Beijing, China, in 2009 and 2012, respectively, and the Ph.D. degree in electronic engineering from City University of Hong Kong in 2015. In 2015, he joined the Institute of Lightwave Technology at Beijing Jiaotong University, where he is currently a Full Professor with the School of Electronic and Information Engineering. His current research interests include millimetre-wave antennas, base station antennas, and leaky wave structures. Dr. Li was awarded the Outstanding Research Thesis Award from City University of Hong Kong in 2015. He received the Best Paper Award at the 2015 IEEE Asia-Pacific Conference on Antennas and Propagation (APCAP), the Best Student Paper at 2013 National Conference on Antennas, and the Best Student Paper Award (2nd Prize) at the 2013 IEEE International Workshop on Electromagnetics (iWEM). He was selected as a Finalist in the student paper contest of 2015 IEEE AP-S Symposium on Antennas and Propagation (APS). He has served as a Reviewer for the IEEE Transactions on Antennas and Propagation, the IEEE Antennas and Wireless Propagation Letters, and the IET Microwaves, Antennas & Propagation. Lei Wang received the Ph.D. degree in electromagnetic field and microwave technology from the Southeast University, Nanjing, China in 2015. From 2014 to 2016, he was a Research Fellow and Postdoc in the Laboratory of Electromagnetics and Antennas, Swiss Federal Institute of Technology (EPFL) in Lausanne, Switzerland. From 2016 to 2017, he was a Postdoc in Electromagnetic Engineering Laboratory of KTH Royal Institute of Technology in Stockholm, Sweden. From 2017 to 2020, he was an Alexander von Humboldt fellow in the Institute of Electromagnetic Theory of Hamburg University of Technology (TUHH) in Hamburg, Germany. From March 2020 to present, he is an Assistant Professor in the Institute of Signals, Sensors and Systems of Heriot-Watt University in Edinburgh, United Kingdom. His research includes the antenna theory and applications, active electronically scanning arrays, integrated antennas and arrays, substrate-integrated waveguide antennas, leaky-wave antennas, and wireless propagations. In 2014, he was awarded the National PhD Scholarship in China and the Swiss Government Excellence Scholarship to conduct research at EPFL. In 2016, he was granted the Alexander von Humboldt fellowship to carry out research at TUHH. In 2020, he received the Principal Investigator grant from German Research Foundation (DFG). In 2022, he was also funded by the British Royal Society. Moreover, he received the Best Poster Award in iWAT-2018 and the Best Paper Award in UCET-2020. He also supervised students winning the Honourable Mentioned Best Student Paper in APS-2021 and Best Student Paper in UCMMT-2022.

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