To design a low radar cross section (RCS) antenna, the major concern is not only to reduce scattering, but also to maintain its radiation parameters, viz. gain, voltage standing wave ratio (VSWR), etc. This paper presents a simple configuration of low RCS microstrip patch array with a periodic structure-based superstrate. The ground of the array is designed as reduced ground plane with high impedance surface elements, viz. rectangular patch and Jerusalem cross. The configuration of superstrate consists of multilayered, viz., two-layered and three-layered structures having partially absorbing and reflecting surfaces. In both the proposed configurations, the array gain of 12.5 dB is maintained with reduced structural RCS over the entire in-band frequency range. The reflection coefficient (~ −20 dB) and VSWR (~ 1.1) of the array are maintained. It is shown that the proposed superstrate-based patch array design has significantly reduced in-band RCS (−18 dBsm) at its resonant frequency.

Propagation modeling of small/big pebbles and air-dry/wet sand environments for wireless sensor networks has not been extensively studied in the 5G frequency band. This study is necessary for the proper coverage planning and efficient operation of wireless sensors in various applications such as monitoring summer sporting activities, and environmental/ground surveillance on coastal pebble/sand environments, or tracking pebble mobility and including the rescue of the flood-type avalanche in Gravel-Bed Rivers. In this study, empirical path loss models are proposed for wireless sensor networks in pebble/sand environments at two discrete frequencies, namely 3.5 and 4.2 GHz. The theoretical models and proposed models are compared to indicate the accuracy of proposed models in predicting the path loss in these environments. Additionally, R-squared and RMSE values of eight different generated models are calculated in the range of 0.931–0.877 and 2.284–2.837, respectively. These comparisons indicate that empirical model parameters have a significant effect on the path loss model.

The in-depth exploration in the future 5G technology symbolizes a revolution in technology for antenna designers to encounter the all time increasing need as well as demand for higher data rate wireless communications. The paper gives out an exhaustive review of the evolution and characteristics of the 5G spectrum allocations, the MIMO antenna design with regard to mutual coupling reduction techniques and safer user applications. It precisely covers almost all the aspects of 5G which mainly include the types of antenna designs and their performance parameters related to MIMO design. The paper also presents a brief description of massive MIMO technology for base station applications. The main aim of the paper is: (1) to emphasize the frequencies allocated for the 5G including sub-6 Ghz and mm-wave bands; (2) to underline the suitable antenna designs for MIMO applications for mobile devices and base stations; (3) to highlight the mutual coupling effects in MIMO designs and its reduction techniques; (4) to consider the gaps in the literature and the challenges for reducing SAR effects for the safety of the users. This review paper has been an attempt to explore the evolution of 5G bands and antenna designs for 5G applications, comparison based on the literature, and the techniques implemented for enhancing the MIMO antenna performances.

A new wideband multiple-input/multiple-output (MIMO) antenna system able to operate in a frequency band ranging between 3.3 and 7.1 GHz is proposed for fifth-generation (5G) new radio applications for future smartphones. The design structure contains four pairs of compact microstrip-fed slot antennas, located at the corners of an FR-4 printed circuit board. Each pair of antennas consists of a radiator with two concentric annular slots, fed by two L-shaped microstrip-feeding lines and provides polarization and radiation pattern diversity function due to the orthogonal placement of their feed-line. In order to reduce the mutual coupling characteristic, we have inserted a rectangular slot under each microstrip feed-line. Besides, we have coupled and linked the two rings by a small gap to combine and move the resonant modes so as to achieve wideband coverage. The measured and simulated results show that the proposed design achieves the desired performance, such as isolation >12 dB, a total efficiency >48%, and an envelope correlation coefficient <0.07. In addition, the radiation pattern, the total efficiency, the realized gain, and the channel capacity are also studied. According to the reached results, the proposed MIMO antenna may be a suitable application-oriented design for 5G mobile communication.

In this paper, a novel ultra-wide-band antenna backed by a suspended ground for an enhancement of bandwidth and a frequency selective surface (FSS) reflector for the enhancement of gain is proposed in order to meet the specific coverage requirements for broadband applications. The impedance bandwidth of the proposed antenna is 13.4 GHz (1.8–15.2 GHz). To enhance the gain of the antenna, a FSS reflector is employed below the suggested ultra wide-band antenna. A novel FSS, which consists of 5 × 5 array of elements, located at a distance of 1.01 λ (36.14 mm) below the proposed antenna. There is 4–5 dBi improvement in antenna gain after application of the FSS. In order to observe the signal correlation of the proposed antenna, the time domain analysis using similar antennas in face-to-face and side-to-side scenarios has been performed using the EM simulation tool CST-STUDIO. The simulated results of the proposed antenna exhibit good agreement with the experimental results of the prototype model antenna.

In this paper, the influence of the cutting plane as well as the orientation of the cavities in cross-coupled W-band waveguide filters are investigated. When waveguide filters are manufactured with the commonly known CNC (computer numerical control) milling technique, at least one cutting plane is required. The position of this cutting plane has an impact on the composition of the cavities, the manufacturing accuracy, and on the maximal number of transmission zeros (TZs) introduced by a direct source to load (SL) cross-coupling. Similar filter set-ups therefore may show different performances depending on the position of this cutting plane. To examine all these effects, three similar fourth-order W-band filter set-ups are realized with distinct cutting planes and different oriented cavities. The filters are compared in terms of the sensitivity to manufacturing tolerances, the maximal number of TZs introduced by a direct SL cross-coupling as well as their spurious mode performance.

A low-profile, wideband dual-polarized 1 × 2 multiple-input-multiple-output (MIMO) antenna with frequency selective surface (FSS) decoupling technique is presented. Low profile is realized with two different artificial magnetic conductor (AMC) cells out of which one operates at 3.5 GHz and other with dual band at 3.1 and 4.5 GHz. The proposed antenna height is maintained at 0.125λ0 which is significantly reduced when compared with the conventional perfect electric conductor (PEC) ground plane. Wideband dual polarization is enabled by two pairs of bow-tie antenna elements surrounded by a square ring placed in the orthogonal orientation. To mitigate the near-field coupling in 1 × 2 MIMO an FSS wall is constructed with wide band stop characteristics from 2.85 to 4.75 GHz. Measured results show by inserting FSS wall vertically, coupling reduction is improved by 27 dB over the existing coupling and the antenna exhibits a bandwidth of 57.14% (2.95–4.95 GHz) for VSWR<2 with port isolation of more than 25 dB for entire band of operation.

In this paper, an efficient dual function radar-communication system is proposed to improve the system's resource utilization. In this work, we considered a scenario where the location of the communication receiver is known prior but the radar target is moving and its location is changing with time. Therefore, we proposed a closed-loop design that allows an adaptive selection of appropriate information embedding strategies during tracking operations. We used two strategies that utilize the amplitudes and/or phases of the transmitted radar waveforms toward the communication direction according to the position of the communication receiver during each scan. Hence, during each radar illumination, the system carries out a target-tracking task and simultaneously maintains the communication symbols transmission toward the intended communication direction. The simulation results verify the effective performance of the proposed approach in terms of target detection and tracking performance and angular bit error rate (BER). Furthermore, the proposed amplitude phase-shift keying signaling strategy can transmit different communication symbols to different users located within the sidelobe region and also provides a significant improvement in data rate transmission and BER performance compared with the existing sidelobe-based communication strategies.

In this paper, we present a 28 GHz antenna array in package which covers the n257 and n258 frequency bands designated for 5G applications. The dipole antenna array is placed on one of the two re-distribution layers in the fan-out eWLB (embedded Wafer Level Ball Grid Array) package. For TX and RX, separate but identical antenna arrays are placed on each side of the die. The paper presents a novel horn-shaped heatsink which not only dissipates the heat, but also improves the radiation performance. The four-elements dipole array has the impedance bandwidth of almost 6 GHz (24–30 GHz) and shows a maximum realized gain of 9.5 dBi. Beam-steering in ± 35 deg is achieved in the azimuth plane (H-plane) by providing different phases to the dipoles through the chip. The measurements nicely agree with the simulation results.

In this paper, a general method for synthesizing dispersive coupling matrix (DCM) of coupled-resonator bandpass filters with the general dispersion is proposed. Compared to the dispersion-less coupling matrix synthesized by classic approaches, the DCM incorporates the identity of the physical realization and provides an accurate description of the filter. Moreover, the DCM can accurately describe the complex resonators and coupling elements with intentional strong dispersion. A DCM consists of two parts: the frequency-invariant part that corresponds to the coupling matrix at the center frequency and the frequency-variant part that reflects the dispersion of the physical realization. By iteratively adjusting the transmission zeros governed by the given filter topology and ignoring the transmission zeros brought by the dispersion, the synthesis method can directly obtain the frequency-invariant part so that the overall DCM leads to an equal-ripple response. To illustrate the general synthesis method, two design examples are presented, which include a filter with unintentional weak dispersion and a filter with intentional strong dispersion. The examples are validated by EM simulation, demonstrating the effectiveness, accuracy, and generality of the proposed synthesis framework. Key words: dispersive coupling matrix, filter synthesis, microwave bandpass filter

In this paper, a novel metasurface-based Fabry−Perot cavity antenna loaded with toroidal metal structures is presented. The antenna is compact, wideband, and has high directivity and a high front-to-back ratio. The idea of the antenna is based on loading a microstrip narrow band patch antenna resonating at 4.5 GHz by a single layer metasurface superstrate and with a toroidal metal structure. The metasurface superstrate comprises a periodic array of square patch cells. Compared to conventional microstrip antenna, the front to back lobe ratio is increased from 7 to 20 dB and the directivity is increased by 7 dB. Also, the antenna impedance bandwidth is 34% which is increased four times. This is the first-ever antenna with enhanced bandwidth and directivity using a single layer of metasurface and that too made up of periodic cell array and has application as an energy harvester.

The paper has proposed a multilayer, polarization rotation featured, low radar cross-section (RCS) antenna using electromagnetic band-gap (EBG)-based frequency selective surface (FSS) at 8.25 GHz. Cross-shaped EBG unit cells offer zero reflection phase and −25 dB reflection magnitude at 8.25 GHz. The FSS layer consists of eight cross-shaped EBG unit cells sandwiched between two substrates to offer high absorptivity at the desired band. The circular patch antenna resonating at 8.25 GHz is placed on the top substrate having a lower dielectric constant. Four circular-shaped patches are etched at the four corners of the top layer and are coupled with two feed lines which are aligned 90° to each other at the bottom layer and interconnected diagonally to achieve polarization rotation. The proposed antenna offers a gain of 6.72 dB and an in-band RCS of −21.4 dBsm. Incident energy is backscattered into eight directions separated by angle ϕ = 45°. The proposed antenna has the RCS reduction band of 7.7–9.4 GHz. It offers normalized polarization rotation ratio more than 0.8 within the −40° to 40° angular region at the frequency band 8–8.5 GHz. The measured result using the fabricated prototype agrees well with the simulated one.

To solve the problems of low gain, narrow bandwidth, and poor radiation directivity of conventional ground penetrating radar antenna, this paper proposes an ultra-wideband and high-gain antipodal tapered slot antenna (ATSA) with planar metamaterial lens. As a constituent part of this lens, a new non-resonant metamaterial unit cell is introduced and analyzed by the full-wave simulation tool. The single-layer planar lens composed of the designed unit cells with different sizes is placed in the maximum radiation direction of the ATSA to greatly enhance its radiation capability. The proposed planar lens antenna has a wide impedance bandwidth of 107.4% (2.41–8 GHz) and −3 dB gain bandwidth of 54.5% (4–7 GHz), respectively. The gain increases averagely by 6.0 dB in the whole operating frequency band, and the peck gain reaches 15.4 dBi at 5.5 GHz. And its excellent performance shows a high application prospect in ground penetrating radar and microwave imaging system.

This paper presents wideband split-ring antenna arrays based on substrate integrated waveguide (SIW) for Ka-band (26.5–40 GHz) applications. The antenna array is fed by a 2.92 mm coaxial connector (K-connector) and the power is equally distributed to each split-ring resonator. The designed coplanar waveguide (CPW), SIW, CPW-to-SIW transition, coaxial-to-CPW transition, and two-stage SIW power divider are described in detail. By using a thin Rogers 6002 substrate with silver epoxy-filled vias, a transition prototype is designed, fabricated, and tested in a back-to-back configuration. A wideband split-ring resonator is developed as a single element and four possible arrangements of antenna arrays are introduced. By combining the designed components and routing paths, two full layouts of the antenna arrays with four split-ring resonators are addressed. As a demonstrator, a 2×2 antenna array prototype in a compact format is designed, fabricated, and tested. The fabricated antenna array achieves a measured directivity of 15.0 dBi with a fractional bandwidth of 23.0% centered at 30.5 GHz.