In this paper, a ultra-wideband (UWB) bandpass filter with stopband characteristics is presented using a multi-mode resonator (MMR) technique. An MMR is formed by loading three dumbbell-shaped (Mickey and circular) shunt stubs placed in the center and two symmetrical locations from ports, respectively. Three circular and arrowhead defected ground structures on the ground plane are introduced to achieve UWB bandwidth with a better roll-off rate. The proposed filter exhibits stopband characteristics from 10.8 to 20 GHz with a 0.4 dB return loss. The group delay and roll-off rate of the designed filter are <0.30 ns in the passband and 16 dB/GHz at lower and higher cut-off frequencies, respectively. The dimension of the filter is 0.74λg × 0.67λg mm2 and was fabricated on a cost-effective substrate. All simulated results are verified through the experimental results.

This paper presents a compact multilayer substrate integrated waveguide (SIW) bandpass filter with wide stopband. The square SIW cavity resonators in multilayer substrates are horizontally and vertically coupled with magnetic coupling. By properly designing the coupling structure, the couplings between the higher-order resonant modes can be suppressed. Compared with the conventional single-layer SIW filters, the proposed multilayer SIW filter also exhibits a compact size. To verify the design concept, a double-layered SIW filter is fabricated and measured. The measured results agree well with the simulations. The measured results show that the upper stopband of the filter is extended to 2.73 times of the center frequency 8.02 GHz.

In this article, we present a simple, low-cost solution for the gain enhancement of a conventional pyramidal horn antenna using additive manufacturing. A flat, metamaterial lens consisting of three-layer metallic grid wire is implemented at the aperture of the horn. The lens is separated into two regions; namely epsilon-positive and epsilon-near-zero (ENZ) regions. The structure of the ENZ region is constructed accounting the variation of relative permittivity in the metamaterial. By the phase compensation property imparted by the metamaterial lens, more focused beams are obtained. The simulated and measured results clearly demonstrate that the metamaterial lens enhances the gain over an ultra-wide frequency band (10–18 GHz) compared to the conventional horn with the same physical size. A simple fabrication process using a 3D printer is introduced, and has been successfully applied. This result represents a remarkable achievement in this field, and may enable a comprehensive solution for satellite and radar systems as a high gain, compact, light-weighted, broadband radiator.

A significant excitation of toroidal moments in cylindrically arranged dogbone metallic inclusions is validated and presented in this paper. The antiparallel poloidal currents excited on the front and back faces of the proposed cylindrical dogbone inclusions create strong magnetic field confinement at the center generating intense toroidal moments on the structure. The significant excitation of toroidal dipole moment causes an improvement in the scattering cross-section from the resonant system. The resonant mechanism is analyzed using the multipole scattering theory, and we used the scattering measurement techniques to characterize the structure experimentally in the microwave regime.

A frequency diverse array (FDA) radar has attracted wide attention, due to its capability to provide a range-angle dependent beampattern and to scan the spatial without phase shifters or rotating arrays. However, FDA systems will suffer from low echo signal energy or high sidelobe peaks when detecting targets by beamforming based on existing receivers. To reduce the sidelobe peak of detection results while increasing the echo signal energy, in this paper, we propose an FDA radar system based on diversity linear frequency modulation waveforms. Correspondingly, we propose a receiver architecture with a time-variant beamforming chain. The proposed system retains the ability of the FDA system to automatically spatial beam scanning, owing to the frequency increment across elements. By increasing the pulse duration of transmitted signals, we enhance the echo signal energy. By applying the artificial bee colony algorithm to design the bandwidth of each chirp signal, the proposed system reduces the sidelobe level of detection results while increasing pulse width. Numerical simulation results are presented to demonstrate the effectiveness of the proposed system.

The growing need of the compact and portable antennas with high speed and low latency wireless communication is the present and future demand of the voice over Internet protocol, on-demand bandwidth, and multimedia applications. Fifth-generation (5G) covers certain low-frequency bands under 6 GHz spectrum, and most of the high-frequency bands under 60 GHz. 5G is the part of the millimeter wave spectrum (30–300 GHz) and is introduced to overcome the problem of spectrum shortage due to exponential enhancement of wireless applications in industry, medical, airborne, radar, satellite, and research fields. The International Telecommunication Union's objective of wireless communications promises to provide higher data rates up to 10 Gbps for 5G mobile users and connectivity to the artificial intelligence devices, along with high spectral efficiencies and enhanced coverage. The users for the 5G require around 5 and 50 Gbps of data rates for low and high mobility, respectively. Beamforming in 5G is the modern powerful technique for the coverage of the intended user/direction using the narrow beam width radiation patterns. A brief survey on 5G beamforming techniques, i.e. analog, digital, hybrid, switched, and adaptive etc. and its types, working algorithms, design of compact antennas, gain, and size/type of the substrates is carried out in this paper. The study of the hybrid coupler, branchline coupler, Wilkinson power divider, and Butler matrix in beamforming is required for 5G smart antennas. Different beam widths like ±15, ±35, ±45, and ±55° etc. are produced for the intended directions using a variety of beamforming techniques. From lower to higher frequency band beamforming applications with Roger's Duroid 4003/4350/5880, tectonic, and aluminum oxide dielectric substrates are discussed here. Various beamforming techniques with their merits, demerits, and applications are included in the paper for the knowledge extension of the beamforming antenna designers and research community.

A single-layer ultra-wideband (UWB) stop-band frequency selective surface (FSS) has several advantages in wireless systems, including a simple design, low debugging complexity, and an appropriate thickness. This study proposes a miniaturized UWB stop-band FSS design. The proposed FSS structure consists of a square-loop and metalized vias that are arranged on a single layer substrate; it has an excellent angle and polarization-independent characteristics. At an incident angle of 60°, the polarization response frequencies of the transverse electric and magnetic modes only shifted by 0.003 f0 and 0.007 f0, respectively. The equivalent circuit models of the square-loop and metallized vias structure are analysed and the accuracy of the calculation is evaluated by comparing the electromagnetic simulation. The 20 × 20 array constitutes an FSS reflector with a unit size of 4.2 mm × 4.2 mm (less than one-twentieth of the wavelength of 3 GHz), which realizes an UWB quasi-constant gain enhancement (in-band flatness is <0.5 dB). Finally, the simulation results were verified through sample processing and measurement; consistent results were obtained. The FSS miniaturization design method proposed in this study could be applied to the design of passband FSS (complementary structure), antennas and filters, among other applications.

In this paper, a backfire suppressed aperture coupled circularly polarized (CP) stacked patch antenna for universal ultra-high frequency (UHF) radio frequency identification applications is presented. Cross-polarized backfire radiation patterns were successfully suppressed by a planar high impedance surface (HIS) reflector. The size of the fabricated antenna is 250 × 250 × 26.9 mm3 (0.71λ0 × 0.71λ0 × 0.076λ0) and its peak gain value of 7.1 dBi is measured. The distance between the antenna and the HIS reflector is only 4.8 mm (0.014 λ0). The HIS reflector suppressed cross-pol backfire radiations by about 10 dB. Detailed antenna and HIS reflector design are discussed thoroughly in this paper. The presented backfire suppression technique using the HIS reflector is scalable to other applications and frequency bands. This paper demonstrates the feasibility of the HIS structure at UHF band.

Substrate-integrated waveguide (SIW) technology has recently drawn attention to its benefits in the microwave field, such as integration in planar microwave circuits, low manufacturing cost, and high-quality factor compared to other technologies. In this paper, a broadband and high gain SIW cavity-backed L-shaped slot antenna structure has been designed and made for X-band applications. Three pairs of L-shaped half-wave resonators are placed on the lower wall of the cavity (backed-slots) to further expand bandwidth and improve gain. The final antenna designed operates on a band ranging from 9.4 to 10.5 GHz with a bandwidth of 11%. Moreover, the gain reaches a value of 9.5 dBi. The final antenna is realized on a Rogers RT/Duroid 5870 substrate. The gain, the reflection coefficient, and the radiation patterns are measured and compared to the EM simulation results and a very good agreement is obtained. The proposed cavity-backed L-shaped slot antenna gives a good compromise between a high gain and a large bandwidth.

In this article, a dynamically switchable ultra-wideband (UWB) planar monopole antenna employing defected ground structure (DGS) with a folded stepped impedance resonator (SIR) that can operate as either a UWB mode or the single band-notched mode is introduced. The UWB monopole antenna contains a novel whirligig-shaped radiating patch and a chambered conductor as a partial ground plane. The switchable UWB antenna uses one PIN diode as switching elements in the DGS-SIR structure without any biasing network. When the state of diode is OFF, the planar monopole antenna changes to the UWB mode, and when the diode is turned ON, a frequency notch is created at 5–6 GHz. The state of diode is set to the “ON” state dynamically in the presence of a 5–6 GHz RF signal that is detected by using a wireless power management unit (PMU) that contains a broadband rectenna and a DC-DC passive booster. The rectenna consists of a novel cypress-shaped monopole antenna as a signal receiving part and two sub-rectifiers which are connected to a 3 dB branch-line coupler with a grounded isolation port. The antenna switches from UWB to single band-notched when an RF input signal (≥8.5 dBm) in the 5.25 GHz is sensed by the RF PMU with a conversion efficiency of 26% and DC output voltage of 0.36 V, and it fades immediately in real time when the external RF signal is eliminated. In the three-tone signals, the efficiency and input signal improvements are about 10% and −5.5 dBm in the low-power levels, especially, and so develop and enhance the performance of the dynamic reconfigurability.

Achieving robust and fast two-dimensional adaptive beamforming of phased array antennas is a challenging problem due to its high-computational complexity. To address this problem, a deep-learning-based beamforming method is presented in this paper. In particular, the optimum weight vector is computed by modeling the problem as a convolutional neural network (CNN), which is trained with I/O pairs obtained from the optimum Wiener solution. In order to exhibit the robustness of the new technique, it is applied on an 8 × 8 phased array antenna and compared with a shallow (non-deep) neural network namely, radial basis function neural network. The results reveal that the CNN leads to nearly optimal Wiener weights even in the presence of array imperfections.

A dual-band stacked antenna integrated with a rescue helmet is proposed for WLAN applications. The cross slot aperture feeding technique enables the antenna to support dual-band operation at 2.4 and 5 GHz. On the ground plane, two slots are formed, perpendicular to each other and of different lengths. Four dielectric layers of different permittivity are stacked over this cross slot to attain the desired frequency bands. Under free space condition, the radiator yields 7% (2.31–2.48 GHz), 15.87% (5.12–6 GHz) 10 dB return loss bandwidth (BW). The use of low loss dielectric layers on the slots also provides a high antenna efficiency and moderate gain at both frequency bands. The small dimension of antenna encourages its use as a wearable helmet antenna. Antenna performances are observed under wearable condition. For assessing human exposure to RF electromagnetic fields, SAR evaluations are conducted at both 2.4 and 5 GHz WLAN bands.