In this study, surface impedance boundary condition (SIBC) incorporated in conformal finite-difference time-domain (CFDTD) to efficiently simulate the thin conductive layer of periodic structure is proposed. The method is based on the vector fitting (VF) technique to approximate the frequency-dependent surface impedance of the thin conductive layer by a series of partial fractions in terms of real or complex conjugate pole-residue pairs. A discrete-time equation of SIBC is then derived and incorporated in CFDTD update equations by Laplace transform and time central difference methods. The proposed method takes the following advantages, (i) the time step is no longer bounded by the fine resolution of the thin layer; (ii) The complicated recursive convolution approach is avoided, and the approximation of the frequency-dependent surface impedance of the thin conductive layer can be easy to handle with the VF method. This method is numerically verified for the time- and frequency-dependent scattering characteristics of several periodic structures behaved under normal and oblique incident waves.

The synthesis of a simple electronically reconfigurable annular ring monopole antenna using a designing optimisation process based on particle swarm optimisation (PSO) and artificial bee colony (ABC) algorithms is proposed. Several antenna dimensions are selected as objective functions of PSO and ABC and their best solutions are considered as the optimised dimensions of the antenna geometry. Two radio-frequency p–i–n diodes, connecting the antenna feeding line to two microstrip stubs are used to change the antenna frequency response from ultra-wideband (UWB), from 3.1 to 10.6 GHz, to narrowband (NB) operation about 5.8 GHz. The antenna design and simulation are performed using Ansoft high-frequency structure simulator software. PSO and ABC algorithms are written in Java language. Thereafter, antenna prototypes are fabricated and measured for validation purposes. Simulation and measurement results are obtained showing good agreement. The measured optimised impedance bandwidths of the UWB and NB bands are up to 128 and 23%, respectively. Additionally, simulated and measured radiation patterns are very similar when the reconfigurable antenna is operating in OFF-state (UWB sensing antenna) and in ON-state (NB transmitting antenna) modes, indicating the proposed antenna geometry as a promising candidate for applications such as UWB and cognitive radio.

A shared aperture artificial composite material (SA-ACM) with broadband absorption and partial reflection feature is proposed. The SA-ACM is composed of artificial composite material absorber (ACMA) and partial reflection surface (PRS), where the ACMA and PRS share the same aperture in the vertical dimensionality. The ACMA consists of quasi-three-dimensional fractal tree loaded with lumped resistances and metallic pattern on the bottom side of a dielectric slab, and the PRS consists of etched parallel slots in a metallic plane. The SA-ACM as the role of superstrate is applied to the mircrostrip antenna. The Fabry–Perot resonator cavity constructed by the PRS and the metallic ground of the mircrostrip antenna can achieve high gain, while the ACMA can obtain the low radar cross section (RCS) characteristic by absorbing the incident wave. Simulation and experiment results demonstrate that the gain of the antenna with SA-ACM is enhanced over 3 dB in the operation frequency band, and the backscattering RCS is obviously reduced by 10 dB in a wide frequency range.

A novel feed system for circular arrays enabling beam steering over 360° in the azimuth plane with a stable scanned beam is proposed. It consists of rotating and fixed parts. The rotating part is a two-dimensional offset fed parabolic reflector which is inside a circular parallel plate waveguide. The power coming from the rotating part is coupled with the elements of the circular array through a fixed coupling structure. The rotating part is light and its moment of inertia is significantly less than the conventional rotating antenna systems for beam scanning. Therefore, the beam can be steered much faster than the current rotating antenna systems. The details of the structure including the rotating and the fixed parts are presented and a design procedure for the fixed coupling section is suggested. The performance of the feed system is investigated by simulations and measurements. It is shown that the simulations and experimental results are in good agreement.

This study presents a multiple-input–multiple-output (MIMO) antenna for compact small-cell base stations operating in the long-term evolution (LTE) standard. This MIMO antenna comprises four unit antenna elements, targeted at the frequency band of LTE700/850/900 (698–960 MHz). In comparison with the state-of-the-art MIMO antennas covering these frequency bands, the proposed design enables the maximum number of antenna elements, yet it occupies a compact volume of only π × 80² × 50 mm³ (π × 0.18² × 0.11 λ ³), which is the most miniature electrical configuration to date. The unit antenna element consists of a monopole antenna with a sleeve attached to each side, fed by a coaxial cable. Since the spacing between antenna elements is so small (0.24 λ), this work uses four miniature and wideband defected ground structures (DGSs) to reduce the mutual coupling; moreover, although the DGSs adversely alter the impedance matching of unit element, the authors applied multi-objective fractional factorial design to achieve isolation enhancement and bandwidth preservation simultaneously. Accordingly, the optimised MIMO antenna shows desired –10 dB impedance bandwidth and sufficient isolation in the targeted frequency band. In addition, the radiation patterns, antenna efficiency, and the signal correlation coefficients are measured and evaluated.

A novel ultrawideband pattern diversity antenna, with sum and difference radiation patterns used in monopulse tracking, is proposed. To achieve pattern diversity, a crescent-shaped radiating element is fed from two sides at an angle close to 90°, by two coaxial feeds. The ground is a corner-shaped cavity. The sum and difference patterns can be obtained by in-phase and out-of-phase excitations of the two coaxial ports. A single-element antenna is first studied to show the operating principle of the design. A modified design with two elements is then proposed and studied through simulations and measurements of a fabricated prototype, showing excellent radiation characteristics over a large frequency bandwidth. The application of this antenna as a reflector feed is then studied. It is shown that the antenna can theoretically feed a prime focus reflector with 60% (for the sum pattern) efficiency over a 48% fractional bandwidth (5.5–9 GHz), with a maximum efficiency of 75%. The antenna is compact (5 × 4.1 × 2 cm³) and has low cost and simple fabrication. To the best of authors’ knowledge, this is the first attempt to feed a reflector with this class of antennas.

In this study, a modified antipodal Vivaldi antenna (AVA) with low cross-polarisation is proposed for ultra-wideband communications. The bandwidth offered by conventional single petal AVA is enhanced by adding another petal. This dual petal antenna occupies a small volume of 60 × 60 × 0.8 mm³ and provides operating bandwidth from 2.4 GHz to frequencies >14 GHz. The proposed antenna configuration provides low cross-polarisation level which is <−15 dB over the entire operational band. The designed antenna offers realised gain >5 dB and an average group delay variation of 0.5 ns. The prototype antenna is fabricated and tested to validate the simulation results.

The theoretical study and experimental results of a compact tunable left-handed (LH) zeroth order metamaterial ferrite resonator is presented. The resonator has been designed and realised using a LH coplanar waveguide transmission line on a ferrite substrate. The theoretical analysis of the operation principles of the resonator has been provided. The detailed measurement setup and experimental results are discussed. The resonator is only 0.19 λ _g (65% reduced in size comparing with conventional one) at 4 GHz. The resonator has fractional bandwidth of 0.1% and average insertion loss, approximately 4.5 dB. Moreover, the zeroth order metamaterial ferrite resonator has tuning capability and requires lower DC magnetic bias.

This study presents a two element multiple-input-multiple-output (MIMO) reconfigurable antenna for ultra wideband (UWB) applications. Each individual antenna is composed of a modified square radiating patch. The array can be reconfigured by either placing antenna elements orthogonally for corner installation or by placing them back-to-back for compact three-dimensional (3D) modules. The design is fabricated on FR4 substrate having dimensions 40 mm × 37.5 mm × 1.5 mm. Port isolation greater than 20 dB in the complete band is achieved by introducing an efficient decoupling structure in the ground plane. Performance parameters such as S-parameters, radiation patterns, envelope correlation coefficient, total active reflection coefficient and channel capacity loss indicate that the proposed MIMO design is a suitable candidate for high data rate UWB applications. The proposed solution is suitable for non-planar designs around wall corners or for compact 3D structures where side-by-side placement is not practical due to size constraints.

A split ring loaded monopole antenna structure for triple-band operation is proposed in this study. Multi-resonance is achieved by attaching two rings to a monopole on a circular ground plane. The connected rings improve the directivity and support multiband response. The authors present a discussion on the multi-resonance operation and investigate the antenna characteristics by changing the dimensions of the structure. Results of the parametric analysis are used to determine the effect of changing the dimensions of the antenna on the three resonant frequencies and also arrive at empirical design equations. An equivalent circuit of the antenna is proposed and is used to explain multi-resonance behaviour of the antenna. Simulated and measured performances of the proposed antenna are compared and a good agreement is observed between the two.

Radio-frequency (RF) design techniques for the development of signal-interference microwave bandpass filters (BPFs) with hybridised microstrip and surface-acoustic-wave (SAW) elements are reported. They make use of two original types of transversal filtering sections (TFSs) with embedded one-port SAW resonators. The SAW resonators operate either as non-resonating nodes or as an in-band SAW BPF in their bi-path circuit topologies. These new TFS approaches extend the suitability of signal-interference BPFs to small-fractional-bandwidth (FBW) applications, while offering significant advantages in terms of filtering selectivity and occupied circuit size when compared to traditional solutions for narrow-to-moderate/wide-band specifications. Broader FBWs than those attainable through most of the available all-acoustic-wave BPF configurations are also feasible with them. Moreover, a single type of SAW device is employed in the entire BPF structure, which results in a practical benefit regarding robustness to deviations in the prototype fabrication and assembly processes. The proposed filter design concepts are experimentally verified through the fabrication in microstrip technology and characterisation of two multi-TFS-series-cascade-based high-order BPF prototypes with very-narrow and moderate FBWs.

This study proposes a frequency selective surface (FSS) design to be used in Wi-Fi shielding applications as either a band reject or band pass dual-band single-layer filter. The proposed design consists of a combination of basic elements, that is, ring loops/slots, and is tuned at both 2.4 and 5.2 GHz Wi-Fi frequency bands. It has a relatively stable frequency response in the aforementioned Wi-Fi bands for incidence angles ranging from 0° to 45°. Both band reject and band pass designs are presented, along with their unit cell dimensions. Simulation and model validation through measurements demonstrate the performance of the proposed FSS design. Active variants are also proposed and briefly evaluated, in simulation environment, which should allow for applications where an on–off switching is desired at 2.4 and 5.2 GHz Wi-Fi bands.

This study presents a technique to design antenna arrays with shaped radiation pattern. The efficiency of the proposed technique is demonstrated for the optimisation of a linear microstrip array for radio base stations of mobile communication systems. The main beam is shaped so as to provide uniform distribution of power inside a sector of a cell and to minimise the power radiated above the horizon. The synthesis of the shaped pattern is achieved by a combination of genetic algorithm and sequential quadratic programming. In contrast to several papers found in the literature that consider only isotropic antennas, the proposed approach takes into account all the radiation properties of small arrays, such as the mutual coupling between the array elements and edge effects. Finally, the design of the feeding system to produce the synthesised pattern is discussed. Measurements are used to validate the proposed technique.

Theoretical analysis of a stair-shaped waveguide-fed hemispherical dielectric resonator antenna (DRA) using method of moments (MoM) is presented in this study. Waveguide steps are used to enhance the coupling of direct coupled waveguide-fed DRA. Theoretical analysis is performed by solving coupled integral equations obtained by imposing magnetic field boundary conditions at various apertures using MoM. DRA and the steps are analysed using Green's function. For waveguide, modal analysis is performed. In order to determine the effects of various design parameters, parametric analysis has been carried out using the code developed. Experimental results confirm the validity of the proposed theory. The antenna has a maximum gain of 5.7 dBi at 9.09 GHz with a 3 dB gain bandwidth of 11.08%. Reasonable agreement is observed between the measured and simulated radiation patterns. The proposed coupling enhancement technique guarantees more than 98% coupling.

In this study, a dual-band waveguide filter based on micromachining fabrication process in terahertz is proposed. The proposed filter employs a pair of four-stage rectangular waveguide resonators based on the parallel structure and the centre frequencies of the two passbands are designed at 240 and 300 GHz with the relative bandwidths of 4.2 and 3.3%. The isolation between the two passbands has been achieved over 40 dB. The simulated and measured results are in good agreement and the proposed filter can be applied in the millimetre wave communication systems.

This study proposes a post-distortion linearisation technique for 5 and 60 GHz complementary metal–oxide–semiconductor (CMOS) power amplifiers (PAs). The technique improves the output 1 dB gain compression point (OP_1dB) and power-added efficiency (PAE) of the PA when the lineariser is turned on. The 5 GHz PA that is fabricated in tsmc^TM 0.18 μm CMOS achieves a 16.3 dB gain, a 20 dBm OP_1dB and a 32.6% PAE. The linearised 5 GHz PA improves the OP_1dB and PAE by 2.3 dB and 3.2% as compared to the PA without lineariser. The difference between the OP_1dB and saturated power (P _sat) is <0.2 dB. The 60 GHz PA was implemented in a 90 nm CMOS process with a chip area of 0.57 mm². The PA achieves a 14.8 dB gain, a 16.8 dBm OP_1dB with a 16.3% PAE and a 15 GHz 3 dB bandwidth. The power difference between the OP_1dB and P _sat is <0.3 dB. The linearised 60 GHz PA improves the OP_1dB and PAE by 3.2 dB and 5.8% as compared to the PA without lineariser.