In this study, a coplanar waveguide-fed reconfigurable antenna using crescent-shaped fractal geometry is presented. The frequency reconfigurable approach is obtained using radio-frequency positive intrinsic negative diodes, resistor, and inductors. The proposed approach has successfully allowed reconfigurable switching up to eight frequency bands between 1.46 and 6.15 GHz. Good results have been obtained in terms of stable and omnidirectional radiation patterns. The realised gains of the antenna system, in these frequency bands, vary from 0.52 to 5.67 dBi. The designed antenna has the advantages of a multiband structure and compact size over the previously reported structures. The proposed antenna is suitable for future wireless application systems.

The spatial division represents an efficient approach to increasing data throughput in wireless networks. It can be implemented by spanning the channel in spatial domain with orthogonal beams generated by a Fourier transform (FT) beamforming network and using each beam to simultaneously convey an independent data stream from the base station (BS) to several users. This study presents a feasibility study of a simplified BS front-end architecture based on a spatial discrete FT beamforming network, with the number of the output ports being lower than the number of the elements in the antenna array. It is shown that connecting each output port of the FT network via power dividers to several array elements, thus implementing a ‘sample-and-hold’ phase interpolation, preserves the orthogonality, gain and beam-width in a narrow scanning sector. Therefore, in applications where few orthogonal beams are required to span a narrow spatial sector, the proposed approach can offer a reduction of the hardware complexity. The simulated radiation patterns for a phase interpolation Fourier beamforming network are demonstrated. The measured antenna radiation patterns are reported for the architecture implemented with a Fourier Rotman lens and Wilkinson power dividers feeding a microstrip antenna array, which confirm the simulated trends.

This work presents a four-element, eight-port cylindrical dielectric resonator antenna (cDRA) with multi-directional pattern diversity for wireless access point applications. Initially, a dielectric resonator is fed by two different feeding techniques to create two simultaneous orthogonal patterns. This primary radiator is combined with similar radiators in a cross-configuration to obtain an eight-port multiple-input-multiple-output (MIMO) antenna system. The antenna elements are placed in such a way that each radiator of the proposed antenna can radiate in a different direction so that the field correlation between the antenna elements is minimum. The proposed diversity antenna consists of four antenna elements and eight polarised ports. Isolation between the ports is improved by generating orthogonal modes in each cDRA and exciting each cDRA to radiate in a different direction. The similarity in simulated and measured results confirm that the proposed eight-port diversity antenna is useful for the frequency ranges between 5.6 and 5.9 GHz for wireless local area network (WLAN) applications.

This study analyses the influence of a metasurface coupled to a rectangular microstrip antenna. The proposed metasurface converts linearly polarised electromagnetic waves into circularly polarised electromagnetic waves. The rectangular microstrip antenna is designed to operate at 2.45 GHz. The metasurface is coupled to the antenna in order to provide a structure capable of radiating circularly polarised electromagnetic waves without the need for adjustments to the antenna feed. In this analysis, the metasurface was designed from unit cells, distributed in a periodic arrangement acting as a polariser, when coupled to the upper part of the antenna. The geometry of the unit cells and their distribution in the arrangement were calculated in order to obtain an axial ratio below 3 dB in the design frequency range. Prototypes of the microstrip antenna and the metasurface were constructed to obtain experimental results and it was possible to observe a good convergence between the simulated and measured results.

An in-house full-wave numerical code COCHLEA based on the Finite Difference in Time Domain (FDTD) method has been developed and used to study the dispersion properties of corrugated waveguide structures. The mathematical formulation of the corresponding electromagnetic problem is presented in detail. Numerical results are derived for axially corrugated waveguides similar to those appeared in gyrotron stacked beam tunnels and are compared with those obtained by the in-house semi-analytical numerical code FISHBONE.

This work presents a wideband 60 GHz microstrip patch antenna (MPA) array based on low-temperature co-fired ceramic (LTCC) multilayer technology. Four parasitic patches are stacked above the driven MPA. An additional resonant mode is excited by the proposed parasitic surrounding stacked patches (PSSPs) to enhance the bandwidth of the MPA. The structure of the PSSPs increases the gain of the MPA due to its array configuration. The proposed PSSP structure can find many applications in designs for large-scale antenna arrays because the PSSPs and the driven microstrip patch are distributed at different layers. This arrangement allows a direct feed network to the MPA without the blockage caused by the PSSPs. A 2 × 2 antenna array was designed and fabricated with the PSSPs to further increase its gain. The proposed array can achieve a gain of 10.5 dBi and a wide bandwidth of 27.3% at 60 GHz.

In this work, a marching-on-time solver for time-domain simulation of partial element equivalent circuit (PEEC) models of electromagnetic systems is presented. The PEEC method is based on the electric field integral equation and the continuity equation. It describes separate magnetic and electric field couplings in terms of partial inductances and coefficients of potential. When the propagation delay is taken into account, the enforcement of Kirchhoff current and voltage laws results in a set of delayed differential equations. Typically, in the PEEC method, currents and charges are expanded with rectangular basis functions in both time and space. In this work, higher order basis functions are used to expand currents and charges in time. The resulting TD-PEEC solver is tested by comparison with other solvers that operate in both the time- and the frequency-domain and exhibits a satisfactory accuracy and better stability properties.

Wireless sensor networks (WSNs) consist of nodes with a limited power source. Reducing the energy consumption is an effective way to extend the lifespan of the sensor node. In this contribution, a new approach of designing of a new class radio frequency sensor–antenna for low energy consumption and low-cost wireless sensor nodes is proposed. Unlike the architecture of a conventional wireless sensor node, the proposed approach does not require any processor device that consumes power. It consists of integrating a capacitive sensor in a narrowband antenna that can send raw information, through its operating frequency which can be tuned depending on the value of the physical parameter to measure. As a result, a corresponding table between the resonant frequency of the sensor–antenna and the measured physical parameter is obtained. Furthermore, by using a dual-band antenna, the proposed configuration is able to measure, simultaneously, two physical parameters. A prototype of the sensor–antenna has been simulated and its behaviour has been validated with measurements. From the obtained results, it can be noted that the use of the proposed sensor–antenna can be a good alternative to reduce considerably the complexity and hence the cost of wireless sensor nodes for WSN applications.

The authors have studied analytically by simulation and by experiment the impact of a conducting medium on the mutual inductance between two coils, in particular as related to the attenuation of magneto-inductive (MI) waves. To illustrate the physics, the distributions of both the magnetic field and the Poynting vector are determined. They show that the plane wave approach used in the literature for the theoretical description of MI attenuation has only limited validity. It is further found that the mutual inductance becomes a complex quantity, its modulus declining monotonically as a function of conductivity or medium thickness. Their results will be relevant for the design and optimisation of MI waveguide links in conducting media, in general, and particularly when the attenuation is caused by soil conductivity. The results can also be useful for practical applications including in vivo communication and wireless power transfer for medical implants.

This study presents a tunable dual-band gallium nitride (GaN) power amplifier (PA) operating in L-band. The first band is aimed near the lower edge of the L-band, 1 GHz, and the second band is aimed near the upper edge of the L-band, 2 GHz, which is located around the second harmonic of the first band. A pin diode-based tunable load matching circuit is proposed and designed in order to present the optimum fundamental and harmonic load impedances to the transistor in both operating bands for maximum efficiency and output power. A method of controlling the tunable load matching circuit according to the input frequency is proposed by the design of a band-selective power detection circuit incorporated into the source matching circuit of the PA. The implemented PA delivers 41.5 dBm output power with 81.3% drain efficiency (DE) at 920 MHz and 41 dBm output power with 60.2% DE at 1720 MHz.

The concept and analysis of a graphene plasmonic nanoantenna over a SiO₂/Si substrate at terahertz (THz) band are presented. The performance enhancement of the antenna is proposed by dynamically controlling the surface conductivity of graphene using an electric field effect. The controlling ability of graphene via gate voltage enables frequency reconfiguration of the antenna over a wide range of 2.56–4.98 THz. The performance merits of the antenna are its high directivity, broadside radiation pattern, low reflection coefficient, stable impedance and high miniaturisation. The proposed graphene plasmonic antenna has potential to be used for THz communication.

A dual-wideband dual-sense circularly polarised (CP) planar slot antenna with a simple structure is presented and studied in this study. By etching a simple spiral slot on the planar ground, adopting an L-shaped strip as the radiator, and employing smooth transition structure, the dual-wideband impedance bandwidths (IBWs) and dual-wideband 3 dB axial-ratio bandwidths (ARBWs) of the proposed CP antenna can be easily realised. The measured 10 dB return loss IBWs is 111.1% (0.88–3.08 GHz) for the lower band [left-hand CP (LHCP)] and 42.1% (3.64–5.58 GHz) for the upper band [right-hand CP (RHCP)]. The measured 3-dB ARBWs is 92.9 and 39.4%, with respect to 1.83 GHz (LHCP) and 4.6 GHz (RHCP), respectively. The radiation patterns of this antenna are good within both two CP bands. This antenna is very suitable for multi-band communication systems for its dual-wideband CP performance.

In this study, the authors have designed and proposed a monolayer graphene-based metasurface, performing as a broadband polarisation converter over a wide angle of incidence. The unit cell of the metasurface consists of split ring generated over a perforated single layer of graphene grown over metal-backed silicon dioxide. The structure converts incident linearly polarised wave into its cross-polarised component over a wide bandwidth with peak polarisation conversion exhibits value to unity. The plane of polarisation of the incident electromagnetic wave is rotated orthogonally on reflection from the proposed metasurface over full width at half maxima bandwidth around 72.83% compared with centre frequency of 2.16 THz, which can be maintained up to 40° incident angles under both transverse electric and transverse magnetic polarisations. The proposed structure is as thin as ∼λ/7.3 with respect to lowermost frequency and the period of the structure is about λ/10.7. The frequency band of polarisation conversion can be tuned by altering chemical doping as well as the electrostatic grating of the graphene sheet. The structure can find applications toward sensing, spectroscopy etc.

A linear polarisation converter surface with broadband and high-efficiency properties is proposed. The proposed frequency selective surface (FSS) polariser consists of two-corner-cut diagonal rectangular printed on a substrate backed by a pinwheel loop shape ground. The combination of two-corner-cut diagonal rectangular patch and pinwheel loop shape ground contributes to bandwidth expansion. Numerical results indicate that the FSS structure is capable of converting x or y polarised incident wave to the relevant cross-polarised wave in a wide frequency range from 7.5 to 24.5 GHz with a fractional bandwidth of 106% for −10 dB reflection amplitude. In addition, the and reflection coefficients of the proposed unit-cell present slight variation for different incident angles up to 40°, which verifies the applicability of the proposed FSS structure in a practical environment. For experimental validation, a prototype of the FSS structure is fabricated and reflection parameters of the proposed structure are measured which agree well with corresponding simulations. Due to the minimal dependency on the angle of incidence and non-uniform ground plane the proposed polarisation converter unit-cell can be a possible choice for secure building applications.

In the study, a coupled line trans-directional (CL-TRD) coupler that allows for equal complex termination impedances with arbitrary power divisions is presented. It is composed of two pairs of coupled lines with an overall electrical length of 90°, two types of shunt capacitors and three pairs of short-circuited stubs. By adjusting the electrical length of each short-circuited stub, as well as the values of the shunt capacitors, arbitrary equal complex termination impedances and power divisions are achieved. Moreover, since capacitors are shunted between the coupled lines, the proposed CL-TRD coupler has an inherent feature of dc blocking. Design equations for the CL-TRD were derived using even–odd mode decomposition analysis. To validate the theory, several numerical examples extracted from the design procedure were listed. Eventually, two prototypes of the proposed CL-TRD coupler are fabricated, and experiments are carried out. The measured results indicate that the coupler is able to realise arbitrary power divisions for equal complex termination impedance with good matching and isolation.

Wearable e-textile near-field communication (NFC) radio-frequency identification (RFID) antennas fully integrated with garments using embroidery techniques, which enables everyday clothing to become connective to wireless communication systems, is presented. The e-textile wearable antennas have been designed through full electromagnetic wave simulation based on the electrical properties of conductive threads and textile substrates at the high frequency band, allocated for NFC wireless communications. The e-textile wearable NFC antenna performance under mechanical bending as well as human body effects have been experimentally studied and evaluated; the antennas can operate under significantly bending angle and body effects attributed to its broad operating bandwidth. This is highly desirable and distinguished to conventional NFC antennas; the proposed e-textile wearable NFC antennas can be placed almost any place on clothes and still capable to communicate at the desired operating frequency of 13.56 MHz. The maximum read range of the e-textile wearable NFC tags is measured to be around 5.6 cm, being compatible to typical commercially available metallic NFC tags. The e-textile wearable NFC tags can lead to numerous potential applications such as information exchange, personal security, health monitoring and Internet of Things.

The constitutive parameters of metamaterials composed of asymmetrical inclusions are inaccurate when calculated using Maxwell Garnett theory. In this study, the authors discuss some particular cases of metamaterials composed of asymmetrical inclusions to demonstrate this statement. The unit cell single element dyadic is derived numerically using the induced currents by incident plane waves and is computed by point matching method of moments. The composite effective parameters approximated by Maxwell Garnett model were used to obtain the reflection and transmission coefficients from a bulk slab. The derived results were compared to the simulated numerical results with High Frequency Structure Simulator (HFSS) software. The scattering parameters from symmetrical and asymmetrical inclusions were compared and a parametric study was conducted. The scattering parameters are in good agreement when the material under consideration is composed of symmetrical inclusions and hold even for higher frequencies than Maxwell Garnett model theory limits. For asymmetrical inclusions the model provides inaccurate scattering parameters results. This discrepancy is especially pronounced when the illuminating field polarisation coincides with the inclusion asymmetry direction. These observations should be considered when Maxwell Garnett model is used for effective parameters analytical approximation.

This study presents a new design for the substrate integrated waveguide (SIW) long slot leaky wave antenna (LWA) with narrow beam, low side lobe level (SLL) and low cross-polarisation. To meet these specifications, the changes of phase constant, , along the structure are compensated, the required leakage constant, , is determined more accurately by considering the loss and a straight slot along the antenna is used. In order to calculate the leakage constant of the leaky-mode more accurately, thru-reflect-line method along with HFSS simulations is used. By properly determining the shape of SIW walls, the amount of leakage along the slot and the phase constant can be controlled. This results in a desired aperture amplitude and phase distribution. The presented LWA realises a Taylor distribution with a predefined SLL of −25 dB. Such a design also leads to a low cross-polarisation level and narrow beam at centre frequency of 17 GHz. The measurement results are consistent with the simulation results.

In this study, a method of moments (MoM) of electric field integral equation (EFIE) formulation using the loop co-tree basis functions has been implemented, analysed, and applied to the electromagnetic solution of perfect electric conducting objects at low frequency. The EFIE with Rao–Wilton–Glisson (RWG) basis functions suffer from low-frequency instability. Even for an appropriate frequency, the EFIE with RWG basis functions is ill-posed. However, the matrix form of the EFIE with loop co-tree basis functions is well-conditioned when combined with an iterative solver such as a type of Krylov subspace methods. The approach is shown to be very effective for resolving both the low-frequency instability and the ill-condition of the MoM matrix with a generalised minimal residual method.

A radio frequency (RF)-based medical device implanted deep inside a human body can be wirelessly powered using RF energy. However, this method experiences loss in the form of attenuation and absorption, which is caused by the lossy human body. The need for a path loss (PL) model is, therefore, necessary to characterise these losses. This study presents a novel PL method, which is based on the measurements, obtained from a series of experiments on two different Dorset breed ovine models. Also, the uncertainty analysis of the equipment has been performed and the available RF power inside the body at a proposed implant location has been computed. The results are then used to develop and propose an equivalent model for PL in an anechoic chamber. Overall, an approximate PL of 3.5 dB/cm in the ovine body was measured at 1.2 GHz. Moreover, it was concluded that with the inevitable errors in the equipment, the total PL for 6 cm deep implant location was ∼21 dB with an uncertainty of 2.034 dB. The PL model presented is novel and has a great potential for applications in the design and development of RF-based implantable devices, especially leadless pacemakers.