The simultaneous improvement of radiating and scattering performance of an antenna is normally considered as a contradiction. In this study, wideband radar cross-section (RCS) reduction and gain enhancement are both achieved for a conventional patch antenna by loading metamaterial in an improved way. First, two artificial magnetic conductor units are selected to construct a chessboard-like metamaterial surface (CLMS) which has wideband low RCS property. Then the CLMS is applied to a conventional patch antenna for the purpose of RCS reduction. Through a detailed investigation of three loading manners, a metamaterial-based patch antenna with both improved RCS and gain is proposed and experimentally validated. Both simulated and measured results show that, compared with the original antenna, the gain of the proposed antenna is enhanced almost over the whole operation band, and the wideband (in-band and out-of-band) RCS reduction is achieved concurrently.

The cross-polarisation discrimination (XPD) of a ± 45° dual-polarised base station antenna is enhanced by adding vertical parasitic elements. First, it is established that high XPD can be achieved by employing four vertical parasitic elements between a simple ± 45° dual-polarised radiator and ground. Second, based on the proposed method, a simple ± 45° dual-polarised antenna for a long-term evolution 1.71–2.17 GHz base station is designed, fabricated and measured. Compared with the antenna without vertical elements, XPDs are improved by about 7 dB in the horizontal plane due to the addition of the vertical parasitic elements. Third, with a wideband quadrature hybrid, the proposed antenna is shown to radiate circularly polarised waves. In particular, the 3 dB axial ratio beamwidth of the proposed antenna is increased from 84° to 195° with the addition of the four vertical parasitic elements.

Surface impedance boundary condition (SIBC) is a potential way to improve the efficiency of the finite-difference time-domain (FDTD) method. However, it is still seldom used in FDTD simulations, especially for complicated problems. In this study, the authors propose a novel SIBC, the perfect electric conductor (PEC) backed SIBC (PEC–SIBC). It is a combination of SIBC and PEC. This character makes it possible to integrate PEC–SIBC with the conventional FDTD method. The authors derive the updating equations of PEC–SIBC for a one-dimensional (1D), 2D and 3D problems. Then, the authors verify the validity of PEC–SIBC with a 1D example and analyse the complexity with a 2D example. The comparison for a 1D configuration indicates that PEC–SIBC is a little more accurate than the traditional SIBC. For a 2D example, the SIBC is used to replace a lossy dielectric medium located in the middle of the problem domain. The complexity analysis indicates that PEC–SIBC is much easier and more practical to use than the traditional SIBC.

Two empirical indoor-to-outdoor path loss models to facilitate femtocell network deployment are derived from continuous wave power measurements. A large set of indoor–outdoor transmitter locations in two residential streets in an urban setting and operating at 900 MHz, 2 GHz, 2.5 GHz and 3.5 GHz have been used to derive the model parameters by using singular value decomposition (SVD). The path loss models have been compared and validated against existing models as well as independent measurement data and good comparison is shown. The root mean square error of the residual path loss data obtained from the measurement data, which directly relates to the channel shadowing characteristics, is compared and validated with known results and has led to new model parameters being proposed. The expressions derived from the modelling can be used in system-level simulators, as well as for shadowing interference analysis of two-tier heterogeneous networks operating in indoor–outdoor scenarios at or close to the operating frequencies considered. In this study, the models extend the operating frequency range compared to related models and introduce SVD as a convenient means of deriving parameters from measured path loss data.

The problem of posture detection is of considerable significance for assisted living (AL). In most cases, radio channel models for wireless body area network (WBANs) are fixed when a specific body posture is considered. To the best of the authors’ knowledge, little work has been done on the reverse body posture information extraction using WBAN radio channel characteristics. This study aims to classify human postures from on-body narrowband wireless channel information. It is demonstrated that by applying the random forest (RF) classification technique, the action of the human body can be detected. The classification error is perfectly acceptable for RF algorithm. Two propagation environments were compared and the results indicate that the classification error is less in the anechoic chamber (21.39%). In summary, this study provides a novel approach to detect human body postures by using body-centric wireless channel information, and will be beneficial for AL.

A passive network possessing an input impedance which is positive real in nature poses difficulties with regard to obtaining an optimal impedance for power amplifiers (PAs) with two uncorrelated frequencies. In this context, this study presents a sub-optimal dual-band matching method via real frequency technique. Theoretical analysis is presented for voltage waveform with four independent variables and boundary conditions are given for not lower than zero region. To increase the possibility of acquiring a matching network, a sub-optimal but broad design space is defined according to acceptable performance. It is more practical to match dual-band or broadband PAs because a single fundamental harmonic impedance point could be corresponding to a set of the second harmonics in the new design space, vice versa. Thus, the sub-optimal solution space is bounded from region-to-region between the required fundamental and second harmonic impedances. An enhanced cost function based on driving point function that could better describe harmonic impedance mismatching degree is also proposed for harmonics control. Finally, a dual-band PA is designed to demonstrate the general matching method. The peak efficiency is about 70% and saturated output power is 41 dBm in both bands.

This study addresses the characterisation of the polarisation states of the re-radiated signal from a natural tree performed in the vicinity around deep signal fades, at 20 GHz. The polarisation pattern method has been used to investigate the effects of depolarisation and to further characterise the changes in the polarisation state of the re-radiated signal occurring as a result of the currents induced in the trunk, branches and leaves, which are significantly influenced by the complex permittivities of the various materials making up the tree. Results presented in this study will explain the signal behaviour observed in the various regions around the tree. The results also show that depolarisation, in addition to absorption and scatter, is an important factor formulating accurate models for propagation in vegetation.

There is a continuing need for reducing size and weight of satellite systems, particularly in light of recent efforts to increase the functional role of small- and nano-satellites (for instance SmallSats and CubeSats). To this end, a family of arrays is presented, demonstrating ultra-wideband operation across the numerous satellite communications frequencies up to the Ku-, Ka-, and millimetre-wave bands. The first design is demonstrated to operate from 3.5–18.5 GHz with VSWR < 2 at broadside, and validated through fabrication of an 8 × 8 prototype. Additional designs operating from 7–37 and 9–49 GHz, both with VSWR < 2.1 at broadside, are verified via simulations only. These designs are optimised for low cost, using printed circuit board fabrication and appropriate materials selection for space application. E- and H-plane scanning patterns are provided. Finally, multi-physics simulations of the array in high-power transmit are conducted. Temperature trends and distribution are given, with the array showing favourable distribution of thermal energy at up to 2 W of RF input power per element, and a peak temperature only 40°C above ambient.

A novel guiding channel that is electromagnetic bandgap (EBG) based is presented. The structure is formed in a two-dimensional slab of a square lattice of multi-layer metallic open square rings. Dispersion diagrams and eigenfield distributions of the in-gap-guided mode are shown. The channel guide offers many advantages such as easy and flexible design for band-select applications while retaining its electromagnetic properties, simple fabrication (no drilling through the substrate) and compatibility with integrated circuit technology. Two EBG waveguide configurations with a feeding structure on a circuit board (PCB) were designed, fabricated and tested. It is shown experimentally that the length of the EBG guide does not impact the insertion loss significantly, which establishes low insertion loss of the guide.

The high-frequency applicability of reflector antennas is limited by the distortions and the errors on the reflector surface. Different techniques can be used to compensate the degradation of the radiation characteristics if the surface distortion profile is known. Here, a fast and accurate iterative method has been proposed to determine the surface distortion profile which is based on a newly developed variational formula. The surface fitting has been done in the general form of B-splines, which is suitable for both local and global distortions. Different types of the distortions have been studied to check the accuracy of the method. It has been shown that for distortion amplitudes lower than half of wavelength, the proposed algorithm converges with a high accuracy. In comparison with the previous works, the proposed algorithm has higher accuracy and faster convergence and it needs a lower number of far-field measurements. Despite the microwave holography, in the proposed method there is no need to compute the aperture fields.

In this study, a two-element multiple-input–multiple-output (MIMO) antenna with heptaband characteristics is proposed. The total size of the proposed antenna is 82 × 40 × 0.8 mm³. The geometry of the radiating element is derived from a hybrid Quadric–Koch island fractal for size miniaturisation. The antenna operates at the bands of 0.95–1.02 GHz (GSM 900), 1.73–1.79 GHz (GSM 1800), 2.68–2.85 GHz (LTE-A), 3.66–3.7 GHz [ultra-wideband (UWB)], 4.20–4.40 GHz (UWB), 5.50–5.65 GHz [worldwide interoperability for microwave access/high-performance radio local area network] and 5.93–6.13 GHz (UWB). The spacing between the two radiating elements is maintained at 5 mm. The proposed heptaband swastik arm MIMO antenna exhibits an S ₁₁ ≤ −6 dB and S ₂₁ ≤ −17 dB over the entire bands of operation. It provides high isolation without requiring any additional structures. The antenna also offers stable radiation characteristics with circular polarisation at the operating bands of 3.66–3.7 GHz and 5.93–6.13 GHz and linear polarisation at 0.95–1.02 GHz, 1.73–1.79 GHz, 2.68–2.85 GHz, 4.20–4.40 GHz and 5.50–5.65 GHz bands. The diversity performance of the antenna is studied in terms of the envelope correlation coefficient which is <0.05, and capacity loss which is <0.5 bits/s/Hz at all the operating frequencies, making it suitable for use in fifth generation communication systems incorporating the existing bands.

This study presents eighth mode substrate integrated waveguide (EMSIW)-based dual band resonator antennas with two different types of slots etched on the top patch surface. Two types of slot structures have been discussed. This antenna resonates at dual band frequencies of 2.4 and 5.8 GHz. The proposed structure has been fabricated and the simulated and measured results seem to be in good agreement. Different variations of this slot have been studied and the simulated results have been presented. The second proposed design involves a triangular complementary triangular split ring resonator slot etched on the EMSIW. This structure operates below the original cut-off frequency of the EMSIW at 1.8 GHz. The simulated and the measured results are shown to be in good agreement.

Correlation processing has previously been used in radio communication systems to measure and characterise multipath environments, but it can equally well be applied to the elimination of multipath effects. The authors review the historical development of the technique in the context of its earlier use, and the background to its implementation as a method of eliminating or at least substantially reducing, the effects of multipath in antenna ranges. Correlation processing, which can be implemented either as a one-step, real-time process or used as a post-measurement option, also suppresses the effects of coherent and non-coherent interfering signals, so that the accuracy of antenna pattern measurement is greatly improved. At centimetre and millimetre wavelengths, it is possible to make accurate measurements in a normal laboratory environment without the need for an anechoic chamber. A simple proof-of-concept practical system is described, and example results are presented to demonstrate the validity of the technique and to compare it with time gating.

In this study, the authors propose a time-modulated frequency diverse array (FDA) by combining time-modulated array (TMA) and FDA for physical-layer security. The proposed scheme has two-folds: first, the information codes are specifically designed for the FDA elements switching alternatively, which produces a specified directional pattern projecting the phase-modulation signal as a constellation in the desired range–angle section while distorting in other areas. Second, a switching scheme is adapted for the TMA according to the spreading sequence associated with the transmitted signal and the desired receiver position. In such a fashion, only the receiver located at the pre-specified range–angle position can demodulate the received signal successfully but the distorted signals radiated to other undesired range–angle positions will be difficult to demodulate by the eavesdroppers even if with the knowledge of the spread sequence. Numerical results show that, due to the range–angle-dependent characteristics, the proposed method offers more secure transmission performance and outperforms the traditional phased-array-based directional modulation method.

Slot cut variations of shorted sectoral microstrip antennas are proposed which yields bandwidth of more than 900 MHz (>75%). Formulation in resonant length at modified shorted patch modes is presented. Frequencies calculated using them show close agreement with simulated results. Using proposed formulations, procedure to design slot cut shorted sectoral antennas at different frequency is presented, which gives wideband response. Thus, proposed work gives guidelines to design shorted compact antennas at any given frequency.

This study addresses the modelling of a dual band (28 and 38 GHz), circularly polarised slotted-patch-antenna for highly demanded millimetre wave multi-input multi-output (MIMO)-systems in fifth generation (5G) networks. A computer-aided-design model is derived by means of an artificial neural network (ANN) which allows obtaining the physical dimensions of a single-fed antenna, satisfying both near- and far-field goals, without resorting to time-consuming electromagnetic simulation. This mathematical model can be implemented in any CAD-tool, as demonstrated within the framework of advanced design system. This allows, for the first time, to carry out optimisations of strategic importance for future 5G non-linear-radiating-systems, especially operating at millimetre wave, directly addressing their far-field behaviour. The model performance is validated by some examples and measurement results. A further important advantage of this approach is that the trained ANN-model can be further adopted to fast, but accurately, investigate the complex relationships between antenna layout and its near-field and far-field performance, such as the resonance conditions and the polarisation behaviour. Indeed arbitrary orthogonal-polarisations (LHCP/RHCP) have been achieved by the aid of the ANN-model of the same topology. This result can be adopted to implement a combination of two independent radiation patterns for the antenna pair: this feature is attractive for MIMO applications. This is confirmed by measurements showing antenna-coupling reduction with the MIMO-array exploiting polarisation diversity.

In this study, a methodology is proposed for extending the high-efficiency power range (HEPR) of symmetrical Doherty power amplifiers (DPAs). This method takes advantage of the output impedance of peaking stage. Traditional symmetrical DPAs maintain high efficiency over a 6 dB power range with an assumption of the output impedance of peaking stage being infinity. This study reveals that the HEPR can be further enhanced by tuning the output impedance of peaking stage away from infinity and changing the phase delay of the impedance inverter after carrier power amplifier. A 2.2–2.3 GHz symmetrical DPA with an HEPR of 9 dB is fabricated to validate our theory. The experimental results possess drain efficiencies of 45.3–49.5% at 9 dB output power back-off level and 62.9–71% at peaking power level over 2.2–2.3 GHz. The maximum output power of the fabricated DPA is 45 dBm with a gain of 11.6–13.5 dB across 2.2–2.3 GHz.

Analogies with physical phenomena indicate that modulation of a material or geometrical parameter of a periodic structure enriches its original band structure. The present work aims to provide an insight into the band-splitting phenomenon in the case of commensurate modulation for a parallel-plate waveguide technology-based geometry. A modulated one-dimensional parallel-plate waveguide signal integrity structure is numerically analysed to exhibit the appearance of band splitting and new bandgaps. The modulation mechanism has a potential in dispersion engineering, as it allows controlling the number and position of the electromagnetic bandgaps and the in-band characteristics of the field propagation. Generation of modes with negative group velocities for a given frequency band is also achievable by this technique.

This study examines the design, realisation and evaluation of a lightweight and low cost hollow oval cross-section torso phantom appropriate for wearable antenna performance assessment. The phantom consists of an empty inner space (hollow) surrounded by a shell with double plastic walls between which there is a tissue simulating liquid. The phantom's plastic shell is made of a low loss cast acrylic and the liquid is a commercially available one with properties calibrated for the frequency range of 2–6 GHz. The proposed phantom is compared, through simulations, with a full liquid torso phantom and a heterogeneous anthropomorphic voxel phantom. In addition, the fabricated phantom is compared with human bodies and a homogeneous anthropomorphic solid phantom, through measurements. The phantom performance is tested in terms of electric field distribution of a wearable antenna on its surface and the path loss between two wearable antennas, on either side of the phantom. It is proved that the hollow phantom performance approximates the full liquid phantom when an RF absorbing material is placed in the central hollow region. The phantom performance in terms of S ₁₁ wearable antenna measurements is evaluated and found in good agreement with real human bodies in the examined frequency range (2–6 GHz). The far-field wearable antenna performance of the proposed phantom shows deviation in gain <1.5 dB, compared with anthropomorphic phantom.

In this study, a mixed split quadrilateral multiband microstrip antenna is designed and its measurement results in terms of different parameters are given. The proposed antenna is fed by a 50 Ω microstrip line. FR-4 (Lossy) is used as a substrate to design the recommended antenna which has a condensed structure of 0.23λ × 0.31λ. This antenna operates at C-, X-, Ku-, and K-band with moderate bandwidth because of its design and feedline. This patch antenna is designed to support models with resonances at 6.30, 7.10, 7.50, 8.70, 12.80, 17.0, and 21.30 GHz, respectively. Regarding the proposed split multiband quadrilateral antenna, results are obtained in terms of reflection coefficient, gain and radiation pattern which have admissible values of reflection coefficient of <−10 dB at each resonant frequency and gain >3.0 dB. It has a reflection coefficient is −20.15 dBi and an efficiency of 86%. The prospective antenna has a compact size, good radiation characteristics and good time domain behaviour to mollify the obligation of the modern communication system.