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A high‐gain, low‐cost empty substrate integrated waveguide (ESIW)‐fed circularly polarized (CP) antenna array is demonstrated for millimetre‐wave 5G applications in this letter. The ESIW is formed by metallized FR4 printed circuit boards. By stacking the metallic aluminium z‐shaped radiating slots atop the ESIW feeding network, CP radiation is achieved. The proposed 1 × 8 antenna array offers a high measured gain of 17.8 dBic and high measured radiation efficiency of 89.1%. The measured impedance bandwidth for |S11| ≤ −10 dB and measured axial ratio (AR) bandwidth for AR ≤ 3 of the proposed 1 × 8 array are 11.4% (36.5–40.9 GHz) and 7.8% (37–40 GHz), respectively.

This paper presents a novel method using a coupling matrix to realize dielectric resonator filters with Transmission Zeros (TZs). This method eliminates the necessity of capacitive coupling in conventional DR filter designs with TZs. Using the proposed technique, the cross‐coupling sign can be changed systematically while the amplitude frequency response is kept unchanged. The method is independent of the filter physical structure and all design steps are based on coupling matrix equation and models. Therefore, unlike the extracted pole technique, this method not only does not require time consuming optimization in full wave simulations but also the extra resonator in this method helps in improving the group delay performance of the filter. After presenting the method, the calculated coupling matrix for a 7 × 4 DR filter will be realized using TE01δ mode of cylindrical dielectric resonators and simulation and measurement results will be compared.

Antennas are the building block of radiative hyperthermia (HT) applicators. This study proposes a compact UWB antenna specifically tailored to meet the requirements for deep HT array applicators. The proposed Open Ridged‐Waveguide (ORWG) antenna, which is an adaptation of a double‐ridged horn antenna, operates over the frequency band of 400–800 MHz. It was experimentally assessed as a single element. The quality metrics considered were reflection coefficient, penetration depth, effective field size (EFS), and mutual coupling. The design shows a 75.5% fractional bandwidth with a reflection coefficient measured to be below −10 dB from 367 up to 820 MHz. The EFS is greater than the physical dimensions of the 3‐by‐4 cm aperture. The mutual coupling between two adjacent elements in the array, measured in a flat phantom arrangement, is lower than −30 dB throughout the entire band. The antenna's performance was further tested in two deep HT scenarios in order to assess the mutual coupling and focussing abilities while in the array configuration. To this end, phased array applicators consisting of 10 and 16 ORWG antennas were simulated in CST, and the results are presented for a homogeneous cylindrical muscle phantom and a realistic patient model, respectively. The good agreement between the simulated and measured results suggests that the antenna can be successfully used for HT.

This study presents a new structure for planar aperture antennas with a very simple configuration by making use of gap waveguide technology. Applying the concept of H‐plane horn in both feeding network and radiating layer results in a flat configuration capable of providing a directive radiation with reasonable impedance bandwidth, high gain and low side lobes. The proposed structure introduces a very simple configuration, which does not face the enhancement of design and fabrication complexity for larger apertures and higher gains. The principle of operation and design procedure for both the radiating layer and feed network are fully discussed in detail. A sample is designed to operate at Ku band, which presents proper radiation performance with the maximum gain of 24.5 dBi, the maximum efficiency of 85% and the side lobe level (SLL) lower than −22 and −13 dB in H‐ & E‐planes, respectively.

In this study, a simple technique is proposed to simultaneously achieve dual‐band characteristics and low mutual coupling between two planar inverted‐F antennas (PIFAs), which is one of the most popular antennas used in handset terminals such as mobile phones. The PIFAs initially operate only at a single band at 2.0 GHz (UTMS band). Although several solutions have been proposed to increase the operating frequency band as well as reduce the mutual coupling between multiple‐input multiple‐output antennas, most of them require adjustment of the original antenna size or configuration. This would be disadvantageous if these methods are applied under design conditions that hinder the adjustment of the original antenna. The proposed method uses a combination of linear parasitic elements (LPEs) and a bridge line (BL) and then loads them onto PIFAs. The loading of LPEs increases the operating frequency band at 2.4 GHz, which can help the PIFAs achieve dual‐band operation. Furthermore, the BL transmits opposite‐phase currents to cancel the coupling current between the two PIFAs. Consequently, the mutual coupling can be reduced to less than −10 dB in the dual band without having to adjust the original PIFAs.

In this work, a wide‐angle forward to backward scanning wideband leaky‐wave antenna (LWA) based on the microstrip spoof surface plasmon polaritons (SSPPs) transmission line (TL) is proposed. The proposed microstrip SSPP TL has tapered corrugations on both sides of the metallic strip conductor. After designing an efficient transition with a good transition occupation ratio and excellent transmission efficiency, a wide‐angle scanning LWA is realised. A methodology of achieving high gain and wide‐angle scanning by adjusting a large number of radiating elements in a compact area is proposed. The proposed antenna operates in the frequency range of 8.9–12.7 GHz having fractional bandwidth of 35.18% with a continuous scanning range from −35° to +51°. The maximum gain of 19.03 dBi with a maximum radiation efficiency of 90% is achieved from the structure. The proposed antenna can easily be integrated with planar wireless communication systems due to the presence of an unbroken ground plane.

A reactive matching (RM) power amplifier (PA) can achieve higher efficiency than a distributed amplifier (DA). In this paper, RM PAs that cover 18–40 GHz are proposed by using one‐order and two‐order synthesised transformer networks (STNs) with GaN on Si technology. In this band, the PAs can provide ≥30 dBm and ≥31.9 dBm output power, with power‐added efficiency (PAE) of ≥17% and ≥15% in the continuous mode, respectively. Further, STNs are analysed using the transmission poles method (TPM) instead of the resonant frequency method (RFM). Thus, the limitations of output power and bandwidth are comprehensively derived. In the one‐order STN, the impedance transformation ratio (T1N) has a maximum value (TMAX_1N), limiting the output power. A two‐order STN is proposed, and the impedance transformation ratio maximum (TMAX_2N) can be improved to (TMAX_1N)2. The result of this paper will be helpful for wideband high‐power amplifier applications.

A novel antenna design technique is proposed, which offers bandwidth enhancement up to the limits defined by element radiation efficiency. The employed technique is referred as frequency pulling (FP) as it mimics the ‘insertion loss design methodology of band‐pass filters’. This is essentially a wideband matching approach pushing the antenna efficiency to the limits set up by radiation efficiency. There are three options towards this trend: (i) first to enhance a single element bandwidth (compact element) exploiting its possibly multiple symmetrical feeding points as distinct resonator ports, (ii) frequency pulled array as to design a small antenna array (less than about 10 elements) where each element acts as a resonator and (iii) second order frequency‐pulled array as to build a small array using compact elements of category (i). Similar to the band‐pass filter design, all antennas or distinct‐port circuits resonate at the same resonant frequency when isolated, cascading two or more of them; FP yields to multiple‐overlapping successive resonances in their overall response. Although the proposed technique is general within this first effort, it is applied to simple patch antenna elements exhibiting multiple symmetrical feeding points, namely two—for rectangular, four—for square and five—for pentagonal. The third option is applied to an array of three compact 4‐feeding point square elements offering triple bandwidth with respect to the already wideband single element. However, this is achieved at the expense of a significant beam squint. Thus, in general, these wideband compact elements should be used within a classical array design. Further bandwidth enhancement using FP to antenna elements with inherent multiple resonances as patches with slots or truncated edges constitutes our next task. Their inherent wider bandwidth in radiation efficiency is expected to allow multiply higher bandwidths when exploited with our FP technique.

A switched‐line substrate integrated waveguide 1‐bit phase shifter is designed and measured at 28 GHz. Reconfigurability is achieved through a set of floating metallized vias whose connection to the waveguide top and bottom layers can be controlled using PIN diodes. The reconfigurable vias are used to dynamically create alternative paths having different phase delays. A three‐metal layer printed circuit board stack‐up is employed to fully integrate the diode biasing network and to ease the routing of the bias signal. The high integration of the proposed configuration makes it well suited for the implementation of reconfigurable Butler matrices. The phase shift offset between the two states of this digital 1‐bit phase shifter equals 262° at 28 GHz, leading to a measured figure of merit of 161°/dB, and input 1‐dB compression point over 20 dBm. The total area is 13.72 × 10.9 mm without feeding lines and bias.

Third‐order intermodulation (IM3) is considered as a typical metric to evaluate passive intermodulation (PIM) in electrical connectors, which may be affected by the second‐order effect in the circuit with cascaded non‐linearities. In this study, the effects of second harmonics and second‐order intermodulation on the IM3 powers were theoretically modelled and experimentally verified. Combined with mathematical modelling, an equivalent circuit model of a passive non‐linear source was presented to investigate the effect of second‐order signals on the IM3 powers. A modified non‐linear transfer function was developed by considering multiple non‐linear superpositions in cascaded non‐linearities. Based on the designed two‐tone test, a mathematical expression was provided with the determination of correlation model coefficients. Numerical simulations were performed by using the proposed model, demonstrating the effect of signals induced by the second‐order non‐linearity on the IM3 powers in series passive components. The IM3 powers resulting from cascaded connectors associated with different signal frequencies were predicted. Coaxial connector samples were designed as cascaded PIM sources in circuits, which were experimentally measured in a GSM 900 MHz frequency band. Good agreement was obtained between the predictions and measurements with error of 0.76 dB in average.

Usually, Radar signature of an object is reduced using microwave absorber surface. Recently, another approach based on time‐modulated frequency‐selective surfaces has been developed to deceive Radar detection by selectively generating harmonics/sidebands. Here, a polarization‐insensitive time‐modulated frequency‐selective surface is proposed that works over a broad frequency range. The design employs a PIN diode‐based switchable frequency‐selective surface to switch between the two states of reflection coefficient. Monochromatic incident electromagnetic (EM) wave, upon reflection from the surface is modulated by a periodic time‐varying reflection coefficient to generate several harmonics/sidebands along with the incident frequency. In the frequency spectrum of the reflected EM wave, the power of the incident frequency component is suppressed below that of the sidebands. The time‐modulated frequency selective surface has the capability to suppress the incident frequency component for any incident EM wave in 2–4 GHz that is S band, as well as generate spurious reflection sidebands.

This study reports a class of balanced‐to‐unbalanced (BTU) and balanced‐to‐balanced (BTB) filtering rat‐race couplers (FRCs) based on multilayer substrate integrated waveguide (SIW) cavities. Input and output ports can perform the functions of balanced and unbalanced terminals to implement various operating states. The theoretical designs consisting of the mixed‐mode scattering matrix and mode analysis are discussed in detail. Good differential‐mode (DM) bandpass filtering (BPF) responses are obtained by two pairs of orthogonal degenerate modes in multilayer SIW cavities. Meanwhile, high common‐mode (CM) suppression and port‐to‐port isolation are realized inherently due to mode distributions of the square SIW resonator. Accordingly, to verify the design concept, two types of BTU FRCs and one type of BTB FRC are designed and fabricated, respectively. Moreover, the measured results agree well with the simulated ones, exhibiting high performances in terms of CM suppression, magnitude/phase imbalance as well as port matching/isolation characteristics.

This study presents a novel approach to design the single‐feed circularly polarized (CP) microstrip antenna by the periodic fractal parasitic structure (PFPS). By adding the PFPS along the radiated patch, a new independent orthogonal mode is generated. When the magnitudes of these two modes are the same and the time‐phase difference between them is 90°, the antenna can realise CP radiation. Parameter studies of the PFPS show how to design the CP microstrip antenna. The proposed CP microstrip antenna, designed for the satellite navigation system, has a 30 MHz 10‐dB impedance bandwidth and an 8 MHz 3‐dB axial ratio bandwidth in the centre frequency of 1.575 GHz. The advantage of this presented CP antenna is that these two modes for realising the CP radiation are independent of each other. This makes it more convenient to simplify the designing and debugging of the proposed CP antenna.

Pattern synthesis of the sparse linear array (SLA) has played an important role when the antenna size is extremely limited. Although grid‐based compressed sensing (CS) algorithms have been widely utilised to synthesise the SLA, the performance is greatly affected by the grid mismatch problem. To solve the problem, a reweighted gridless CS (RGCS) algorithm based on the reweighted atomic norm minimisation and the rotational invariance propagator method is introduced. In the RGCS algorithm, the number of antenna elements can be efficiently reduced through the reweighted gridless convex optimisation, which utilises a reweighted strategy to break the limit of the atomic norm and improves the performance of the SLA. More importantly, in addition to the focussed‐beam pattern, the proposed algorithm can also synthesise the SLA with the asymmetric beam pattern. Numerical experiments show that the RGCS algorithm can save about 18.75%–46% array elements for the uniform linear array.

This work introduces a new low‐noise current‐reused CMOS active inductor (AI) based on Gyrator‐C configuration. In the proposed AI, a simple common‐source amplifier is utilised for boosting effective Gm and improving noise performance of the AI. While the Gm‐boosting technique improves the noise performance as well as the quality factor of the inductor, a resistor is also added to the feedback path for more quality factor improvement. Rigorous analysis of the proposed circuit shows a significant noise performance and quality factor enhancement. In order to verify the concept and confirm the mathematical analysis, the AI is designed and simulated in a commercial 0.18 μm RF‐CMOS technology. The simulation results show that the proposed inductor operates in 1 to 7.2 GHz frequency range, has a 9 nH inductance at 2.864 GHz frequency and 650 μW total power dissipation at 1.8‐V supply voltage. Maximum quality factor of 90 is achieved at 2.864 GHz frequency and a quality factor greater than 40 is obtained from 2.4 to 3.3 GHz. The input‐referred current noise of the proposed inductor is as low as 22 pA/√Hz showing 30.5% improvement compared to the conventional AI. The proposed AI is also tunable and sweeping the tuning voltage results in changing extracted inductance from 4.35 to 15.2 nH with only a chip area of 0.003 mm2. Post‐layout and different Monte Carlo simulation results also confirm the robust operation of the proposed AI against different process non‐idealities.

This work aims at realizing an antenna with a stable radiation pattern for base station applications. A novel high‐efficiency shared‐aperture triple‐band dipole array antenna configuration is proposed in sub‐6 GHz microcell base stations in environments where bi‐directional radiation patterns are preferred. The radiators of the antenna are composed of double‐sided square dipole array structures and two novel long‐side monopoles. To verify the principle, the antenna with an overall size of 0.3λ0 × 0.33λ0 × 0.002λ0 (λ0 is the free‐space wavelength at 900 MHz) is fabricated and measured. The measurements confirm impedance bandwidths of 9.5% (840–924 MHz), 21.9% (3–3.74 GHz), and 12.2% (4.9–5.54 GHz), respectively. The second frequency band covers the 5G sub‐6 GHz NR frequency band n78, mainly used in China and some other countries, and the LTE 42/43 bands (3.4–3.6 GHz, 3.6–3.8 GHz). The first and the third bands are also suitable for GSM 900, 5 GHz WiFi and are potential candidates for future 5G developments. The boresight gain is greater than 1, 6, and 10 dBi in these bands, respectively. The measured results also demonstrate that the proposed configuration has stable radiation patterns across the three frequency bands.

Mode matching is a widely used method in various electromagnetic problems due to its dimensional advantages. However, the numerical implementation of mode matching is challenged by its high computational complexity in solving eigenvalue problems. The Lanczos method approximates the original eigenvalue problem in an efficient way and transforms the mode matching into an implicit form. The price of efficiency enhancement is the loss of eigenmode accuracy and behaviour interpretation. Its numerical efficiency is also influenced by block matrix dimensions while solving multi‐junctional waveguide structures. This work proposes a novel structure eigenmode restoration algorithm based on the Krylov subspace method. The method takes the conventional explicit mode matching strategy, but the efficiency enhancement for solving eigenmodes is prominent. The overall computational complexity is reduced from the original O(N3) to O(N1.5) without the cost of eigenmode accuracy loss. Numerical benchmark cases verify the proposed method’s accuracy and efficiency. The efficiency advantage is outstanding especially in computational time. Extra discussions about eigenmode selection strategies based on accuracy controlling indicate that the proposed method possesses better flexibility in solving complicated waveguide problems.

To minimise the profile of Satcom‐on‐the‐Move ground terminals, in this study, a two‐dimensional beam‐scanning continuous transverse stub antenna that is based on a compact linear source generator (LSG) is presented. A periodic anti‐symmetric two‐step (ASTS) structure cascaded with a broadband network is used to construct a single‐layer compact LSG. The concept of a parallel‐plate waveguide polarisation twister is used to design the ASTS unit that acts as a TE10‐to‐transverse electromagnetic wave converter, to which chamfers and T‐type grooves are added to improve the output plane wave performance over the entire frequency band. A prototype array operating in the band 10.95–12.75 GHz is designed, fabricated, and measured, and the experimental results agree well with the simulated ones. A beam‐scanning range of ±61° in elevation is achieved at the central frequency. Although only 29 of 36 slots (ϕ = 0°) contribute to the radiation, the antenna aperture efficiency is still over 60% due to the superior performance of the single‐layer minimised LSG.

The conventional quarter/half‐wavelength common‐mode filter (CMF) cannot work well for a differential stripline when there is no ground via. A new and effective via‐free CMF design for the differential striplines is proposed in this study. An alternating high‐low impedance structure is developed to increase the coupling between the stripline and the CMF, thus achieving a good filtering effect. A cascaded CMF design is further studied to enhance the stopband without deterioration of the differential signals. Both simulation and measurement results demonstrate the effectiveness of the proposed high‐low impedance CMF. The significant advantage of the proposed CMF is its compactness and simplicity, which has almost no impact on the original layout of differential striplines.

This study presents a new approach to line‐source diffraction by a circular strip. The investigation is obtained by employing both electric and magnetic line sources where the corresponding field components satisfy the Dirichlet and Neumann boundary conditions. The position of the line source in the problem resulted in a very different radiation mechanism. Regarding the boundary condition and the aperture size, the total field characteristics change noticeably. The current density on the circular strip is expressed by Fourier series expansion, and then, orthogonal polynomials including edge condition are utilised to obtain the scattered field. The investigation is done throughout the far‐field radiation pattern, near‐field distributions, and the total radar cross‐section. Then, the results are compared with the analytical and numerical outcomes. For the comparison, the method of moments and the method of auxiliary sources are employed as the numerical tools.

In the study, a miniaturised printed quadrifilar helix antenna (PQHA) with isoflux coverage and wide‐angle circular polarisation is presented. It is composed of a three‐stage stepped impedance spiral arm and open stubs. Each of the three‐stage spiral arms has different pitches and number of turns. For the last‐stage spiral arm, meander lines are used. The proposed PQHA features the characteristics of wide bandwidth, isoflux radiation, small size, and wide‐angle circular polarisation. Moreover, in order to feed the proposed PQHA, a miniaturised quad‐feed network is designed to provide equal amplitude signals with sequential orthogonal phases. For demonstration, a prototype is designed, fabricated, and measured. The overall size of the proposed antenna is 88 × 88 × 132 mm (0.44 λ0 × 0.44 λ0 × 0.66 λ0). The results show that isoflux radiation with a coverage of about ±60° is realised from 1.40 to 1.62 GHz (14.6%) while maintaining more than 10 dB return loss and larger than 200° 3‐dB axial ratio beamwidth.

A switchable polarisation conversion/in‐phase reflection metasurface (PCIPR‐MS) based on a Pin diode is proposed in this study. By changing the working state of the Pin diode, the PCIPR‐MS can be used as a polarisation conversion metasurface (PCM) and an artificial magnetic conductor (AMC), respectively. Furthermore, the designed PCIPR‐MS can be arranged into a checkerboard surface to integrate with the antenna, which means that the PCIPR‐MS can be flexibly switched to control the radiation and scattering characteristics of the antenna in a time‐sharing manner. The simulation results show that the 3‐dB axial ratio (AR) bandwidth is 4.31 to 6.14 GHz (35%) with the application of the AMC as the metal ground plane of the antenna in the radiation state, and the right‐hand circularly polarised (RHCP) gain is enhanced by more than 1.5 dB from 4.65 to 5.80 GHz in a low‐profile compared with the reference antennas. By using the PCM, phase cancelation is achieved in the scattering state of the antenna to realise the low‐RCS characteristics.

In this study, efficient channel characterisation and modelling based on deep learning algorithms are developed and presented in a line‐of‐sight (LOS) scenario. The learning and the validation processes are performed using measurements from only one environment, enabling robust model learning and prediction results. Then, the model efficiency is analysed and validated using measurements from different environments that are not included in the learning process. Finally, the channel characterisation is made with the predicted and measured ones. The designed model achieved a highly accurate channel frequency response prediction within different environments without any prior information. The model root‐mean‐square error achieves up to 2% compared with the latest proposed models in the literature. Hence, an efficient modelling tool is provided for the future wireless communication design in a complex confined environment in LOS scenarios.

This study deals with the evaluation of the longitudinal and transverse coupling impedance of a charge travelling in a drift tube with a perfectly electric conductive angular strip. The problem is formulated in the particle frame as dual series equations, and efficiently solved through the representation of the unknown in terms of Neumann series. Then, the electric parameters are obtained in the pipe frame through the Lorentz transforms. The presented solution can be adopted as the general methodology in case of angular discontinuities in particle accelerators.

An initial‐boundary value problem for the transient electromagnetic field in the presence of a complicated‐shape scatterer is studied. The considered biconical zero‐thickness and perfectly conducting surface with periodic longitudinal slots has many special cases, each of which is of separate practical interest. A new analytical‐numerical technique is presented that is based on the Mehler–Fock integral transform and the method of analytical regularisation. The source of excitation of the slotted bicone is a radial electric dipole with an arbitrary time dependence on the field. The proposed method makes it possible to obtain an analytical and numerical solution of the problem to carry out a qualitative and numerical analysis, and to study the characteristic features of the main scattering. Due to the use of the method, the initial‐boundary value problem is reduced to a Fredholm type of the second kind system of linear algebraic equations (SLAE‐2). The dependences of the condition number on the problem parameters and the estimation of the SLAE‐2 convergence with larger truncation orders are given. The far‐field radiation patterns are plotted for various values of the problem parameters, the behaviour of the near field at the apex of the slotted cone is studied, and the regime of the slot wave propagation is analysed.

Achieving a high‐gain omnidirectional patch antenna with low profile is a challenging task. On the basis of improving antennas by metamaterials, a broadband near‐zero‐index metamaterial (NZIM) is designed to enhance the horizontal gain of the omnidirectional patch antenna. The zero‐index metamaterial is loaded on the substrate of three antennas with different working bands (4.99–5.50 GHz). The results show that the horizontal gain of antennas is enhanced by more than 0.6 dB at various frequencies. The roundness of antennas is also optimised. Therefore, using NZIM can increase the horizontal gain with a low profile (λ/30) structure size. The low profile, high‐gain omnidirectional patch antenna has great potential in the conformal field.

Employing a family of dimpled and slotted spherical resonators, and a set of X‐band waveguide bandpass filters (BPFs) is presented in this study. The resonators originated from spherical resonators with depression (hence, dimples are formed) and cutting slots into the resonator shell. With a large number of experiments, the combination of using the twisted coupling topology, depression and slotting on the resonators results in filters with simultaneous low insertion loss (IL), good out‐of‐band rejection and wide spurious‐free stopband. Several BPFs are designed and 3D printed using the selective laser melting (SLM) technique where they are made in one piece. The monolithic implementation of these filters is allowed by the compatibility of the microwave and the mechanical design, which minimises the required supporting structures inside the filters. The measurement results show ILs of 0.2–0.3 dB and are better than 15 dB return loss. There is a small frequency shift of less than 0.9%, and a measured spurious‐free stopband covering the entire X–Ku bands (8.2–18 GHz) with rejection over 34 dB. Additionally, for the SLM printed devices, the influences of temperature to the S‐parameter responses, as well as polishing, which lowers the losses, are included in this study.

The solution to the problem of electromagnetic (EM) wave scattering from a set of small‐size impedance particles of arbitrary shape is derived by the asymptotic approach. Particles are located in a homogeneous domain with constant ɛ0 and μ0. The solution is derived under the condition b → 0, where b is the characteristic size of the particle; further, the number M(b) of particles tends to infinity at a specific rate. The regularising procedure consists of the derivation of the explicit form of a solution that excludes the necessity to solve the respective integral equation for determination of the fields at the surface of particles and thus avoids integrating the Green function derivatives, which are in the kernel of this boundary integral equation. Practical application of this approach yields an ability to create media with the desired inhomogeneous distribution of effective refractive index n and magnetic permeability μ(x). Explicit analytical formulas are derived for these physical parameters and supported by computations.

Front‐fed parabolic reflectors are among the most commonly used antennas in the industry. While in spaceborne applications, membrane reflectors are very promising due to their lightweight and foldable features. However, considering the large size, small thickness and low stiffness, solar radiation and microwave radiation will have considerable influences on the antennas’ shape accuracy as well as the radiation characteristics. In this article, a theoretical approach is presented to solve the multi‐physical effects of the parabolic antenna. The deformation of the reflector is derived by the shallow shell theory, taking into account the solar pressure, the microwave pressure and the thermal effects due to solar and microwave heating. The far‐field electromagnetic radiation pattern is then obtained by considering the deformation of the reflector. On the other hand, a numerical approach combining the finite element method, the multi‐level fast multipole method, and the large element physical optics is also presented. Numerical examples suggest good agreement between the theoretical and numerical results. The methods have been applied into the analysis of design models in the Space Solar Power Station project. Also, these approaches can be directly extended into other space membrane reflector antennas.

The Cauchy method of analytical regularisation, first in full explanation, is used for the modelling of the plane wave scattering by a flat pre‐fractal system of impedance strips. It is based on a modification of the singular integral equation technique and the correct type of inversion formula of the simplest Cauchy singular integral equation. The approach is effective for the plane H‐polarised electromagnetic (or sonic) wave scattering problems. The principle of strict mathematical orderings of the pre‐fractal strip grating deals with a five‐parts class of perfect fractal sets, which is characterised by a variable positive fractal dimension. Considerable attention is devoted to the asymptotic model of the plane wave scattering by a sparsely filled grating, which has an explicit solution. This result is very important for verification of the mathematical models presented and testing of direct numerical methods. The scattered electromagnetic field in the far zone is successfully analysed with the aid of the asymptotic solution derived for electrically narrow strips.

The radiation of a double‐layer spherical dielectric lens equipped with a conformal spherical‐disk PEC reflector and excited by elementary sources located on the antenna axis of rotation is considered. The sources are either a radial electric dipole or a tangential magnetic dipole. Using the analytical inversion of the static part of the homogeneous‐lens problem with the aid of the Abel integral transform, developed earlier for plane‐wave scattering, each of the considered problems are cast to the Fredholm second‐kind infinite matrix equation with elements containing only the elementary functions. This guarantees convergence and enables accurate characterisation of the considered lens antennas in terms of directivity and total radiated power, in wide frequency range. It is demonstrated that the best directivity is provided by the quasi‐Luneburg lens configuration with dielectric constants of the core and the shell being 2 and 1.1, respectively, shell thickness being 0.1 of the outer lens radius, and reflector size around 30°. This performance, however, is spoiled by the whispering‐gallery‐mode resonances that can be effectively alleviated by the introduction of the moderate losses to the shell material.

The diffraction of an E‐polarized plane wave from two finite sinusoidal gratings is investigated by a rigorous approach, based on the Method of Analytical Regularization. This allows us to analyse the problem in a wide frequency band without limitations on the depth of the sinusoidal corrugation, the wave size of the gratings, or their relative location. The homogeneous version of the equations obtained is used for accurate computation of the complex eigenvalues in an open resonator of Fabry–Perot type with sinusoidally corrugated strips. In the calculations, two types of structures were used: one is formed by parallel translation of the sinusoidally corrugated strips and the other is obtained by rotation of one of the strips in the first structure. For each relative location of the corrugated strips, the resonance excitation of the structure by an obliquely incident E‐polarized plane wave is considered. Based on the calculation of the surface current density for different incidence angles, their specific values for optimal excitation of the complex modes are established.

An accuracy evaluation analysis of a novel in‐building measurement‐based path loss prediction narrowband model is presented here, comparing the performance of Kriging‐aided shadowing prediction against the most traditional assumption of slow fading as a random variable and a classical estimation derived from linear interpolation. Extensive radio measurements were employed using distinct samples to calibrate (tuning dataset) and validate (testing dataset) the model. Path loss predictions are made over the testing dataset locations to compare it against the measured values, thus obtaining an error in the prediction from the difference between measurements and predictions. The results in the seven buildings evaluated show the potential of Kriging‐aided channel modelling with a higher level of confidence than other modelling approaches compared hereafter.

A design procedure is described which enables simultaneous control of the input‐impedance match, directivity, and bandwidth of segmented antennas. Their impedance properties can be accurately determined by a straightforward multiport analysis, which makes it possible to optimise the values of individual reactances with the use of closed‐form objective functions. Two practical omnidirectional segmented monopole antennas have been designed, built, and tested, to verify the proposed design procedure. One is a single‐tuned design with the narrow bandwidth of 1% and realised gain of 6.7 dBi, and another is the double‐tuned design with 42% relative bandwidth and realised gain of 6.2 dBi.

The uniaxial perfectly matched layer (UPML), an important link in the numerical calculation of the discontinuous Galerkin time domain (DGTD), truncates the size of the computational domain and absorbs the outward‐travelling waves. In traditional DGTD, tetrahedral meshes are used to divide the computational domain. These meshes are also used in UPML. As the scope of the computational domain is wide, many meshes will be generated after the UPML division wraps the external part of the domain, thereby resulting in inefficient computation. The use of a UPML in DGTD form based on hexahedral meshes is proposed. The internal propagation and UPML regions are divided by a tetrahedral element and a hexahedral element, respectively. Among the relevant numerical examples given, a few cases are analysed to demonstrate the feasibility of the proposed method: the high efficiency of the UPML with hexahedral mesh, attenuation of electromagnetic waves inside the UPML, and relative error of the electric field values at the observation points.

The Carleman method of analytical regularization is used to model the plane E‐polarized electromagnetic wave scattering by flat grating of a finite number of impedance strips. It is based on a modification of the singular integral equation technique, the perturbation method and the Carleman inversion formula for the simplest logarithmically singular integral equation. Attention is focussed on the asymptotic model of the plane E‐polarized electromagnetic wave scattering by a sparsely filled impedance strip grating. In this case, the corresponding singular integral equation system has an explicit solution. This result is important to verify direct numerical methods. The scattered electromagnetic field in the far zone is successfully analysed with the aid of the asymptotic solution derived for electrically narrow strips.

Analytical expressions are derived for probability distributions of the three‐antenna efficiency measurement. The probability density function (PDF), the expected value, and the standard deviation are derived analytically. The Mellin transform is used in the PDF derivation to obtain the product of random variables, and the analytical distributions for the measured antenna efficiency are obtained. Measurements are performed to verify the results, which agree well with the analytical expressions.

As the market for internet of things (IoT) is growing and due to Ofcom's decision to reassign parts of the very high frequency (VHF) spectrum in the UK for IoT use, a propagation study has been conducted using the newly released VHF spectrum and the currently commercially operated ultra‐high frequency (UHF) spectrum, in order to compare and contrast the suitability of the VHF spectrum for IoT use. The authors conducted their study in a number of different environments (rural, suburban, urban and dense urban), with measurement equipment deployed in a manner suitable for a portable IoT use case. Results are presented in comparison to other propagation studies available in the literature and widely used propagation models such as the Hata model. Shadowing and noise are also measured and examined. It is found that current propagation models do not provide adequate predictions within the considered use case, but found it is possible to calculate log‐distance based models that provide good predictions. Path‐loss is found to be constantly lower at VHF than UHF, but radio frequency noise is consistently higher. The newly released spectrum is found to be suitable for IoT deployments in all the examined environments.

An approach for the synthesis of sum and difference patterns in monopulse antennas is described. The proposed approach provides a significant reduction in the complexity of the beam‐forming network that is fulfilled by reducing the number of array elements and keeping the elements at the edges of the array that share common excitations for both sum and difference modes. An iterative constrained optimisation method is used where the non‐convex lower‐bound constraints on the beam pattern are cast as an equivalent multi‐convex optimisation problem while concurrently minimising a reweighted l1‐norm of the magnitudes of the elements in the beam‐forming weight vector. Thus, better radiation performance of beam pattern (e.g. narrower beamwidth, lower peak sidelobe level), a much narrower spatial aperture and a smaller number of elements can be achieved compared with the case of uniformly spaced arrays. To compensate for array imperfection in practice, robust beam pattern constraints are derived in the optimisation stage using a worst‐case performance optimisation technique. Numerical examples show the effectiveness and advantages of the proposed synthesis approach.

Asymptotic closed‐form expressions for calculating complex transverse electric (TE)/transverse magnetic (TM) whispering gallery mode (WGM) frequencies in homogeneous gyroelectric circular cylindrical resonators of infinite length are derived. In addition, a volume integral equation (VIE)‐cylindrical Dini series expansion method is extended to support the prediction of complex WGMs for continuously varying highly inhomogeneous gyroelectric circular cylindrical resonators. To this end, the entire domain orthogonal vectorial basis of VIE is extended to support very large indices of the involved Dini‐type cylindrical vector wave functions via asymptotic closed‐form expressions. This way, the eigenbasis required to solve the VIE becomes free of numerical instabilities arising when very large orders of the involved Bessel functions are employed. The complex frequencies obtained by the asymptotic closed‐form expressions for the case of the homogeneous gyroelectric resonator, as well as those obtained by the VIE when the multilayered gyroelectric resonator is reduced to one layer, are validated by comparisons with the complex roots extracted by numerically solving the TE/TM characteristic equations obtained from the separation of variables method, using complex root finding techniques. We demonstrate the calculation of very high‐order WGM frequencies for cylindrical resonators composed of homogeneous and highly inhomogeneous permittivity profiles. This asymptotic theory constitutes a rigorous tool that may serve for verifying method of analytical regularisation–based numerical solutions for other non‐circular inhomogeneous cylinders, and for interpreting experimental data for applications such as WGM lasing, refractometric sensing, and magneto‐optic coupling.

To solve the complexity and blindness of the tuning process in the manufacture of microwave cavity filters, this study proposes a self‐learning tuning method based on the poles and residues of the admittance function. First, the improved Cauchy's method based on the differential evolution algorithm is used to extract the poles and residues of the admittance parameters (Y‐parameters) in a non‐ideal environment, and the effect of different port phase shifts and cavity losses on the accuracy of parameter extraction is overcome. Second, a parametric model based on the experience and data fusion via the fuzzy neural network method is established according to the collected non‐linear relation data. Furthermore, problems such as poor data reliability, low modelling accuracy and weak generalisation ability are solved. On this basis, an adaptive optimisation tuning of microwave cavity filters using an implicit space‐mapping algorithm is proposed and problems such as convergence difficulty and dependence on the initial value are solved. The results of the online simulation show that the proposed method has a high tuning accuracy and fast tuning ability.

This study presents a microwave oscillator in which an innovative hybrid microstrip and substrate integrated waveguide filter is proposed for phase noise reduction. By virtue of equivalent circuit, the specific topology of this filter is clearly revealed and verified by both the simulation and the experiment. It can realize a third‐order elliptic bandpass response with two transmission zeros near the passband. Specific phase response can be formed, in which a high group delay can be achieved within a narrow frequency range around the centre frequency. This feature makes the filter performance very flexible for its application in microwave oscillator, and then reduces its phase noise. For demonstration, an oscillator example at 9.0 GHz was deigned, fabricated and measured. The measured phase noise at 100 kHz away from the oscillation frequency is as low as −120.06 dBc/Hz. Furthermore, the proposed oscillator is featured by greater than 60 dB second‐harmonic suppression and at least the third harmonic is suppressed.

To study the problem of radio wave propagation in an environment of dielectric ground and obstacles, especially in low‐lossy obstacles, to consider the propagation inside the obstacle, a new method is proposed to solve the two‐way parabolic equation based on the principle of domain decomposition. First, a mathematical model of parabolic equations for reflection and refraction in an environment with obstacles is established. Then, the area of the obstacle is decomposed into two different subdomains; the discrete mixed Fourier transform is used to calculate the field value in the upper subdomain from the top of the obstacle to the absorption boundary, the finite difference method is used to calculate the field value of the lower subdomain inside the obstacle, and the field strengths and phases of the two subdomains coordinate with the boundary conditions. In the process of calculation, we emphatically consider the multiple reflection and transmission paths inside obstacles to make the algorithm results more accurate. In addition, we verified the stability and convergence of the method. Finally, the simulation results show that the proposed method and the method of moments have good consistency, so the algorithm has the characteristics of strong applicability and fast calculation speed, which provide a novel and reliable method to research the wave propagation problem in a modified environment of obstacles.

Single‐layer ultrathin absorber patch elements are analysed under normal and oblique incidence. A compensation technique for impedance variation with an angle of incidence (θ) is proposed. An equivalent‐circuit model is formulated that provides a realisation for the absorption response (in terms of resonance frequency and absorption ratio) of non‐complicated rectangular patch absorbers using simple equations. The design methodology proposed is based on using multiple subcells of simple patches to add another degree of freedom for the design equations and keep the ultrathin planar feature. Previously reported techniques realising a wide angle of incidence are applied for fair comparison with the proposed methodology. The developed structure features angular stability, polarisation insensitivity, and a planar non‐complicated configuration compared with that of structures reported in literature. Moreover, a prototype is fabricated and measured to validate the proposed approach.

On‐body propagation channels show significant polarization selectivity that has dominant impacts on the on‐body link quality in wireless body area networks. For on‐body relay channels over long distances across the body, the antenna polarization configurations of the relaying nodes are crucial to determine the relay performance, thereby requiring polarization optimization schemes. In this article, narrow band flat‐fading on‐body channels at 2.484 GHz are measured to analyse their polarization distribution above the skin under static postures. The measured on‐body channels show full‐space polarization convergence and dispersion, which are modelled by an extended 3 by 3 channel polarization matrix, and a statistical channel model is also proposed. Multi‐hop polarized on‐body relay channels are analysed on the relay link capacity, which shows polarization dependency between adjacent hop channels when relaying nodes are mounted by linearly polarized antennas. Both regular and iterative polarization selection algorithms are then designed to optimise the antenna polarization configuration of the nodes. Simulations based on the proposed polarization distribution model show significant link capacity increases for two‐hop and tfhree‐hop on‐body relay channels by the optimization algorithms. The results confirm the necessity of proper polarization selection schemes for on‐body relay channels.

An efficient technique for calculating the scattering from curved metasurfaces using the extinction theorem in conjunction with the Floquet and Fourier series expansions is presented. Here, we treat the two‐dimensional metasurfaces that have transversal polarizabilities with no variation along the y‐axis. The boundary conditions at the metasurface are given by the generalized sheet transition conditions (GSTCs) whose susceptibilities are given in an arbitrary local coordinate system. First, we use the extinction theorem to provide integral equations of the scattering problem. The integral equations involve the Green's functions, tangential electric and magnetic fields and their normal derivatives in regions above and below the metasurface. Then, we employ the Floquet theorem that gives us the analytical periodic Green's functions of each region. Next, we employ the Fourier theorem to expand the tangential fields in terms of unknown Fourier coefficients. The GSTCs and the integral equations provide equations to be solved for the unknowns. The method can calculate scattering from both periodic and non‐periodic metasurfaces. The technique is used to analyse different applied problems such as carpet cloaking, illusion, and radar echo width reduction. The method is fast and accurate and can efficiently treat metasurfaces with electrically large curved geometries with dimensions as large as 120 times the wavelength.

A compact broadband circularly polarized square‐slot antenna based on an offset‐fed inverted‐L‐shaped microstrip feed line is proposed. A rectangular slot beneath the feed line and an inverted‐L‐shaped grounded strip on the left bottom corner of the wide square slot produce dual‐wideband circular polarization (CP) coverage in a compact size. Meander lines with a square slot loaded at the right bottom corner of the ground plane result in broadband CP through overlapping of the dual‐wideband axial ratio (AR) bandwidths. The magnitude and phase differences between two field components can be controlled effectively with proper tuning of the square slot dimensions (4.5 × 4.5 mm2), which is helpful for broadening AR bandwidth over the C‐band by minimizing AR values (≤3 dB). The overall dimensions of the prototype are only 26 × 26 × 1.6 mm3. The antenna prototype has been validated experimentally. The measured results indicate an impedance bandwidth of 104% (3.56–11.22 GHz) and a 3 dB AR bandwidth of 76% (3.90–8.72 GHz). A peak gain of 3.4–4.42 dBi is achieved within the AR bandwidth. The proposed antenna is suitable for CP applications in the C‐band.

Wave scattering from a finite hollow cone with perfectly conducting boundaries embedded in a dielectric sphere is considered. The structure is excited axially symmetrically by the radial electric dipole. The scattering problem is formulated in the spherical coordinate system as the boundary value problem for the Helmholtz equation. The diffracted field is given by expansion in the series of eigenfunctions. Owing to the enforcement of the conditions of continuity together with the orthogonality properties of the Legendre functions the diffraction problem is reduced to infinite system of linear algebraic equations (ISLAE) of the first kind. The usage of the analytical regularization approach transforms the ISLAE of the first kind to the second one and allows one to justify the truncation method for obtaining the numerical solution in the required class of sequences. This system is proved to be regularized by a pair of operators, which consist of the convolution type operator and the corresponding inverse one. The inverse operator is found analytically using the factorization technique. The numerical examples are presented. The static and the low‐frequency approximations as well as the transition to the limiting case when the cone degenerates into the disc are considered.

Considered is the plane‐wave scattering from and absorption by a thin circular dielectric disk. The analysis uses a set of the singular integral equations for the effective electric and magnetic currents, derived using the generalized boundary conditions on the disk median section. Following the recently developed analytical preconditioning procedure, these equations are discretized by the Galerkin technique with judiciously chosen expansion functions, which provide for the Fredholm second‐kind nature of the resulting matrix equations. This guarantees the code convergence and high efficiency. It is demonstrated that the developed technique delivers the most important features of thin dielectric disks–the resonances on the natural modes. In the resonances on the slab modes, the disk can be well‐transparent and the shadow is created only by its rim; in the whispering gallery mode resonances, the scattering occurs mainly in the disk plane.