A novel technique incorporating the unitary matrix pencil (UMP) method is extended to the case of sparse linear arrays with wideband frequency invariant (FI) patterns. Multiple pattern data at different frequencies are formulated with the same poles corresponding to the FI element positions by a new sampling strategy. The unitary transform converts the complex computations for block Hankel matrices into real ones, which contributes to the reduction of the computation complexity for the singular value decomposition and the eigenvalue decomposition procedures. Taking advantage of the equivalent matrix pencil obtained by the unitary transformation, a new relation between the element positions and generalised eigenvalues can be built by the UMP method, which contributes to the real solutions for all element positions win wideband. A new tactics is exploited to avoid small element spacing with the consideration of the size of array antennas. Multiple FI patterns are reconstructed with a higher matching accuracy by only varying the excitation distributions. A set of representative numerical examples demonstrate the efficiency and advantages of the proposed method.

A radio frequency identification (RFID) based system is developed in this paper as a novel passive wireless sensor to detect corrosion for structural health monitoring (SHM) of metallic surface through simulation, design, fabrication and experiment. Firstly, a 2D label-type antenna in a new configuration – circular three arm (CTA) element – is designed as an ultrahigh frequency (UHF) RFID sensor tag working on surface of steel samples. To improve gain and power transmission coefficient, a parasitic element has been added into the centre of CTA. The CTA antenna attached with the parasitic element – so called (CTAP) sensor – has a quality factor notably higher than similar metal mountable UHF RFID antennas that has been resulted to revelation of resonance frequency in measured forward interrogation power. A demonstration and validation system are introduced then to detect corrosion based on the shift of revealed resonance frequency in the forward interrogation power to activate the CTAP tag when placed on surface of corroded and non-corroded steel samples. Results show that the presented system could communicate with sensor tag to detect corrosion up to 2 m of read monitoring range at the UHF RFID standard bandwidth with a sensitivity around of 13 MHz.

A polarisation independent artificial magnetic conductor (AMC) miniaturised using interdigital capacitors is investigated. Its performance is analysed and compared with two other previously reported AMCs. Then, a circularly polarised square slot antenna (CPSSA) utilising the proposed AMC as a reflector is implemented, which achieves unidirectional radiation and low profile simultaneously. The CPSSA is fed by a coplanar waveguide and loaded by two L-shaped strips placed around two diagonal corners, providing the required perturbation for circular polarisation radiation. In between the slot antenna and the AMC reflector is an air gap layer. The performance comparison of the whole antenna system employing different reflectors (three AMCs and one perfect electric conductor (PEC)) proves the advantage of the proposed AMC. As for the fabricated antenna prototype, the measured 10 dB return loss impedance bandwidth and 3 dB axial ratio bandwidth are 15.9 and 4.1%, respectively, corresponding to the centre frequency of 2.71 GHz. In addition, an almost constant gain of around 5.5 dBic has been obtained across the circular polarisation bandwidth within an overall antenna volume of 0.54λ ₀ × 0.54λ ₀ × 0.08λ ₀.

The Aharonov–Bohm effect is a well-established quantum phenomenon that relates the behaviour of an electron not only to the local electromagnetic field but also to the associated potentials. An important consequence is that electron beams travelling through an arbitrary medium can carry information not only about the properties of the materials along with their trajectories but also about those of the entire background around them. Based on this established principle, the authors propose an inverse formulation for the estimation of a sample's magnetic permeability based on measurements of the wave function of a split electron beam. For one-dimensional distributions, the analysed concept is shown to lead to the ideal retrieval of the sample's texture for a sufficiently large number of measurements. Two- and three-dimensional permeability distributions can be successfully estimated by employing sparsification transformations associated with compressive imaging approaches. The proposed configuration and formulation may be useful in non-destructive detection and sensing of biological, chemical or ferrimagnetic samples.

This study describes the design of a very compact dual-band-notched ultrawide band (UWB) multiple-input multiple-output (MIMO) antenna. The main features of the proposed antenna are its compact dimensions and high isolation between the two antenna elements. The UWB performance of the antenna is attained by designing two identical triangular-shaped patch elements, which are designed opposite to each other and connected to a tapered microstrip feedline. The ground plane of the proposed antenna consists of a funnel-shaped patch connected with two triangles at the lower end. In addition, two J-shaped slits are etched in the radiator to get the band rejection characteristics at wireless local area network band from 5.1 to 5.8 GHz and IEEE INSAT/Super-Extended C-band from 6.7 to 7.1 GHz. The designed antenna has the smallest dimension of . Results show that the MIMO antenna has a large impedance bandwidth of 31.9 GHz from 3.1 to 35 GHz. The diversity performance of the proposed antenna is also analysed using different parameters such as envelope correlation coefficient (ECC), diversity gain, total active reflection coefficient and so on. Due to the low mutual coupling of <−24 dB and ECC of <0.2 across the frequency band this antenna is a good candidate for portable UWB applications.

This study is a computational study on focused microwave thermotherapy for knee pathological treatment using the time reversal (TR) principle in musculoskeletal disorders. The authors presented a modified TR algorithm with amplitude compensation for an accurate beam focusing of the knee tumour location in a lossy medium. Furthermore, they proposed a new approach called the truncated threshold method, which could be used to apply an effective beam focusing on a tumour location in the knee while the unwanted hot spots are controlled in the normal tissue region. Compared to the other existing methods, this new approach has the advantages of being implemented simply in the unwanted hot spot control and having a similar performance to the beam focusing on the target location. The application of the proposed algorithm and the new hot spot control method to knee pathological tissue achieved acceptable electromagnetic (EM) and thermal results. The anatomical based two-dimensional (2D) knee model for the simulation analysis was implemented using a segmentation result of the Korean human body model obtained from magnetic resonance imaging. 2D finite-difference time-domain electromagnetic and thermal solvers were developed and applied to conduct the 915 MHz focused microwave thermotherapy for knee pathological treatment.

Unequally spaced linear arrays are synthesised for the sidelobe level (SLL) suppression, the null control in some specified directions and the beamwidth (BW) set within predefined limits. Based on the vector mapping method and the position perturbation technique, the constrained vector projection (CVP) is defined to offer flexibility in controlling null depths with multiple constraints. A modified differential evolution (DE) algorithm with the CVP method (in short MDE-CVP) is then performed in the array synthesis, which effectively reduces the optimisation space, avoids the infeasible solutions and enhances the global search ability. Three examples are presented and the first two results are compared with those obtained by learning particle swarm optimisation, invasive weed optimisation and wind-driven optimisation and a modified version of the central force optimisation algorithms. Comparison results indicate that the MDE-CVP algorithm outperforms the existing algorithms in terms of minimum SLL, BW control, null control and convergence characteristics.

Novel compact microstrip quint-mode multi-stub-loaded resonator has been investigated. Meanwhile, its applications to the dual-band bandpass filter (BPF) and quad-channel diplexer have also been proposed. The proposed dual-band BPF is constructed by a single quint-mode resonator, leading to a very small circuit size. Herein, the first three resonant modes are used to form the first passband, while the other two are used to form the second passband. Besides, the proposed quad-channel diplexer is constructed by two different quint-mode resonators, which also occupies a very small circuit size. In this diplexer design, one of the quint-mode resonator is used to realise the passbands for channels 1 and 3, and the other one is used to realise the passbands for channels 2 and 4. For demonstration, a dual-band BPF operating at 0.9 and 2.2 GHz with a size of 0.17λ _g × 0.23λ _g and a quad-channel diplexer operating at 0.9, 1.45, 2.1, and 2.7 GHz with a size of 0.28λ _g × 0.23λ _g have been designed, fabricated, and measured. The electromagnetic simulated and measured results are in good agreement.

This study presents a dual-band circularly polarised (CP) coplanar waveguide fed split-ring resonator-loaded rectangular slot antenna. A wide-frequency ratio (the ratio of the centre frequencies of the two bands) ranging from 1.2 to 2 can be achieved by changing the dimensions of the resonator. To demonstrate the concept, an antenna having two bands, one at 2.4 GHz and the other at 3.5 GHz, is designed and measured input reflection coefficient, axial ratio, and radiation pattern are compared with the simulation results and they are found to be in good agreement. Measured results show that the proposed antenna operates in two frequency bands with overlapping bandwidths of 32.13% (2.22–3.07 GHz) and 2.17% (3.415–3.49 GHz). The axial ratio and impedance bandwidths are used to compare the performance of the proposed antenna with some of the published dual-band CP antennas.

A design criterion for a small, compact, high-aperture-efficient antenna is proposed in this study. The proposed criterion is validated experimentally through fabricating and testing two prototypes for Ku-band and Ka-band applications. The proposed antenna design consists of a resonance slot perforated in an aluminium plate and surrounded by a cavity and two corrugations. The proposed Ku-band antenna has an aperture efficiency of 74.3% and a measured gain of 12.2 dBi at 13 GHz, while the Ka-band antenna resonates at 28.5 GHz with a peak aperture efficiency of 78% and a gain of 12.9 dBi. The gain of both antennas is dramatically boosted by the radiation from the cavity and corrugations, resulting in a significant enhancement in the aperture efficiency.

A simple technique to reduce the mutual coupling between patch antenna arrays is proposed. Mutual coupling phenomena in patch antenna array are investigated and according to the coupling characteristics, a simple rectangular slot(s) placed in between the patch elements is proposed. An equivalent circuit of the proposed structure is obtained through the reciprocity method. In the proposed technique, the series equivalent circuit of the slot is used to reduce the mutual coupling level without changes to the back lobe as well as cross-polarisation levels compared to conventional arrays. The mutual coupling reduction is >20 dB when compared with the conventional array. Applied to a phased array antenna, the reduction in the mutual coupling results in a wider scan range, up to 75°. A parametric study is carried out for different parameters of the slot that affect the mutual coupling and the scan range. A prototype of the antenna array is fabricated and the measured results are compared to the simulation.

The design of novel ultra-thin polarisation rotating frequency selective surface based on substrate integrated waveguide (SIW) technology has been presented. The primary function of the structure is to select the linear polarisation from the impinging electromagnetic wave on it and to rotate the wave into the 90° counter-clockwise direction in the given frequency band. The proposed array consists of periodic Y-shaped slot elements surrounded by SIW cavities. The structure shows a relative bandwidth of 8% with impedance matching better than −10 dB and a very good insertion loss of 0.3 dB. It also offers co-polarisation below −20 dB in the passband. The proposed structure is very thin (0.055λ ₀) compared to the existing SIW-based frequency selective polarising rotators. Also, the conformal analysis of the proposed structure has been carried to study its behaviour for real-time applications when used on curved surfaces. Finally, to prove the efficacy of proposed structure, a prototype has fabricated and measured its performance. A very good agreement is observed between the experimental and simulated results.

The pattern synthesis based on the spectral Green's functions for multi-layered perforated dielectric loaded patch antenna is presented. It is conducted by optimising the dielectric superstrate parameters, i.e. relative permittivity (ε _r) and thickness. To solve the problem of the optimised ε _r unavailable from market, perforated dielectric superstrate (PDS) is utilised. The equivalence between PDS and homogeneous dielectric superstrate (HDS) in spectral domain is investigated and the effective relative permittivity (ε _reff) of PDS is studied. Moreover, by introducing ε _reff, the general optimisation procedure for HDS loaded antenna is also applicable for PDS loaded antenna, which reduces the computational cost significantly. To validate their method, a two-layer PDS loaded patch antenna with E/H beamwidth equalisation was designed, fabricated and measured. Simulations and measurements show that the synthesised antenna has satisfactory performances over the ITU-RS.1068-1994 band from 13.75 to 14 GHz. The half 10 dB E/H beamwidths of 19°/20°, −27 dB cross polarisation and −15.5 dB side lobe level have been achieved.

This paper studies the cause of low-broadside directivity and high sidelobe levels (SLLs) in compact resonant-cavity antennas (RCAs) (footprint ). An approach to determine an optimal near-field distribution to significantly improve the broadside directivity and SLL of RCAs is presented. A near-field to far-field transformation routine developed in MATLAB is used to study the individual effects of amplitude and phase distributions, above the partially reflecting superstrate (PRS) of RCAs. Unlike the direct use of a full-wave simulator design, this new approach allows the designers to understand the individual effects of amplitude and phase distributions in the radiating near-field region, on the broadside directivity and on the SLL in the far-field radiation pattern. A method to realise a desired complex near-field distribution is demonstrated, using a dielectric PRS. The RCA with the new PRS showed a significant improvement of 5.0 and 6.4 dB in the broadside directivity and SLL, respectively, compared with a uniform PRS without compromising the footprint and profile of the antenna. The predictions of MATLAB are validated using computer simulation technology (CST) microwave studio (MWS) and experiments

The application of phase delay property of complementary split-ring resonator (CSRR)-loaded transmission line in the miniaturisation of Butler matrix beam forming network is proposed. The design of left-handed transmission line and the influence of the capacitive stubs in the phase delay properties of the CSRR-loaded transmission line with a gap are presented. The CSRR-loaded transmission line with a 7 mm capacitive stub provides 93° more phase delay than the CSRR-loaded transmission line without the stubs. CSRR-loaded transmission line with capacitive stubs is applied to the phase compensating lines of 4 × 4 Butler matrix configurations, without disturbing the 90° hybrids and crossovers. The CSRR-loaded configuration exhibits a size reduction of 48% of the footprint of the conventional Butler matrix. The output of the miniaturised Butler matrix is connected to a linear microstrip antenna array of four radiating elements and its radiation parameters (beam pointing direction, beamwidth and sidelobe levels) are compared with that of an unloaded conventional 4 × 4 Butler matrix. The miniaturised switched beam-steering antenna exhibits similar radiation characteristics as that of a conventional switched beam antenna.

A joint optimisation of the ground plane and feed structure shapes, and the topology of radiating patch in ultra wideband (UWB) antennas are proposed. The material distributive topology of the radiating patch is configured with symmetric and asymmetric topologies. The ground plane and feed structure of antenna are optimised from a set of discrete feasible combinations. The ground plane, feed structure and radiating patch topology are identified based on the particle's position vector of the improved binary particle swarm optimisation algorithm. Based on this vector, the antenna is simulated and tested by using the MATLAB–Computer Simulation Technology (CST) co-simulation method. The fitness value of the each particle is calculated from the return loss value evaluated by the CST and the particle's velocity and position vectors are accordingly updated. Compared to asymmetric topology, the symmetric topology antenna converges faster because of reduced search space dimension. The symmetric topology UWB antenna is further miniaturised to occupy an area of from based on the observation of surface current distribution. The measurement results of the fabricated prototypes confirm that all the three antennas yield good impedance matching over the UWB band with better pulse preserving capability.

A fast pencil beam synthesis method for large-scale non-uniformly spaced four-dimensional (4D) arrays is presented in this study. It separates a non-uniformly spaced 4D array into several excitations dependent ‘conventional arrays’ with an identical array structure. Far-field radiation patterns of the two ‘conventional arrays’ correspond to the patterns at the centre frequency and the sidebands, respectively. An interpolation in a least square sense is then used to interpolate the non-uniform array structure with a uniform virtual array structure. Therefore, the iterative fast Fourier transform (IFT) method can be used to synthesise the two dependent ‘conventional arrays’ with a uniform virtual array structure. Owing to the efficiency of the fast Fourier transform, the proposed method is measurably faster than existing methods, especially for ultra-large-scale 4D arrays. Numerical examples including non-uniformly spaced 4D linear array (1024 elements) and non-uniformly spaced 4D planar array (32 × 32 = 1024 elements) are presented to validate the efficiency of the proposed method.

This study presents the methodology design and implementation of a novel feed network. This feed network consists of several ring couplers with miniaturised and wideband characteristics. The size of this coupler is significantly reduced by employing λ/6 sections and open stubs. Besides, wideband performance is achieved by introducing a phase inverter. Compared to traditional ring couplers, the size of the proposed coupler is reduced by nearly 90%, and the relative bandwidth (BW) of the demonstrated ring coupler can reach about 80%. Owing to the miniaturised size and wideband characteristics of this ring coupler, this kind of feed network is much smaller than traditional ones with similar BW. On the basis of this feed network, a monopulse array and a low side-lobe level array is designed and fabricated. Good agreements are achieved between simulations and measurements.

Wall clutter mitigation is important in through-the-wall imaging (TWI) because the target information is often buried in the wall's reflections, especially when the target is located close to the wall. A new wall clutter mitigation method is proposed based on the multilevel delay lock loop (M-DLL) technique. The M-DLL structure uses the M-sequences’ phase information sequentially remove the direct wave, wall's primary echo and the multiple echoes, then the rest part of the signal is used to probe and estimate the position of the target. The M-DLL can obtain the precise correlation of the signal, and the rest target echo has an enhanced signal-to-noise ratio. This method is used in TWI as a new technique, and the results compared with other methods show that it has an excellent detectability and a good anti-jamming performance.

An impedance matching bandpass filter (BPF) with arbitrary image impedance (Z ₂) of parallel/antiparallel coupled lines SIR is presented in this study. The proposed structures consist of conventional SIRs and additional transmission lines with an electrical length of π/2–θ ₂ at the input and output ports. Moreover, the coupling coefficients of the coupled line can be controlled by the image impedance and it does not affect the bandpass responses. Since the first spurious frequency of the conventional SIR BPF can be suppressed with a transmission zero by using antiparallel coupled lines, the proposed BPF can provide wide stopband and high attenuation characteristics. For the experimental validation of the proposed filter, three types of BPFs with 20–50 and 50–100 Ω termination impedances were designed and fabricated at the operating centre frequency (f ₀) of 2.6 GHz. By using the antiparallel coupled line instead of the parallel coupled line, the first spurious frequency is occurred at 3.71f ₀ and the stopband rejection considerably improved.