ABSTRACT Four new fractal frequency selective surfaces (FSS) wideband discriminators for instantaneous frequency measurement (IFM) systems based on the balanced Gray‐code are presented in this paper. The use of a balanced binary code in the design of the discriminators enables the implementation of a 4‐bit IFM system using only dual‐band discriminators, operating in the 1.2–5.6 GHz range. The FSS patterns are based on recursive Sierpinski space‐filling geometries, which allow a significant reduction in the size of the discriminators while enabling dual‐polarisation operation. The numerical analysis is compared with the experimental data with good agreement, demonstrating the feasibility of compact low‐frequency FSS‐based IFM.
ABSTRACT In this paper, a low‐cost, flexible, ultrathin absorber comprising a simple square patch‐shaped frequency selective surface (FSS) is proposed for wearable applications. The FSS‐based absorbing structure contains a periodic graphite patch pattern placed on a thin paper substrate and backed with a copper foil‐based ground plane. The dimension and periodicity of the unit cell are 0.31 and 0.36, respectively, whereas the thickness of the absorber is approximately , where is the wavelength of free space at 10 GHz. The characteristics of the FSS absorber are optimised in terms of unit‐cell periodicity and dielectric substrate height using a full‐wave high‐frequency structure simulator (HFSS) software. The effects of the parameters are also examined using an equivalent circuit approach. The proposed absorber shows insensitivity towards the angle of incidence and the angle of polarisation. Finally, a prototype model is fabricated, measured and validated with simulation results. Good agreement is observed between the simulated and experimental models. The proposed absorber finds application in the fields of wearable electronics, medical applications and automotive applications for the effective mitigation of the EMI.
ABSTRACT A broadband reflective Linear‐To‐Circular Polarization (LTCP) converter is proposed in this work. Such a mechanically simple structure converts a linearly polarised incident plane wave into a circularly polarised reflected one with a minimum loss. In its design, an anisotropic metallic grating printed on the interior side of an inhomogeneous superstrate is utilised. Each re‐radiating element of this array is composed of a double concentric ring in which the inner ring is attached to two arc lines. The inhomogeneous superstrate, formed by a low‐cost FR4 dielectric slab with periodically drilled air holes to reduce structure loss, is mounted above a copper plane with an optimised spacing to minimise reflective wave loss. The dimensions of its unit cell are 10.6 × 10.6 × 1.8 mm 3 (0.25 λ × 0.25 λ × 0.043 λ ), where λ represents the free‐space wavelength of the lowest frequency in the operating bandwidth. To validate the numerical results, the designed structure with 12 × 7 cells is fabricated and tested. Measurements demonstrate the Axial Ratio (AR) < 3 dB, indicating near‐ideal circular polarization and the Polarization Conversion Rate (PCR) approaches unity within 7.2∼12.6 GHz. Such a structure can be used in various applications, such as weather monitoring multi‐beam/beam scanning antennas, wireless/radar tracking and navigation systems.
ABSTRACT A compact, high‐selectivity, dual‐band substrate‐integrated waveguide (SIW) filter is proposed in this paper based on an orthogonal hybrid loading scheme. To address the inherent trade‐off among circuit size, frequency selectivity and tuning flexibility in conventional single‐cavity dual‐band SIW designs, two symmetrical capacitive conductive posts and three central inductive metallised vias are orthogonally integrated into a single cavity to reconstruct the first four resonant modes for dual‐passband operation. The capacitive posts modulate higher‐order resonant modes to support mode reconstruction, lowering the lower passband frequency for miniaturisation and quasi‐independent tuning. The inductive vias regulate the inter‐mode coupling strength to generate two controllable transmission zeros and achieve independent bandwidth adjustment for both passbands. A filter prototype is implemented using the multilayer PCB process. It occupies a compact footprint of 0.76 λg 2 and operates at 5.48 and 8.36 GHz with 3‐dB fractional bandwidths of 3.8% and 5%, respectively.
ABSTRACT Simultaneous transmit‐and‐receive (STAR) phased arrays operating in continuous‐wave mode require ultra‐high inter‐array isolation to prevent receiver saturation, particularly in space‐constrained platforms requiring electronic beam scanning. However, achieving high isolation across a wide bandwidth within a compact footprint remains a significant challenge due to the inherent narrowband nature of conventional decoupling structures. This paper proposes a wideband, low‐profile isolation enhancement strategy that integrates a circuit‐guided Symmetric Notch Attenuation Structure (SNAS) with an Active Element Pattern (AEP)‐based beamforming method. The SNAS is engineered as a planar bandstop filter embedded in the ground plane, utilising a mirror‐symmetric meandering geometry to suppress surface waves over a broad frequency range without increasing the array profile. To further mitigate residual spatial coupling and correct beam pointing errors in the compact array environment, a pattern synthesis method with asymmetric sidelobe constraints is developed. Measurements and full‐wave simulations validate the efficacy of the proposed joint suppression strategy, demonstrating an in‐band isolation above 70 dB across a wide fractional bandwidth of 19.6% (8.3–10.1 GHz) with an ultra‐low profile below . These results confirm that the proposed approach effectively overcomes the bandwidth‐isolation trade‐off in compact STAR systems while maintaining accurate limited‐angle scanning capabilities.
ABSTRACT In this article, we introduce an inexpensive leaky‐wave antenna (LWA) design that is simple to fabricate and that is based on a nonradiative dielectric waveguide (NRDWG) platform. Although NRDWG based LWA designs have been demonstrated, these works have focused on coupling a fast wave propagating within the dielectric to a longitudinal slot located at some offset distance from the dielectric which necessitates the fabrication of new metallic parts should the design requirements of the LWA need to be changed. Therefore in this work we show that it is also possible to couple the fast wave directly to free space through a slot cut directly above the dielectric. Furthermore, by leveraging 3‐D printing to flexibly taper the complex longitudinal propagation constant, we also show that the same metallic parts can be reused for LWAs with completely different radiation pattern characteristics resulting in a practical LWA design approach with vastly lowered fabrication costs, lead time and design complexity. To validate this concept, we extract the longitudinal propagation constant from a modified transverse equivalent network model of the LWA and compare it against values obtained from full‐wave simulations. We then use these extracted values of the propagation constant to design, fabricate and experimentally characterise two one‐dimensional (1‐D) LWAs with an untapered and tapered amplitude distribution along the aperture to verify the proposed design concept.
ABSTRACT This paper presents a frequency‐reconfigurable dual‐band nematic liquid‐crystal (LC) reflectarray antenna with angular beam separation for satellite communication. Dual‐band operation at 12 and 28 GHz is achieved through voltage‐controlled reconfiguration of the LC layer. In the proposed design, the reflectarray is illuminated by a feed antenna along boresight, and the reflected main beams are steered to 45° at 12 GHz and 30° at 28 GHz. This angular beam separation helps mitigate the inter‐band interference issue commonly encountered in conventional dual‐band reflectarray antennas. Simulated results demonstrate gains of 22.5 and 29.8 dBi at 12 and 28 GHz, respectively, with projected‐aperture efficiencies of 43.3% and 35.2%. The corresponding sidelobe levels are 15 and 17 dB. To validate the design, a prototype was fabricated and experimentally characterised. The measured gains are 22.1 dBi at 12 GHz and 28.98 dBi at 28 GHz, corresponding to projected aperture efficiencies of 39.4% and 28.8%, respectively. The gain deviations from simulation are only 0.4 and 0.82 dB, mainly attributable to fabrication tolerances. The good agreement between simulation and measurement validates the proposed design and confirms its potential for dual‐band satellite communication applications.
This paper presents a high-efficiency L-band planar array antenna employing a shielded suspended stripline feed network on a conventional FR-4 substrate. Although achieving high efficiency on FR-4 is generally challenging because of its relatively high dielectric loss tangent, the proposed feed network mitigates substrate loss by realising quasi-air propagation characteristics with an effective permittivity close to unity. The antenna consists of a 6 & times; 6 planar array of cross-slot-coupled corner-truncated patch elements for left-hand circular polarisation (LHCP), and a sequential rotation feeding scheme based on four quarter subarrays with 90 degrees phase offsets is adopted to improve the axial ratio at the array level. The fabricated antenna uses a 0.2-mm-thick FR-4 PCB integrated with upper and lower metallic shielding enclosures. Experimental results show a measured peak gain of 23.25 dBi and a system-level antenna efficiency exceeding 84% over the operating band from 1.52 to 1.675 GHz, with a boresight axial ratio below 1.51 dB across the entire band. These results confirm that high-efficiency planar array antennas can be realised on a lossy FR-4 substrate, making the proposed design a promising candidate for L-band satellite ground terminal applications.
ABSTRACT This paper presents a novel dual‐band circularly polarised (CP) filtering antenna designed using electromagnetic coupling theory. First, characteristic mode analysis (CMA) is employed to verify that introducing different types of coupling between two adjacent patches significantly modifies the resonant frequencies of the inherent modes. Consequently, by applying specific coupling configurations between the patches, a desired modal distribution is achieved, enabling dual‐band CP filtering performance across the two operational frequency bands. Subsequently, the analysis of the magnetic field distributions of the characteristic modes leads to the selection of slot‐coupled feeding as the optimal excitation method to effectively activate the desired modes and achieve the intended functionality. Finally, to further enhance the filtering response without degrading the CP performance within the passbands, slots and open‐circuit stubs are integrated into the feedline. This modification offers additional control over out‐of‐band suppression and improves selectivity. Following fabrication and measurement, the proposed antenna exhibits dual‐band impedance bandwidths of 3.3–3.67 GHz (10.6%) and 4.74–5.28 GHz (10.7%). The corresponding axial ratio (AR) bandwidths are 1.4% (3.47–3.52 GHz) and 1.2% (4.86–4.92 GHz), respectively. These results validate the effectiveness of the synergistic integration between the guided patch coupling mechanism and the parasitic feedline elements. The proposed design approach enables the realisation of the multifunctional antenna structure, achieving dual‐band operation, CP and intrinsic filtering characteristics within a single structure.
ABSTRACT The scattering of electromagnetic waves from moving objects with intrinsic material losses is a critical phenomenon in high‐frequency satellite and radar communications. This study presents a comprehensive analysis of wave scattering from a single lossy dielectric sphere subjected to an initial velocity and constant acceleration along the direction of the incident wave. The theoretical framework is formulated using Lorentz transformations, enabling accurate modelling of motion‐induced effects. To incorporate acceleration, the time‐dependent velocity parameter is expanded via a Taylor‐series approach, capturing the gradual evolution of relativistic corrections. Mie theory, combined with a complex refractive index, is employed to rigorously account for both scattering and absorption mechanisms. Numerical simulations implemented with coding in MATLAB provide time‐resolved evaluations of the scattered field, including amplitude and phase distributions, transmission and attenuation coefficients, power density variations, and Radar Cross Section (RCS). The results demonstrate that acceleration significantly influences the temporal evolution of scattering signatures compared with the constant‐velocity case, introducing frequency‐dependent attenuation and phase modulation. These findings offer valuable physical insights into dynamic wave–object interactions and are directly applicable to the design of robust satellite communication links and advanced radar sensing systems operating in realistic time‐varying propagation environments. This approach effectively bridges theoretical modelling with practical conditions, enhancing the accuracy of satellite link analysis under dynamic atmospheric scenarios.
ABSTRACT Pattern synthesis based on complex masks remains challenging for optimization algorithms due to poor conditioning and slow convergence. To address this issue, this paper proposes SRLM, a novel pattern synthesis algorithm based on subspace relaxation and the Levenberg–Marquardt (LM) algorithm. The problem is formulated as a weighted nonlinear least‐squares (LS) problem in the amplitude domain, where array excitations are represented in a reduced null space basis consistent with the mask. The resulting problem is solved using LM. A subspace relaxation mechanism progressively enlarges the solution space, enabling fast initial shaping and refined global optimization. Whitening is applied as a preconditioner to improve conditioning and computational efficiency during the LM iterations. Numerical experiments demonstrate that the proposed SRLM algorithm is competitive with representative state‐of‐the‐art (SOTA) synthesis methods. In the low‐dimensional case, it achieves comparable pattern control performance while maintaining efficient and stable convergence. For high‐dimensional planar array synthesis, the algorithm exhibits robust performance across diverse scenarios, including stochastic constraints, irregular element layouts, and cases incorporating mutual coupling. Consequently, this study establishes a unified framework for high‐fidelity radiation pattern synthesis.
ABSTRACT This work proposes a characteristic‐curve‐based design approach to wideband horizontally polarised (HP) electric dipoles. The orthogonality of limaçons and parabolas is deduced to reveal the mapping relationship between circular sector and crescent. In this way, analytical theory of sectorial antennas can be extended to cover the more generalised crescent antennas, which yields a co‐design approach to crescent electric dipole with balancing device conformal to its orthogonal characteristic curve. Unlike traditional linear dipoles, the crescent electric dipoles feature pure horizontal polarisation in both E‐ and H‐plane as well as wide bandwidth enabled by the shape of themselves and the conformal loop balancing devices: By varying the shapes of crescent dipole and loop balancing device according to the orthogonality of characteristic curves, the dipoles' bandwidth can be adjusted from 47.7% to 59.3% as desired. This suggests that the shape of balancing device should serve as a new degree of freedom in two‐dimensional (2‐D) electric dipole designs. Benefiting from its generality, robustness and simplicity, the proposed approach is cost‐effective and promising for advanced, wideband HP antenna designs and future complex‐shaped antenna designs.
ABSTRACT This paper introduces a Miniaturised Element Frequency Selective Surface (MEFSS) with unusual resonance characteristics that the longer loop determines the higher‐frequency resonance, whereas the short loop supports the lower‐frequency resonance, which provides electromagnetic shielding for the high‐sensitivity Intermediate‐Frequency (IF) receiver on the remote‐sensing module of a radiometer and is transparent at the data‐link frequency of 5.8 GHz. The MEFSS isolates the signals at 2.4 and 3.5 GHz, avoiding the crosstalk with IF signal. The longer metallic strip increases the equivalent inductance and improves resonance stability, as illustrated by an equivalent circuit model based on the field distribution. The planar and curved prototypes are fabricated and tested. The transmission coefficients of the planar Frequency Selective Surface (FSS) at different incident angles agree well with the simulation. In the test, MEFSS exceeds 17.3 dB in average shielding performance under normal incidence in two stopbands for both polarisations, with a maximum insertion loss of less than 1 dB over an incident angle range of 0°–70° in passband. The far‐field patterns, gains, and insertion losses of conformal MEFSS are measured in Compact Antenna Test Range (CATR). The far‐field patterns are consistent with the simulations. The measured insertion losses of conformal MEFSS in the passband are 0.60 and 0.47 dB under TE and TM polarisations.
A compact, wideband dual-polarised antenna with high isolation is presented for in-band full-duplex applications. In order to achieve high isolation, an orthogonal tapered slot antenna (TSA) with symmetric configuration is designed and investigated to realize the proposed aperture-level coupling cancelation (ALCC) concept. An asymmetric top-loading patch is employed to lower the TSA's profile, resulting in a more compact structure and improved impedance matching. The proposed design with electrical dimensions of Phi 0.49 lambda min & times; 0.29 lambda min (lambda min is the free-space wavelength at the lowest operational frequency) is fabricated, measured and analysed. The measurement results reveal that the proposed antenna exhibits a 10 dB return loss bandwidth of 73.5% (2.52-5.45 GHz), an isolation of >= 45.6 dB over the bandwidth from 3.4 to 5.45 GHz (46.3%) and peak isolation of 70.3 dB at 4.7 GHz.
A compact dual-port antenna is proposed for Dedicated Short-Range Communications (DSRC) operating in the 5.85-5.925 GHz frequency band. The antenna employs a symmetric slot-loaded cross-shaped patch configuration with closely spaced central feed points to intrinsically suppress mutual coupling and control surface current distribution. Without using any external decoupling networks, parasitic elements, or complex feeding structures, the proposed antenna achieves high port isolation exceeding 25 dB, reaching approximately 35 dB at the centre frequency of 5.88 GHz. The antenna exhibits an ultra-compact electrical size of 0.43 lambda & times; 0.43 lambda & times; 0.012 lambda on a Rogers RT/Duroid 5880 substrate, making it well suited for space-constrained DSRC on-board unit integration. In addition to compactness and high isolation, the antenna demonstrates highly directional radiation characteristics with a measured front-to-back ratio of approximately 17.6 dB, which is explicitly quantified and experimentally validated. These attributes-an electrical footprint of 0.43 lambda & times; 0.43 lambda & times; 0.012 lambda, measured isolation of approximately 35 dB at 5.88 GHz, a gain of 6.4 dBi, and a front-to-back ratio of 17.6 dB collectively make the proposed antenna a practical and self-contained solution for DSRC on-board unit deployment in vehicular and point-to-point communication platforms.
ABSTRACT The azimuth super‐resolution capability analysis of a Microwave Radar Coincidence Imaging (MRCI) system in complex scenes presents significant theoretical challenges. To address this, in this paper, we propose an Eigen Subspace Characterisation (ESC) method for analysing the azimuth resolution of an MRCI system, particularly in scenarios involving multiple imaging points within a coherent area. The ESC method is implemented by first constructing the eigen subspace from the reference signal matrix of the MRCI system using the Singular Value Decomposition (SVD) method. Within this eigen subspace, differences between reference signals are effectively characterised by the Euclidean distance between their corresponding vectors. Then, a criterion for resolving adjacent imaging points is established, whereby they become resolvable when the Euclidean distance between their reference signal vectors in the eigen subspace exceeds the noise vector norm (i.e., the noise power). Finally, the relationship between resolution and system parameters, including the signal‐to‐noise ratio (SNR), the number of imaging points, and the deployment of the transmitting array, is derived analytically. The proposed ESC method is validated through a series of simulations, providing theoretical insights for resolution analysis of the MRCI system across various imaging environments and requirements.
ABSTRACT A multiobjective phase‐only design method, COMO‐PHO (coevolutionary multiobjective with phase‐only hybrid optimisation), is proposed for synthesising sparse circularly polarised phased arrays. The method introduces element rotation to improve circular polarisation and adopts a dual‐population coevolutionary framework. This framework decomposes the high‐dimensional optimisation—covering element selection, excitation phase and rotation angle—into coordinated placement and rotation optimisation, whilst jointly minimising sidelobe levels and axial ratio. A fast phase‐only solver embedded in the evolutionary process quickly evaluates candidate layouts. Once the placement is fixed, a phase‐only convex optimisation algorithm refines the phase distribution using the solver output as the initial solution, achieving precise beam steering and low sidelobes through controlled phase perturbation and linearised constraints. Simulations confirm that COMO‐PHO balances multiple performance metrics for wide‐angle scanning, offering a cost‐effective solution for sparse circularly polarised phased array design. Furthermore, its effectiveness has been validated using measurements from an circularly polarised phased‐array prototype, demonstrating the method's practical feasibility under real hardware conditions.
ABSTRACT A high‐power RF/microwave termination incorporating cascaded π‐type attenuation networks, a multi‐segment microstrip structure and an electromagnetic constraint metal carrier is proposed. The design aims to improve the voltage standing wave ratio (VSWR), particularly at high frequencies, while enhancing power handling capability by mitigating local overheating. Theoretical analysis is conducted to determine the required characteristics of the attenuation network. Measurement results show that the proposed termination achieves a VSWR below 1.4 over the frequency range from DC to 43.5 GHz. Compared with existing designs, the proposed structure demonstrates improved bandwidth, lower VSWR and enhanced power handling capability.
ABSTRACT A linearly polarised antenna array using different AMC‐inspired radiating units that integrates good RCS reduction and radiation is proposed. First, two AMC units are designed to achieve reflection phase difference within 180° ± 37° from 6.2 to 11.26 GHz. Second, the designed antenna units are composed of 4 × 4 AMC unit cells and excited by the slot‐coupled sources below. Eventually, the low‐RCS antenna array consists of 4 × 4 AMC‐inspired antenna units, which the AMC structures both work as radiators excited by feeds below and reduce RCS through arrangement. By introducing the intervals between antenna units and optimising the geometric parameters of antenna units, the performance of radiation and RCS reduction is improved. The results show that the proposed antenna array has operating bandwidth from 6.2 to 8.5 GHz and peak gain of 15.93 dBi. Bandwidth of RCS reduction over 10 dB is from 6.8 to 11.45 GHz. This study has potential for application in stealth platforms.
ABSTRACT Free space measurement is widely employed to characterise the permittivity of flat materials. However, the requirement of shape and dimension can hardly be satisfied in measurement of materials employed in semiconductor field. In this study, permittivity of SiO 2 samples with various thicknesses and small section were characterised in an improved light path. The proposed light path configuration consisted of two pairs of discrete concave lenses and general horn antennas attached to frequency extenders which were connected to a vector network analyser. Reliability and far field approximation were verified by optical simulation and real tests. In addition, the performance of NRW algorithm and NIST algorithm were compared, indicating that the permittivity of SiO 2 glass ranged from 3.15 to 3.8 in WR10 band (75–110 GHz) and that NRW took no advantage over reducing thickness dependence of permittivity compared to NIST algorithm. Moreover, the NIST model was compatible to the proposed light path configuration for characterising small thin materials with arbitrary thickness. This study may guide characterisation of dielectric property for cases that large cross section of material cannot be satisfied.
