Iet Microwaves Antennas & Propagation最新文献

A novel method based on machine learning is proposed to estimate the electromagnetic radiation level at the ground plane near fifth-generation (5G) base stations. The machine learning model was trained using data from various 5G base stations, enabling it to estimate the electric field intensity at any arbitrary radiation point when the base station provides service to different numbers of 5G terminals which are in different service modes. The inputs required for the model include the transmit power of the antenna, the antenna gain, the distance between the 5G base station and 5G terminals, terminal service modes, the number of 5G terminals and the environmental complexity around the 5G base station. Experimental results demonstrate the feasibility and effectiveness of the estimation method, with the mean absolute percentage error of the machine learning model being approximately 5.98%. This level of accuracy showcases the reliability of the approach. Moreover, the proposed method offers low costs when compared with on-site measurements. The estimated results can be utilised to reduce test costs and provide valuable guidance for optimal site selection, thereby facilitating radio wave coverage or electromagnetic radiation regulation of 5G base stations.

The authors design and experimentally analyse an anisotropic metasurface for the simultaneous conversions of linear cross and circular polarisations in the C and partial X bands. The proposed compact structure utilises a dual-cut square-ring resonator and a square patch on an FR4 substrate above a ground plane. The full-wave simulations exhibit over 92% efficiency in cross-conversion over a fractional bandwidth (BW) of 52.3%. The proposed metasurface efficiently converts linear to circular polarisation in two different bands, achieving left-hand circularly polarised and right-hand circularly polarised efficiencies exceeding 93% and 89%, respectively. The compact unit-cell with dimension 0.22λ × 0.22λ attains stable polarisation transformation efficiency rates exceeding 80% up to 40° incident angles. The design is adaptable for efficient wide BW across both lower and higher (millimetre-wave) frequencies through scaling. A prototype of the proposed polarisation converter (PC) was realised and validated effectively in measurements, aligning well with simulations. This PC features a novel and simple structure with multifunctional wideband operation, stable incident angle output, polarisation insensitivity, and scalability, making it an efficient front-end for various radio frequency applications, including polarisation control and polarisation transformers.

A mathematical model is presented for the optimisation of an Electromagnetic Halbach Array (EHA) for wireless power transfer. The mathematical model is based on Biot-Savart's law of magnetic field in a current-carrying square loop. The model is validated through static electromagnetic simulations in ANSYS Maxwell and found to be accurate and effective when predicting the magnetic field within the EHA box. To verify the analytical and simulation results, an EHA was fabricated, and experimental verification was carried out. The results show that the mathematical model can be widely used in the analysis and design optimisation of EHAs of any size and with diverse parameters in the case of single-layer square spiral planar coils.

The design and optimisation of a five-cascade layer Rotman Lens based on waveguide with a graded refractive index at K_a-band frequency are presented. The proposed lens incorporates 5 layers within the cavity, utilises a modified three focal Rotman lens design methodology to achieve perfect phasing for true time delay beam steering, and optimises each layer's parameters such as refractive index, vertex location, and contour angle to improve insertion loss between input ports and output array elements. This approach enhances energy concentration between input and output ports, reducing spillover to dummy ports. The graded layers are formed using periodic cubic metallic posts with varying heights within a parallel plate waveguide. The results show an average improvement in the insertion loss of 16% and a peak improvement of 48% across the bandwidth when compared to conventional Rotman lenses. The normalised average phase error across the array elements for all beam ports excitation is 4 × 10⁻³ and the peak degradation of the main beam during beam scanning from 0° to ±22° varies from 0.7 to 1 dB at different frequencies operating in the K_a-band (24–30 GHz) with potential applications for 5G communications. The simulation results demonstrate good agreement with the measurement results.

A design of a broadband Doherty power amplifier (DPA) using a novel harmonic control network (HCN) is presented. The DPA structure focused on manipulating harmonic components using this new HCN to enhance the bandwidth of operation and power efficiency. The proposed HCN combines a third harmonic suppression network (THSN) with the second harmonic impedance inverter circuit (SHIIC). In the design, the second harmonic component of the main transistor and the peaking transistor are short-circuited by their associated SHIIC. Additionally, the carefully designed SHIIC and THSN circuit placed between the main and peaking transistors realise open circuit conditions at the third harmonic for both transistors at the saturation state and back-off regions. Side by side with the aid of the properly designed post-matching tuned network (PMTN), these harmonic loading conditions make both amplifiers to operate in continuous Class-F modes with enhanced-added efficiencies over wideband conditions. As a result, a succession of highly efficient DPA modes can be formed over a continuous frequency range in the full Doherty region, resulting in a reduced-size enhanced power efficiency wideband DPA. For verification, a wideband DPA operating from 1.1 to 1.8 GHz was designed and implemented using the proposed topology. The measured drain efficiency of the implemented DPA at 6-dB back-off is 52.5%–67.5%, while the saturated power across the specified bandwidth is 43–43.7 dBm. The fabricated DPA exhibits an average efficiency of 48.7%–59.4%, when supplied by a 40-MHz long-term evolution (LTE) signal with a 7.5-dB peak-to-average power ratio (PAPR). Performing digital pre-distortion (DPD) improves the adjacent channel power ratio (ACLR) from −25.8 dBc to −47.6 dBc.

A novel use case for two data-driven models, namely, a Transformer and a convolutional graph neural network (CGNN) is proposed. The authors propose to use these models for emulating the dynamics of electromagnetic (EM) propagation and scattering. The Transformer translates a past sequence into a future sequence by constructing representations from the past and using it to predict the future, taking all of its own previous predictions as input at each step of prediction. The CGNN updates the current state of attribute vectors of each node by passing it information (messages) from all of its neighbouring nodes. We train these models with FDTD simulations of plane waves propagating and scattering from PEC objects. The authors demonstrate that, within the bounds of computational resources, the Transformer can be utilised as a surrogate for EM dynamics, providing 14× speed-up, while the CGNN can be utilised as a next-frame predictor, providing 9× speed-up. When comparing the accuracy of these two models with the authors’ previously developed Encoder-Recurrent-Decoder (ERD) model, it is observed that the error for both the Transformer and the CGNN remains within the same bound for the ERD model. To the best of the authors’ knowledge, this work is the first to utilise the Transformer as a surrogate for EM dynamics.

The authors present spatio-temporally resolved wideband measurements of Sub-Terahertz (Sub-THz) reflection coefficients in the frequency range of 92–110 GHz. A stochastic model for single reflection fixed links that is capable of modelling random scattering from small-scale discontinuities such as those encountered in complex structures in walls and partitions of buildings is presented. The model auto-regressively produces filter coefficients that are fed into an Infinite-Impulse-Response (IIR) filter which convolves them with the spatio-temporal series in order to generate the next output sample based on previous observations. The IIR filter allows for flexible stochastic generation of samples, and its parameters can be adjusted as needed to suit different channel conditions. A total of 20 suitable start-up filter coefficients are generated from 21.7 % of the sample size for each complex delay tap distribution, which corresponds to 50 complex instances of the channel. These coefficients are then utilised to validate the remaining measured sample set. The model is in quantitative agreement with measurement statistics and can be used to construct relatively simple modified reflection coefficients that can be used in micro-cellular ray-optical network planning tools.

Instead of using expensive highly performing conventional pyramidal absorbers, in some cases, the use of absorbers based on very cheap eco-friendly materials is an attractive alternative. The authors introduce a novel technique to improve the absorption bandwidth of these intrinsically low-quality absorbers by adding a Salisbury screen at the base of the pyramidal absorber, which adds two extra design parameters. The first is the resistive sheet surface impedance, and the second is the resonance frequency of the Salisbury screen. By tuning these two parameters, better impedance matching can be obtained. This is verified with the help of a transmission line model, full-wave simulations, and measurements. The method is applied to two different lower-quality absorbers found in the literature. In the first absorber, the operating bandwidth is increased at the lower frequency side by 40%, and in the second one, this is about 90%. Finally, a prototype of an eco-friendly combined absorber is designed and fabricated. The measurement validates the expected behaviour.

A novel 4 × 4 Butler Matrix (BM) with continuously tunable phase covering phase range from −180° to 180° is proposed using substrate integrated waveguide technology. The proposed BM consists of two H-plane hybrid couplers, two E-plane hybrid couplers and four 180° tunable phase shifters. With this simple configuration, the proposed BM can realise a continuously tunable progressive phase difference from −180° to 180° in a one-dimensional (1-D) or two-dimensional (2-D) form. For demonstration, the proposed BM has been fabricated to feed a 1 × 4 or 2 × 2 broadband microstrip antenna arrays. The fabricated prototype has a 16.7% fractional impendence bandwidth centred at 6 GHz. The measured pattern for the 1-D array showed a continuous beam scanning range from −39° to 39° in H-Plane, while the measured radiation patterns for the 2-D array showed continuous beam range from −27° to 30° in elevation and 0°–180° in azimuth. The measurement results were in good agreement with the simulation results.

A compact Archimedean spiral antenna with high gain for electrostatic discharge (ESD) detection is investigated. A frustum-shaped cavity proposed in this work is formed to optimise the maximum antenna gain. Additionally, a helix arm is loaded to enhance antenna gain in lower band. The measured results demonstrate that the proposed antenna, which employs a unidirectional radiation mode, exhibits a maximum antenna gain ranging from −0.74 dBi to 9.46 dBi and a voltage standing wave ratio less than 3 across a frequency range of 0.65–5 GHz. A test system consisted of proposed antenna and a commercial ESD detection antenna is established. The verified results indicate that the proposed antenna has a performance close to the commercial antenna, so that proposed antenna with compact size is preferred to be utilised for ESD detection.