IET Microwaves Antennas & Propagation

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The micro-motion target can perform micro-Doppler modulation on the electromagnetic wave due to motion components in different directions, leading to the defocus effect of the target imaging feature along the azimuth direction. This phenomenon has been widely concerned and investigated in the field of target recognition and anti-recognition. However, the micro-motion target fails to transform image features along the distance direction according to its slow mechanical modulation speed. In comparison, electromagnetic metasurfaces can flexibly control the characteristics of electromagnetic waves with faster modulation speed through electronic control. It can realize the image feature transformation along the range direction. Inspired by this idea, a joint modulation method of rotational micro-motion and electronic control through an active frequency selective surface (AFSS)-loaded rotating Luneberg lens reflector is proposed. The signal models of micro-motion modulation and non-periodic AFSS modulation are established separately. On this basis, the imaging properties of the joint modulation are further analysed and the two-dimensional (2D) defocusing phenomenon of the image is discovered. Moreover, the simulation of the measured synthetic aperture radar (SAR) data with different modulation methods is conducted to demonstrate the effectiveness of the proposed method. © 2022 The Authors. IET Microwaves, Antennas & Propagation published by John Wiley & Sons Ltd on behalf of The Institution of Engineering and Technology.
 
Abstract In this paper, a novel compact antenna test range based on transmitarray (TA), transmitarray compact antenna test range (TACATR), is proposed. A non‐negligible parameter, central cell reference phase (CRP), is defined to harmonize the disturbance field. The CRP not only determines the physical structure of TA, but also can adjust the phase of the desired plane wave field of the quiet zone (QZ). Several strategies are proposed to enhance the performance of QZ of TACATR. The strategies include selecting CRP and the layout of TACATR. To validate the feasibility of TACATR, a 169.6 mm × 169.6 mm TA working at 28 GHz is designed, simulated and fabricated. The measured amplitude peak to peak value (P2P) of the middle QZ plane field is 1.5 dB and the phase P2P is 9.4°.
 
Physical layout photograph of the proposed QVCO
Output frequency versus the control voltage of QVCO
Phase noise and power consumption variation versus tuning voltage
Monte-Carlo simulation results (a) Phase noise variations, (b) Phase noise histogram, (c) Oscillation frequency histogram, (d) Total power consumption and (e) Phase error
This paper presents a new low-power and low-phase noise Colpitts quadrature voltage-controlled oscillator (QVCO). The proposed QVCO contains two differential Colpitts VCOs that use gain-boosting technique to enhance the negative transconductance and relax start-up condition. Both switching and tail transistors are configured in class-C to improve ISF and phase noise performance of the circuit via better noise modulation function. In addition, flicker noise would be reduced because of the switched-biasing mechanism of tail transistors. Phase noise analysis shows that changing the bias voltage of switching as well as tail transistors can improve the phase noise performance of the core VCO with overlap reduction of the transistors active time. Consequently, due to no extra noisy elements for coupling and class-C biasing of all transistors, low-phase noise quadrature signals are generated. To verify the performance of the proposed technique, post-layout simulation is performed in TSMC 0.18μm RF-CMOS process at supply voltage of 1.2V. Post-layout simulation results show that the phase noise of the proposed QVCO is-120.6dBc/Hz at 1MHz offset from5.73GHz operation frequency. The total power consumption of the QVCO is 3.1mW and the tuning range is from 5GHzto 5.73GHz.
 
Configuration and results of proposed 90° and 135° couplers (a) 90° coupler, (b) 135° coupler (R 2 = 0.9 mm and R 1 = 1.4 mm)
Results of proposed Butler matrix (a) Magnitude of S-parameters with exciting port 1, (b) Magnitude of S-parameters with exciting port 2, (c) Output phase difference with exiting each port
Configuration of the proposed antenna with modified Butler matrix feed network
Comparison between simulated and measured S 11 and S 22
In this work, a beam-steering array antenna for pattern stabilisation is presented. The antenna element consists of two layers with a unique structure of microstrip to coplanar waveguide (CPW) transition to cover a frequency range from 20 to 30 GHz and a stable pattern at all operating frequencies. To increase the gain of the antenna element, an elliptical broadband Fabry-Perot cavity structure is used. The element manages 3 dB gain bandwidth of almost 23-26 GHz with a 14.4 dBi peak. In addition, a modified Butler matrix consisting of two 90° and two 135° couplers without phase shift section is used to attain a stable pattern. The proposed Butler matrix performs a stationary phase with a phase error of less than ±6° over a frequency band from 21 to 29 GHz. The integration of the proposed element and feed network leads to a beam-tilting antenna capable of managing its patterns through the input ports in the operating frequency range of 20-29.5 GHz (9.5 GHz) and 20.6-29.6 GHz (9 GHz) for ports 1 and 2, respectively.
 
In this work, a unique compact size high gain circularly polarised (CP) array antenna is proposed. The array is considered as a 2 × 2 metasurface-based CP patch antennas which fed by a cascade network. The single element consists of two different layers of substrate that metasurface layer excited by aperture slot on metallic ground layer in middle layer between metasurface cells and microstrip line, meanwhile this layer isolates destructive effects of feed lines on radiation layer. For achieving high gain at a compact size several techniques such as (i) use of broadband feed network, (ii) utilising of adjacent cells with unique structure as mutual coupling reducer, and etc. have been simultaneously applied. In order to validation of simulated parameters the proposed array has been fabricated and measured. The measurement results for the proposed compact size array antenna are the impedance bandwidth (|S 11 | < −10 dB) of 4.22-7 GHz (49.55%), a 3-dB axial ratio bandwidth of 4.71 to 6.28 GHz (28.5%), and a peak gain of 16.57 dBic at 5.5 GHz.
 
Reflection and transmission coefficients for a FSS
Reflection and transmission coefficients for TE and TM polarisations at frequency f ¼ 10 GHz
Illustrates the plots of the distribution and angle of the electric and magnetic fields a Magnitude of the electric and magnetic fields for d ¼ 27.5 mm and f ¼ 10 GHz b Angle of the electric and magnetic fields for d ¼ 27.5 mm and f ¼ 10 GHz
Obtained results for both TE and TM polarisations a Reflection and Transmission coefficients for TE and TM polarisations (with loss, d ¼ 27.5 mm) b Reflection and Transmission coefficients for TE and TM polarisations (without loss and d ¼ 16.2 mm)
This study presents a generalised and easy-to-use method based on Hill’s equation to analyse the electromagnetic wave propagation in inhomogeneous planar layers with arbitrary permittivity profiles. We explore the effects of polarisation, incidence angle, layers thickness and permittivity profiles on reflection and transmission characteristics. Hill’s method is as an efficient tool to obtain an analytical solution to the propagation equations and is applicable to all arbitrary profiles of permittivity and permeability. Using Hill’s method and an arbitrary profile of permittivity, numerical simulations for both polarisations transverse electric (TE)- and transverse magnetic (TM)-polarised waves are performed to compute the reflection and transmission of Brewster’s angle polarisers having optimised thicknesses. The proposed polariser is analysed and optimally designed in microwave domain systems. The results obtained agree well with other published data.
 
A circular microstrip patch antenna with epsilon (�)-shaped clusters of shorting pins (ECSP) is proposed in this paper to emphasize the improvement of cross-polarization (XP) suppression across different elevation planes of radiation at = 0�, 45�, 90�, and 135� rather than considering only the principal radiation planes. The near-field approach has been adopted to understand the basic of XP radiation from the antenna. The placement of two ECSP structures symmetrically along the H-plane of the conventional circular microstrip patch antenna (CMPA) has been experimentally studied. It is found that the inclusion of ECSP structures within the conventional CMPA effectively suppresses its XP radiation not only across the principal radiation planes by 57 dB at the boresight and 26 dB over the tracking angle ± 45� but also across the diagonal radiation planes by 58 dB at the boresight and 2 dB over the tracking angle ± 45�. A detailed analysis on the evolution of ECSP structures from arcshaped clusters of shorting pins (ACSP) is presented. Symmetric distribution of antenna fields in the near-field region results in better XP suppression in the far-field region along the principal and diagonal radiation planes. The proposed design offers a considerably high gain of 8 dBi at the boresight. The fabricated prototype of the proposed design shows satisfactory agreement with the simulated results.
 
A novel dual-band waveguide slot array antenna with low cross-polarisation is proposed. The main structure of the proposed antenna consists of two waveguides in which the top broad wall of the smaller waveguide, a WR-28 Ka-band waveguide, is connected to the top broad wall of the larger waveguide, a WR-90 X-band waveguide. The WR-28 waveguide operating in the higher frequency band of the proposed antenna acts as a ridge inside the WR-90 waveguide operating in the lower frequency band. For achieving a suitable side lobe level (SLL) and cross-polarisation in the lower frequency band, the slots are placed close to the higher frequency waveguide and side ridges are placed beside each slot. A 2D graph is presented in which the normalised resonance conductance versus the offset of a side ridge from the outer edge of the lower frequency waveguide is shown, while the slot is located between the higher frequency waveguide and the side ridge. By applying proper coefficients to this graph, appropriate offsets from the outer edge of the lower frequency waveguide for the side ridges are obtained so that a low SLL can be realised.
 
A novel antenna design was configured via a five-stage process. At each stage, a new modified square patch with a half size of that used in the previous stage was added to shape the dual fractal-structure antenna. The antenna had measured dual operating bands (2.4–2.65 and 4.8–6.4 GHz) that meet the specifications of the wireless fidelity (Wi-Fi) and worldwide interoperability for microwave access (WiMAX) applications. The structure of the antenna was carefully analysed to achieve an axial ratio bandwidth of 3 dB, which was ∼35% of the first operating band and 30% of the second operating band. Right-hand and left-hand circular polarisations were achieved by switching the inputs. The proposed antenna can be used with portable communication devices because of its compact size (18 × 18 × 0.8 mm3) and maximum efficiency (85%) compared with other related antennas.
 
A gallium-nitride high electron mobility transistor (GaN-HEMT) three-way asymmetric Doherty power amplifier using an unequal three-way Gysel power divider (GPD) is presented in this study. Considering the output power capacity of transistors, the peaking amplifier was designed to have an output power capacity that is 1.5 times larger than that of the carrier amplifier through the use of a parallel combination of two different power amplifiers. A peaking amplifier that is 1.5 times larger can provide a back-off level of 7.5 dB for another peak efficiency. Two different power amplifiers in the peaking amplifier can work as a single power amplifier through an optimization of the input power division ratio (PDR). To split the input power for the carrier and the two peaking amplifiers, an unequal three-way GPD with an optimized PDR was employed. The implemented Doherty power amplifier for the 2.14 GHz band exhibited a drain efficiency (DE) of 59.5% and an adjacent channel leakage ratio (ACLR) of 27.1 dBc at an average output power of 37.2 dBm (7.2 dB for the output power back-off) for the 2.14 GHz downlink long-term evolution signal, which has a peak-to-average power ratio of 6.5 dB and a signal bandwidth of 10 MHz.
 
A novel wideband bandpass-to-all-stop reconfigurable filtering power divider is proposed in this study, which allows for four-order bandpass-to-all-stop reconfigurable operating function and equal power division. Its circuit configuration includes the cascaded coupled-line sections with tight coupling to extend the impedance transforming. Furthermore, with the introduction of the half-wavelength open-circuit stubs, which controls the bandwidth, extra transmission poles located at the cut-off frequency are generated, thus resulting in high frequency selectivity. Moreover, by using a single resistor between input coupled-lines, the high all-passband isolation can be achieved. The grounding are then loaded to the output coupled-lines to enable bandpass-toall- stop operating functionality. For demonstration, a prototype operating at 2 GHz is designed, simulated, and measured with a 15 dB bandwidth of 51%, 19 dB stopband rejection up to 5 GHz, and 14.5 dB all-passband isolation, which shows a good agreement between the simulated and measured results.
 
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.
 
In this work, a connected circular monopole antenna array is presented for ultra-compact, wideband, multiple-input-multiple-output (MIMO) wireless access point applications. Two configurations of the circular array are considered; one with 4 and another with 8 antenna elements using the same substrate size. The frequency bands covered were from 2.3 to 5.3 GHz (LTE 2.3- 3.8 GHz, Bluetooth 2.4 GHz, WLAN 2.4, 5.2 GHz and WiMax 2.3 GHz). The obtained measured bandwidth was 3 GHz and 2 GHz for the 4 and 8 element arrays, respectively. The antenna system is fabricated on a commercially available Roger 4350 substrate with Er equal to 3.5 and height of 1.52mm. The radius of the array is 32mm (the most compact with wideband characteristics). The length of a single element is only 15mm which is lambda/8 at the lowest frequency. The design is planar, low profile and very compact. Good MIMO performance is achieved
 
A new low profile, multiband, multipolarised single feed antenna is designed. A corner truncated patch loaded with slots and a complementary split ring resonator is used to achieve three frequency bands with different polarisations. The antenna exhibits right-hand circular, linear, and left-hand circular polarisation at three different bands, respectively. To obtain circular polarisation, orthogonal modes are excited by truncating corners of the patch and etching slits. Single feed, single layer geometry with Y-shaped feed structure is employed. The antenna is fabricated on a low-cost FR4 substrate with an overall size of 70 mm × 70 mm, and the proposed design has been verified experimentally. The selected frequency bands are centred at 2.313, 2.396, and 2.478 GHz, and they have −10 dB impedance bandwidths of 2.14, 2.50, and 2.42%, respectively. The first frequency band exhibits right-hand circular polarisation with a 3 dB axial ratio bandwidth of 0.85%. The second frequency band exhibits linear polarisation, whereas the third frequency band exhibits left-hand circular polarisation with a 3 dB axial ratio bandwidth of 0.81%.
 
In this article, a circular microstrip tunable wideband frequency and polarisation reconfigurable antenna are investigated. The wideband tunability is achieved by placing the four varactor diodes between circular microstrip patch and circular ring. The impedance bandwidth of 2.09–2.96 GHz (34.52%) is realised by tuning the capacitance value from 4.15 pF (0 V) to 0.94 pF (6 V). Both frequency tunability and polarisation reconfigurability are controlled in the antenna using the proposed cascaded branch line coupler feed network. The four single pole double throw and two dual in-line package switches are used in the feed network to control the tunable impedance bandwidth of the antenna over a wideband from 1.25 to 2.95 GHz. The vertical linear polarisation and horizontal linear polarisation (H-LP) are achieved by any one of the feed port excited in the antenna. Similarly, the right-hand circular polarisation and left-hand circular polarisation are realised by exciting both the feed ports in the antenna with±90° phase difference. The impedance bandwidth of proposed feed network is well matching with the tunable bandwidth of the patch antenna. Simulated and experimental verification results show good agreement.
 
A standing-wave slot array covered with a two-layer dielectric slab based on substrate integrated waveguide (SIW) technology, which is circumferentially conformed to a cylinder, is designed, fabricated and measured at Ka-band. The radiating part consists of seven slotted cylindrical SIWs, each of which has four-element longitudinal slots. Several such slot arrays fed by Rotman lenses are grouped on a cylinder along the circumferential direction to cover a 2D field of view. The microstrip Rotman lens is employed as the beamforming network with seven input ports, which can generate a corresponding number of beams to cover −35° to 35° along 1D (E-plane). Another dimension (H-plane) is covered from −52° to 52° with four beams generated by the slot arrays group. A prototype antenna is fabricated, the measured results demonstrate its ability of scanning over a 2D field of view with multiple beams. This type of multibeam conformal antenna is a good candidate for high-speed vehicles or communication base stations requiring electronic beam-scanning capabilities and considering conformal reasons.
 
In this study, a planar arbitrary phase-difference coupled-line coupler having wide bandwidth, tight coupling coefficient, and small phase variation are proposed. The proposed coupler consists of two quarter-wavelength coupled-line sections connected with two different transmission lines. Moreover, the interconnection transmission line can be designed as a small phase variation phase shifter with the introduction of a shorted stub. The detailed theoretical analysis is provided to illustrate the design concept. The circuit parameters of this novel arbitrary phase-difference coupler can be easily determined by the derived closed-form equations. For verification, two 3 dB couplers with 45°/135° and 60°/120° phase differences are fabricated and measured. The 45°/135° coupler achieves 34.2% fractional bandwidth for 45° phase difference with 2° phase variation and 36.3% fractional bandwidth for 135° phase difference with 3.4° phase variation. Moreover, the 60°/120° coupler achieves 30% fractional bandwidth for 60° phase difference with 0.9° phase variation and 32.9% fractional bandwidth for 120° phase difference with 1.5° phase variation.
 
A tri-band planar monopole antenna with one linearly polarised band, two circularly polarised bands, and bidirectional radiation patterns is presented in this study. The antenna consists of a monopole with seven arms etched on the top side of a FR4 substrate, and a slitted ground plane on the bottom side. Measured results show that the antenna has a S 11 <−10 dB impedance bandwidth of 9.7% (2.05–2.26 GHz) for the lower band, 11.3% (3.41–3.82 GHz) for the middle band, and 60.2% (4.89–9.11 GHz) for the upper band. The circular polarisation (CP) is radiated within the middle (left hand CP) and the upper (right hand CP) bands, and the measured broadside 3 dB axial ratio bandwidths are 4.9% (3.53–3.71 GHz) and 58.6% GHz (4.69–8.58 GHz), respectively. The proposed antenna has a total size of 25 × 35 × 1.6 mm ³ and can be employed for the 2.1 GHz UMTS, whereas the CP bands can be used for the 3.5/5.5 GHz WiMAX, 5.5 GHz Wi-Fi, and 5.2/5.8 GHz WLAN applications.
 
Schematic of the defocusing operation and phase factor obtained by offsetting the sub-reflector
Convergent curve of PSO and optimised phase factor (a) For random distortion, (b) For gravitational distortion
Real surface distortion and retrieved surface distortion by Misell algorithm using different phase factors
The deformation of reflector surface can be obtained by Misell algorithm using two far-field amplitude patterns: one with the antenna in focus and the other with the antenna defocused. For reflector antennas with active surface, the defocused pattern can be measured by adjusting the panels to get a more effective phase factor instead of offsetting the sub-reflector. The shape and size of phase factor have a great influence on the accuracy of convergent solution. To improve the performance of Misell algorithm, this study proposed an optimisation method for phase factors under different surface deformations. This method utilises Levenberg–Marquardt algorithm to get an estimation of the aperture distribution and then applies the particle swarm optimisation to optimise the phase factor parameterised by Zernike polynomials. Numerical simulations show that the proposed method is an efficient tool to achieve applicable phase factors under different surface deformations. Using the optimised phase factor, Misell algorithm can get a better convergent performance than traditional Misell algorithm.
 
This study proposes the design of the periodic lossy magnetic (PLM) surface with a low profile for enhanced array characteristics. The proposed PLM surface consists of multiple rectangular grooves with equal spacing, and the grooves are filled with lossy magnetic materials to increase the surface impedance between array elements to mitigate the mutual coupling. To verify its feasibility, the authors fabricate a two-element circularly-polarised patch antenna array with the PLM surface inserted into the ground plane and measure antenna characteristics in a fully anechoic chamber. The results demonstrate that the PLM surface is capable of reducing the mutual coupling so that the active array patterns can be improved with higher front-to-back ratios and lower active reflection coefficients for large steering angles.
 
In the study, an improved method for measuring the axial ratio of circularly polarised antennas based on the polarisation rotation is proposed. In the measurements, an auxiliary antenna can work in different rotation senses of polarisation (left-hand or right-hand) according to the status of the antenna polariser. By measuring received signals of the circularly polarised antenna under test when the auxiliary antenna work in the left-hand or right-hand modes at two different axial angles, the axial ratio of the antenna under test can be determined from the polarisation efficiency equations. The measurement of a typical circularly polarised antenna is performed in the anechoic chamber by using this method. Through the comparison between the measured results and the simulation results, the validity of this method is demonstrated. This method improves the measuring efficiency and has important engineering application.
 
A novel magneto-electric dipole antenna with wide H-plane beamwidth, triple-linear polarisation and frequency reconfiguration is proposed for 5G communications. In this study, the radiated dipoles are designed with the arc-shaped planes to exhibit wide H-plane performance. A bent cross-dipole feed with PIN diodes and varactor diodes inserted into its four arms at different altitudes is utilised to excite and control the antenna. Consequently, a measured wide beamwidth ranging from −67° to 67° can be achieved in H-plane. In addition, the height of the proposed antenna decreased about 30% due to the bent designs. Finally, by controlling the PIN diodes and varactor diodes, the three kinds of linear polarisations and frequency reconfiguration functionalities for 2G/3G/LTE/5G applications can also be easily obtained. With the above-mentioned features, the proposed antenna well suits for intelligent base-station communications.
 
In wireless system deployment, the system planner must deploy base station antennas at the most appropriate locations to maximise system performance. In this study, the effect of transmitting antenna deployments on coverage in a single room office environment at millimetre wave frequencies is investigated using a three-dimensional implementation of Geometrical Optics. Multiple antenna deployments and artificial environmental modifications (convex reflectors) are also investigated in order to quantify how they might improve coverage compared to a single antenna deployment. A quantitative estimate of shadowing inside the office volume indicated that there are significant non-illuminated regions which cannot be eliminated by simply increasing the transmitted power. It is shown that such regions can significantly be reduced in size if an appropriate deployment of multiple antennas or passive reflectors is adopted.
 
This study presents the study of beamforming capabilities of arrays installed on a non-conductive unmanned aerial vehicle (UAV). The main purposes of this study are the application of a beamforming algorithm by including the airframe in the optimisations and the study of simplifications of the aircraft model, so as to allow performing full-wave simulations, even though the UAV is much larger than the operating wavelength. To validate the simplified electromagnetic model, antenna arrays have been designed and installed onto the UAV. Radiation pattern measurements demonstrate that the proposed simplifications yielded very good radiation pattern predictions and can be used as guidelines for simulation of other kinds of non-conductive aircrafts.
 
This study investigates the channel performance of a 28 GHz multiple-input–multiple-output (MIMO) system in a subway tunnel, using ray-tracing-based simulations and experimental measurements in a realistic tunnel environment. The transmitter (Tx) array was deployed at three positions, i.e. in the centre of the tunnel, close to the sidewalls and under the ceiling. The performance was significantly better in the centre and sidewall positions. Good agreement is obtained between simulation and measurement results. Furthermore, it is inferred that the MIMO channel capacity for the Tx–receiver distance of 60 m is higher than that of 15 m. The angular spread calculated for the 60 m case is also larger than that of the 15 m case.
 
The measured power for different polarization angles.
This study presents a polarisation-independent metasurface harvester composed of an ensemble of novel electric-field-coupled inductive-capacitive (ELC) resonators. The ELC resonator has full symmetry in a way that its behaviour is highly insensitive to the polarisation of the incident wave. Loading the resonators with resistors (which model the input impedance of a power combining circuit in a harvesting system), it is shown that the metasurface absorbs the incident electromagnetic wave energy, with nearly unity harvesting efficiency, irrespective of its polarisation while simultaneously delivering the absorbed power to the loads. As a proof of concept, a metasurface harvester composed of a 9 × 9 resonator array working at 2.45 GHz was fabricated. Near-unity harvesting efficiency of the metasurface was demonstrated using full-wave numerical simulation for a wide range of polarisation angles. Laboratory tests showed strong agreement between the simulation results and the measurements.
 
A gaseous plasma antenna array (PAA) is an aggregate of plasma discharges and possibly conventional metallic radiating elements, and it constitutes a promising alternative to metallic antennas for applications in which fast reconfiguration of radiation pattern, and gain is desired; such properties can be achieved by exploiting the electronic switch on/off condition of plasma discharges, and tuning of the plasma parameters. Here, the authors present a reconfigurable PAA that features a central metallic half-wavelength dipole working around 1.45 GHz, surrounded by a planar circular lattice of cylindrical plasma discharges. Customised plasma discharges have been realised, and filled with argon gas at 2 mbar so as to have a complete control on the plasma discharge properties (e.g. plasma frequency, collisional frequency). The magnitude of the reflection coefficient, and the gain pattern on the H-plane have been investigated numerically and experimentally; numerical and experimental results exhibit a good agreement and show that the central intrinsically omnidirectional antenna can provide simple beamforming capabilities upon turning on a subset of plasma discharges; as these plasma discharges are turned on, the authors have observed a maximum gain of -5 dBi, a half-power beam width of 80 ∘ , and an angular steering resolution of -15 ∘ .
 
In this study, a printed dual-band multiple-input multiple-output (MIMO) antenna is presented for Worldwide Interoperability for Microwave Access (WiMAX) and wireless local area network (WLAN) applications. Two symmetric printed inverted-F antennas (PIFAs) are considered here as radiators. One of the open-circuited radiating arms of the PIFA is loaded with a folded resonator to achieve dual-band characteristics. Two symmetric stepped 'L'-shaped open ground slots have been introduced to enhance the isolation at the lower operating band. This also realises diagonal orthogonal radiation patterns of the antenna elements a results low correlation between them. Measurement reveals that the proposed MIMO antenna system has two 10 dB return loss bandwidths in the ranges of 2.6-3.08 and 5.48-6.12 GHz with isolation > 21 dB over the operating bands. Respective gains are 2.5 and 3.48 dBi at the lower and upper resonant frequencies, respectively. Promising diversity performances are also investigated in terms of different MIMO parameters (total active reflection coefficient, envelope correlation coefficient, diversity gain, mean effective gain, and channel capacity loss), which are found within their acceptable range. Furthermore, the characteristics of the antenna have been investigated in the presence of a large ground plane, USB connector, and housing to ensure the applicability of the proposed antenna in handheld wireless devices.
 
Herein, a dual-band circularly polarised (CP) antenna for adjustable frequency ratio is proposed. The proposed antenna consists of two square-ring patches with a gap on the stacked substrates. A capacitor is inserted within the gap of each square-ring patch. The two square-ring patches have two resonant frequencies, because the additional resonant frequency is generated by the gap. The two first resonant frequencies of each square-ring patch combine to form the low band and two second resonant frequencies combine to make the high band. In the low band, as the gaps of the two square-ring patches are located on different diagonals, the two first resonant frequencies are orthogonal to each other. In the high band, as the second resonant frequency is perpendicular to the first resonant frequency, the two second resonant frequencies are also orthogonal to each other. In the measurement results, the low and high bands achieve −10 dB bandwidths (BWs) of 3.75 and 2.49%, 3 dB axial ratio BWs of 0.99 and 0.53%, and gains of 1.89 dBi and 0.64 dBi, respectively. By appropriately changing the values of two capacitors, frequency ratio has an available range from 1.065 to 1.336.
 
A decoupling metamaterial (MTM) configuration based on fractal electromagnetic-bandgap (EMBG) structure is shown to significantly enhance isolation between transmitting and receiving antenna elements in a closely-packed patch antenna array. The MTM-EMBG structure is cross-shaped assembly with fractal-shaped slots etched in each arm of the cross. The fractals are composed of four interconnected-'Y-shaped' slots that are separated with an inverted-'T-shaped' slot. The MTM-EMBG structure is placed between the individual patch antennas in a 2 × 2 antenna array. Measured results show the average inter-element isolation improvement in the frequency band of interest is 17, 37 and 17 dB between radiation elements #1 & #2, #1 & #3, and #1 & #4, respectively. With the proposed method there is no need for using metallic-via-holes. The proposed array covers the frequency range of 8-9.25 GHz for X-band applications, which corresponds to a fractional-bandwidth of 14.5%. With the proposed method the edge-to-edge gap between adjacent antenna elements can be reduced to 0.5λ0 with no degradation in the antenna array's radiation gain pattern. Across the array's operating band, the measured gain varies between 4 and 7 dBi, and the radiation efficiency varies from 74.22 and 88.71%. The proposed method is applicable in the implementation of closely-packed patch antenna arrays used in SAR and MIMO systems.
 
In this study, a circularly polarised (CP) patch array antenna based on reactive impedance surface (RIS) is presented. The proposed 4 × 4 patch array antenna possesses a RIS layer which is utilised to replace the conventional ground plane in order to enhance the bandwidth. Meanwhile, patches are cut with two diagonal corners and a rectangular slot for the fulfilment of wideband CP radiation and fed by a one-to-sixteen network simultaneously. Furthermore, in order to explain the effect of RIS in the axial ratio (AR), the CP antenna elements with (or without) RIS are designed. There is the same trend in simulated and measured results. Measured results show the proposed antenna exhibits good performances, such as wideband impedance matching, low AR, high gain. It is testified that pure CP radiation and symmetry broadside patterns are achieved in a wide frequency band, indicating the proposed antenna can exhibit significant application in global satellite communication systems and 5G Wi-Fi application.
 
A tunable low-pass to bandpass switchable filter is presented in this study. The proposed structure consists of two concentric resonators. Along with varactors, outer resonator is also loaded with a PIN diode switch. The switching conditions of the PIN diode - OFF lead to a low-pass filter (LPF) and ON lead to a bandpass filter. At the ON condition of the switch, the outer resonator behaves as two resonators which are coupled to each other through two ways - inductively coupled through via hole and capacitively coupled through the varactor loaded between the resonators. The coupling coefficients between the two grounded resonators are controlled using varactors to achieve constant fractional bandwidth (CFBW) tunability. The third CFBW tunable band with bandpass characteristic having three transmission zeros across the passband is achieved using inductively coupled inner resonators. A filter is designed, simulated and fabricated. The response shows a tunable LPF having a tuning range of 0.55-1.4 GHz. One tunable CFBW bandpass response with a tuning range of 0.850-1.25 GHz is achieved with a CFBW of 64%. The third tunable band with bandpass filtering behaviour is achieved with a tunable range of 1.04-1.6 GHz with a CFBW of 13.4%.
 
Test board experiment (a) Model and (b) Simulation results of a test board. The width of holes in the x-axis is from 10 to 3 mm. The height of holes in the z-axis is from 20 to 4 mm
Model and simulation results of human body and gun using depth information (a), (c) Model meshes without and with a gun; (b), (d) Images of model (a) and (c) with depth information. Bright yellow represents close to the antenna array, while dark red represents far from antennas array. The area within white rectangular is most likely to detect the abnormal objects
A high-resolution imaging system for short-range personnel security screening based on a novel multiple-inputmultiple- output (MIMO) array is presented. It overcomes the problem of specular reflection which occurs in the most state-ofthe- art mono-static-based configuration. Prior to the state-of-the-art portal-based architecture, this system is able to reconstruct three-dimensional (3D) image of the whole body and detect a concealed weapon in secret while passenger walking through the hallway or intersection, which is a potential for realising a reconfigurable system to monitor the marching crowd. The architecture consists of four transmitter columns and two receiver circles, illumination area which covers the most concerned part of the human body. The reduced number of sensors and no moving part characteristic make authors' system cost efficient and robust. A ray-tracing technique is applied to fast estimate its illumination and imaging performance. Several numerical experiments based on the frequency modulated continuous wave model with 10-25 GHz frequency sweeping are proposed to reach both azimuth and range 1 cm resolution. 3D results of these experiments show that authors' system works well on wholebody imaging and threat object detection.
 
In this study, a method to estimate the far-field boundary of the high-power ultra-wideband (UWB) pulsed antennas is presented. By analysing the properties of the UWB pulsed antennas, it shows that the radiation space should be separated based on the behaviours of time or phase differences caused by the distribution of radiation elements of the antenna. In the time domain, characteristic parameters of the radiated pulse can be different due to various waveforms, which means that the farfield condition is waveform dependent. So, for all the various waveforms, it is difficult to formulate a universal far-field condition with a certain time-domain parameter. Therefore, the far-field boundary is better to be estimated in the frequency domain, which is obtained by substituting the cut-off frequencies of the UWB system into the classical far-field conditions. The cut-off frequencies are determined by the 90% energy edge frequencies of the radiating field. Finally, a reflector Impulse Radiating Antenna (IRA) system is developed to verify the proposed method. The electric fields of the IRA from the near- to far-field regions are measured, which indicates that the far-field boundary calculated by the proposed method agrees with the experimental result well.
 
S-parameter variation (a) S 21 for different L 11 of the SIRs, (b) Corresponding S 11 , (c) S 21 for different L 24 of the SRs, (d) Corresponding S 11
Fabrication process of the proposed DBBPF using an advanced IPD technique
Morphology evaluation of the fabricated chip (a) Altitude morphology by microscope, (b) Surface roughness by AFM,, (d) Uniformity control of strip-to-strip distance and strip width, where (c) and (e) correspond to the thickness of two random positions by SEM
Physical sizes of the proposed design (μm)
Compact and high-performance radio-frequency devices require several optimisation processes in the design, fabrication, and packaging, as well as a thorough reliability verification. The authors propose a compact planar dual-band bandpass filter comprising mixed electric-magnetic coupling open-loop resonators. To increase the coupling performance and minimise the structure size, the resonators were topologically optimised into both external stepped-impedance resonators and internal half-wavelength spiral resonators. Besides the electric length of the resonators, the centre frequencies and transmission zeros were tuned to the desired values by controlling the coupling coefficients and quality factors. Moreover, in their design, the in-band flatness and bandwidth can also be adjusted. To validate their approach, they fabricated a filter on a gallium arsenide substrate using the advanced-integrated passive device process. Then, the filter was packaged by attaching it to a printed circuit board using a chip-on-board technique. The measurement results were consistent with those obtained from a simulation. A series of tests verified that the utilised fabrication and packaging techniques notably enhance the device reliability and its long-term stability even in harsh environments.
 
A compact ultra-high-frequency (UHF) antenna is developed in this work. First, by means of shifting symmetrical octagonal monopole into asymmetrical one, the impedance matching performance of antenna at high frequencies is improved; then, by cutting the radiation patch and ground plane in part, the antenna is reduced by 37.9% in size while maintaining its bandwidth; finally, a G-shaped slot on the ground plane is utilised to create an additional resonance, decreasing the low cutoff frequency without enlarging the antenna volume. The proposed antenna only occupies 124 mm × 77 mm × 1.6 mm and achieves 3:1 voltage standing wave ratio from 575 MHz to more than 4.5 GHz, along with simple structure, good radiation patterns, moderate peak gain, and strong pulse handling capability. Furthermore, the electrical dimension, ratio bandwidth, and bandwidth dimension ratio are compared with those of existing broadband antennas. The partial discharge (PD) experiments have been carried out for four typical insulation defect models in gas-insulated switchgear (GIS), and the results demonstrate that it is more sensitive to PD signals arising from floating metal, free metal particle, and void in the epoxy resin as compared to the disc coupler.
 
Characteristic mode (CM) theory provides a physical and intuitive guidance for discussing electromagnetic radiating or scattering issues and is a useful tool for the antenna design. The effective mode sorting is the key to the CM analysis. However, due to the mode degeneration or mode swapping in the observed frequency range, the mode tracking must be performed to obtain the correct mode sorting. Current mode tracking algorithms are mainly based on the correlation of eigenvectors, but they are computationally expensive and time-consuming. Here, the eigenvector number is limited by selecting the eigenvalues, to speed up the tracking realisation. At the same time, the factors affecting the correctness of the mode tracking are discussed, including the selected eigenvalue number and the mesh fineness. Some numerical examples are given to verify the reliability of the improved algorithm.
 
This paper presents two design improvements over conventional microstrip antenna in terms of multiband operation, gain and cross-polarization level (XP). The first design (Type-I) consists of a microstrip line on one side and an elliptical slot (ES) on the other side of the substrate. The second design (Type-II) consists of a parasitic elliptical patch inside the ES in addition to the first design. A theoretical analysis using equivalent circuit model is performed for both antenna designs for estimating the dimensions of each part of the proposed structures. It is found that the size of the wide ES determines the lower operating frequency whereas coupling between the parasitic patch and microstrip line decides the multiband operation. Experimental results show that the Type-I antenna resonates in three frequency bands at 2.49, 3.7, and 9.76 GHz with peak gain and XP level of 8.66 and 44.43 dB, respectively. With proper selection of the dimensions of the parasitic patch, the Type-II antenna generates five frequency bands at 1.87, 6.42, 10.76, 12.67, and 13.66 GHz. An exceptionally high gain of 16.79 dB with an XP level of 36.15 dB is reported in Type-II antenna. The theoretical and simulation results show a good agreement with measured results.
 
In this study, two types of dielectric resonator antennas (DRAs) with dual-polarisation are proposed and investigated for multiple-input multiple-output (MIMO) applications. The first design is a dual-slant polarised DRA and exhibits the measured impedance bandwidth of 8.2% (5.27-5.72 GHz) and 11.2% (5.05-5.65 GHz) for Port1 and Port2, respectively. Across the operating frequency band, the isolation between the input ports is better than 22 dB and the measured gain of the antenna varies in the range of 5.99-6.38 dBi. The second design developed here is a dual-linearly polarised DRA which is able to provide horizontal and vertical polarisations and is realised by integrating a two-stage rat-race coupler with the DRA for excitation purpose. In this case, the measured impedance bandwidths are 11.81% (5.18-5.83 GHz) and 15.18% (4.93-5.74 GHz) for Port1 and Port2, respectively, and the inter-port isolation is better than 20 dB throughout the operating frequency band. The antenna exhibits the measured gain variation of 5.94-6.48 dBi for Port1 and 5.54-5.86 dBi for Port2. Furthermore, for both the antennas, the envelope correlation coefficient is below 0.0004 throughout the bandwidth and the total active reflection coefficient is better than -10 dB, confirming the good diversity and radiation performance.
 
The study deals with the novel polarisation-independent 20-bit chipless radiofrequency identification (RFID) transponder that is based on a circularly arranged array of electrically small dual-spiral capacitively-loaded dipole scatterers with improved robustness of radar cross-section (RCS) response. A single scatterer exhibits a better performance in terms of electrical size and RCS than the earlier introduced U-dipole type scatterers occupying the same footprint area. In order to investigate the frequency stability and amplitude uniformity of RCS curve, two arrangements of scatterer arrays were analysed: rearranged side-by-side and circular. In comparison to the other arrangements, the latter provides an excellent amplitude RCS stability over the whole operational frequency band and, at the same time enables the polarisation independence of identification. However, it requires the two-channel orthogonal polarisation measurement.
 
This study presents a novel compact high gain dual reflector antenna design with a switchable feeder for ultra-wide band (UWB) applications. The design comprises a main parabola, a replaceable feeder antenna that is chosen depending on the requirements in a given situation, and an axially displaced elliptical sub-reflector. The functioning of two different feeder antennas with manual switching option, the partly dielectric loaded K-Vivaldi and double-ridged horn (RH), is discussed in this research study. When the K-Vivaldi is employed, the dual reflector antenna is more appropriate for impulse radiation with 33:1 operational bandwidth, ranging from 600 MHz to 20 GHz. The double-RH feeder antenna displays more efficiency than the KVivaldi one; however, the bandwidth in this case goes down to 12:1, ranging from 1.6 to 20 GHz. The sub-reflector, situated inside the main parabola aperture with onset type of feeders, reduces the depth of the structure to 16 cm. The antenna dimensions are 60 cm × 60 cm x 16 cm. Experiments have shown that the designed antennas display good efficiency and high gain performances in UWB operations.
 
A simple solution to efficient pressure measurement in sports balls using a force sensitive resistance sensor connected to an ultra high frequencies (UHF) radio frequency identification (RFID) tag is presented. The proposed solution has the advantage of increasing the pressure range measurement without changing the configuration of the employed RFID chip. It also offers a small and compact sensor structure due to miniaturised antenna design. The designed antenna is placed between the ball inner and outer layers. Prototypes were fabricated on a flexible substrate to make their integration easy into a sport ball. This smart tag concept was fully validated through measurements on different types of sports balls, and the results successfully compared to data from electronic ball pressure gauge.
 
Top-cited authors
Ke Wu
  • Montreal Polytechnic
Maurizio Bozzi
  • University of Pavia
Apostolos Georgiadis
Will Whittow
  • Loughborough University
Per-Simon Kildal
  • Chalmers University of Technology