The authors proposed a unidirectional, high-gain antenna loaded with the artificial magnetic conductor (AMC) back-cavity, which generates the reflection phase of 0° for gain enhancement in a low profile. The working bandwidth of the AMC unit cell is extended to 5.1–7.4 GHz (36.5%) by applying the fractal technology. The radiation performance improvement, wider bandwidth and increased gain are obtained for the composite antenna mounted on the fractal AMC reflector with the distance of 2 mm (0.04 wavelength at the central frequency of 6 GHz). The simulated results indicated that the operational bandwidth of the proposed antenna ranges from 4.7 to 8 GHz (52.0%). Besides, an average gain enhancement of 4 dB as compared to the original antenna without AMC back-cavity is achieved over the entire operating band. In addition, the proposed antenna is manufactured and tested, the measured results coincide well with the simulated counterparts.

Nowadays, the advancements in three-dimensional (3D) printing technology have made it possible for sophisticated structures to be fabricated fast, accurate, and affordable. In this study, a new method is proposed for the manipulation of near-electric and magnetic fields based on 3D coils. The manipulation of near-fields is widely used in various electromagnetic problems. The 3D coils lead to 3D distributions of current paths. By changing the shape of the coil (i.e. the current path) the desired electric field or magnetic field can be achieved. During the design procedure, at first dyadic Green's functions are used to calculate the electric and magnetic fields. Also, proper basis functions are used to define the current paths. Following that, with the help of genetic algorithm, the coefficients of the basis functions, and thus the shape of 3D coils are derived for the realisation of the desired field. Finally, this method is used to realise a normal distribution on a curved path. The coils are fabricated with fused deposition modeling (FDM) 3D printing technology and by attaching the conductive wires to the printed structure. The results of design, numerical simulations, and experimental tests indicate the high performance of this technique for manipulation of near-electric and magnetic fields.

This study presents the design and experiment of a terahertz reflective phase shifter based on liquid crystal. The proposed voltage-tunable phase shifter takes advantage of the variable permittivity of the liquid crystal, adjusted by an external electric field, providing tunable reflective phase shift of terahertz (THz) waves. We analysed the performance of this phase shifter using a simple isotropic and homogeneous model considering the angle of incidence. The phase variation was achieved in a range greater than 360° and frequencies from 349 to 361 GHz. An array constituted of 900 patch elements was fabricated using the photolithographic process. It provided tunable phase range of 350° over the frequency range of 353.5–359.3 GHz at 40 V, where a maximum phase shift 362.6° was achieved at 357 GHz. The LC dielectric constant was obtained by fitting the computed reflectance spectrum to the measured result. We compared accurate model, considering both inhomogeneity and anisotropy of the liquid crystal, with the simplified model. In addition, the differences were analysed between the measurements, accurate and conventional model. The results show the liquid crystal phase shifter is great for beam scanning antenna array in THz.

A field-controlled switchable frequency selective surface (FSS) with broadband absorption characteristic is proposed here, which comprises a switchable screen and a periodic resistive surface. The structure permits transmission of small signal with little attenuation and reflects high-energy signal spontaneously within the operating band around 2.1 GHz. Also it can absorb out-of-band electromagnetic wave from 6 to 17 GHz. The simulated results demonstrate that the FSS has favourable switching and absorption performance over 0–30° for incident wave, verified by experiment. This novel FSS integrating different functions has various prospects of applications in electromagnetic protection, electromagnetic compatibility, electromagnetic interference along with stealth technology.

A reconfigurable multiple-input multiple-output (MIMO) antenna design for the smartphone with a metal frame capable of covering hepta-band LTE/WWAN is proposed. The proposed antenna has four monopole slots (two long slots and two short slots) etched on the top and bottom side of the system ground, respectively. Two PIN diodes are inserted into the two long monopole slots. When these two PIN diodes are OFF, the two long monopole slots can generate the desired lower-band (824–960 MHz) which can be applied in a 2 × 2 MIMO system. When these two PIN diodes are ON, the corresponding lengths of these two long slots are decreased to the same length as the short slots. Then these four ports can excite the desired upper-band (1710–2690 MHz) which can be used for a 4 × 4 MIMO system. In addition, the performances of the proposed antenna including the return loss, isolation, efficiency, envelop correlation coefficient, mean effective gain, and channel capacity are presented.

Reconfigurable frequency-selective surfaces (RFSSs) are of significant interest in applications, such as secure communication systems or tunable radomes, to improve indoor communication and smart antennas. In order to change the frequency-selective surface (FSS) characteristics, and therefore its frequency response, conventional methods include loading with active semiconductor devices such as varactors, PIN diodes, Schottky diodes and radio-frequency microelectromechanical system. Another possibility is the use of mechanical adjusts, such as spring resonators or mechanical rotation. When PIN diodes are used, commonly only the reverse and forward regions, OFF and ON states, respectively, are considered. In this study, the implementation of an RFSS is described, using PIN diodes as active components. The RFSS is based on the four-arms star geometry, and initial design equations and procedures are presented. Numerical and measured results are shown for different project stages, with a very good agreement. In addition to obtaining two distinct resonant frequencies, due to the OFF and ON states, a third situation is included, considering the PIN diode threshold region when the FSS becomes practically transparent, which is an interesting feature with potential applications.

A stripline fed tapered slot antenna (STSA) with integral coupler operating as an active array element over 1.5 octave bandwidth is presented. The proposed STSA with integral coupler features wideband transition which allows a direct 50 Ω stripline feed and enables the coupler to be integrated along with the feed. The coupler helps in taking a sample of signal for online performance monitoring and active phased array calibration. The measured results of antenna element have exhibited good radiation pattern performance with low cross-polarisation in principal plane and low beam squint. Using this, a 16-element H-plane linear phased array antenna designed for the frequency range of 6–18 GHz and azimuth scan coverage of 60° is also presented. The active reflection coefficient of the array is calculated from measured mutual coupling coefficients and the array pattern over different scan angles is demonstrated. The grating lobe free scan coverage of 60° is obtained. The antenna array finds application in airborne systems.

In practical applications of wideband antenna-array processing, the signals to be processed may include either circular or non-circular or both circular and non-circular ones. The problem of direction-of-arrival (DOA) estimation for wideband circular/non-circular source signals is addressed in this study. The presented method is based on the incorporation of the envelope alignment and inverse power techniques, involving no steps such as matrix eigendecomposition and preliminary DOA estimation. The performance of the method in terms of the resolution capability and DOA estimation accuracy has been evaluated by extensive simulations and compared with some existing popular wideband DOA estimators.

Millimetre (mm)-wave communication is a viable solution to the future 5G cellular services. For different temperate locations, rain attenuation does not contribute significantly to the overall path loss at the mm-wave band for a small cell size of 200 m. However, in various tropical locations, the rain attenuation effect cannot be ignored even for this small cell size of the system. If the authors increase either frequency or path length, the attenuation increases significantly under tropical raining conditions. In this study, time series of predicted rain rate values are used for real-time prediction of rain attenuation over terrestrial paths.

A technique for rapid re-design of miniaturised microstrip couplers with respect to operating conditions as well as material parameters of the dielectric substrate is proposed. The dimension scaling process is based on a set of pre-optimised reference designs, obtained for an equivalent circuit model of the coupler at hand. The reference designs are utilised to construct an inverse surrogate model which – upon suitable correction – yields the optimum geometry parameter values of the structure re-designed for required values of operating conditions and substrate parameters. An iterative fine-tuning process necessary to account for the surrogate model inaccuracies is also developed. The proposed methodology is validated using a compact rat-race coupler example. The obtained results demonstrate that accurate scaling is possible at a low cost of up to three EM simulations of the structure. For a considered example, the scaling is realised within a wide range of operating conditions (1–2 GHz for the operating frequency, −3 to 0 dB for power split) and substrate parameters (0.7–1.5 mm for the thickness, and 2.5–3.5 for dielectric permittivity). Numerical results are validated experimentally.

A circularly polarised filtering dielectric resonator antenna is proposed. Left hand circularly polarised electromagnetic field is generated by virtue of a rectangular ring slot present on the ground plane of the dielectric laminate, beneath the dielectric resonator antenna, which is fed by a 50 Ω microstrip line. Additionally, a semi-elliptical shape bandpass filter is incorporated in the feed line to produce high selectivity, flat gain and wide stopband. A detailed analysis of the filter and the antenna has been carried out. The design offers fractional bandwidth of 6.13% ranging from 7.9 to 8.4 GHz, gain around 5.3 dBic and 3 dB axial ratio bandwidth of 3.68%. The simulated radiation efficiency is >80% in the pass band. The proposed structure has been fabricated and the results are compared with the simulated values.

This study describes an experimental investigation into improving the detectability of self-resonant structures. Passive self-resonant structures have been used as an alternative to active devices in many applications including radio-frequency identification (RFID) due to their long lifetime and low manufacturing cost. They can be used to store information in terms of the presence or the absence of resonance peaks at a given frequency window. This work investigates practical validation of gain enhancement techniques, aiming to improve the resonance characteristics of passive RFID tags to increase their read range and allow them to operate in highly lossy mediums. Two passive resonator designs are studied: a uniform cross-shaped resonator and a folded (gammadion) cross having a physical aperture 45% less than that of the uniform cross. Arrays of elements have been used to improve the detectability of the structure. Measurement settings have also been investigated in order to achieve higher levels of detectability.

A new design technique of a dual-mode ring-resonator suspended-substrate stripline filter is reported here to achieve low passband insertion loss, high quality factor, and good spurious response. A fourth-order bandpass filter was designed and fabricated at the operational frequency of 2.07 GHz with two ring resonators packaged in a metallic cavity where each ring resonator is one wavelength long. The transmission zeros of the dual-mode filter are generated due to the phase cancellation between the two paths in a ring, thus a sharp-skirt selectivity filter response was achieved. Perturbation notch structure was implemented on each ring resonator to provide the electromagnetic coupling of two degenerate modes, thus a dual-mode response of the filter can be synthesised. Measurement results of the first dual-mode filter prototype showed a return loss and an insertion loss of better than 16.42 and 0.926 dB, respectively, while an out-of-band rejection of up to 55 dB was achieved. The novel filter design technique proved that no cross-coupling is required to obtain the transmission zeros of the filter, thus the sharp-skirt response was achievable.

This study describes two stand-alone iterative algorithms that fully solve the issue of generating non-standard input impedances in probe-fed circularly polarised rectangular microstrip antennas (CPRMA). Both were derived from a new cavity model-based methodology that controls, at the frequency of operation (), the input reactance and resistance levels without disturbing the circular polarisation (CP) characteristics of the antenna. The first algorithm finds the antenna dimensions and probe position for an arbitrarily selectable input impedance and the second algorithm determines the possible choices of input impedance for a given dielectric substrate and . To highlight the applicability of both algorithms, two prototypes were designed and built: a right-hand CP (RHCP) active antenna with input impedance , and an RHCP reference antenna (RA) with input impedance . These antennas were utilised in the design and characterisation of a co-designed global positioning system active integrated antenna. The experimental and theoretical results show excellent agreement.

This study was conducted to design a wideband septum polariser as a feeder of the antenna to generate right-hand circular polarisation (RHCP) and left-hand circular polarisation (LHCP) concurrently. The proposed septum polariser overcomes the conventional bandwidth limit and covers 40% bandwidth from 8 to 12 GHz. Moreover, a quad-ridged horn is chosen as a radiator of the feeding network due to its waveguide structure, wide bandwidth, and dual polarisation. Therefore, the proposed structure consists of a quad-ridged horn fed by a square waveguide, a waveguide transition, and a wideband septum polariser. The final design has an axial ratio <1.5 dB over the X-band. This configuration can be used as a feed for reflector antennas due to the wideband and dual polarisation characteristics. Furthermore, the structure is highly recommended for high-power applications because of its waveguide feature.

A set of two compact ultra-wideband (UWB) antennas are presented and investigated in this study. Antenna 1 is a dual-polarised antenna consisting of a hexagonal wide slot, hexagonal stub and rectangular stub in the ground plane for getting UWB radiation characteristics and wide axial-ratio bandwidth. Antenna 1 radiates for 3.1–10.8 GHz band with 3-dB ARBW of 67.76% (4–8.1 GHz). Antenna 2 is a multi-notch UWB antenna and is designed by protruding out a rectangular spiral stub in the ground plane of antenna 1. It has been demonstrated in this work that a desirable number of band elimination notches (up to 14) can be easily achieved in antenna 2. Antenna 2 has been successfully fabricated and experimentally verified for seven band elimination notches with simulated and calculated band elimination notch frequencies in good agreement with each other.

A new geometry of wideband circularly polarised antenna with an asymmetric meandered-shaped monopole is proposed in this communication. The wideband characteristics of the antenna are accomplished by the two asymmetric meandered-shaped perpendicular arms of monopole and defected ground structure having slanting edge. The proposed prototype offers good performance in terms of impedance bandwidth (close to 102.04% in the frequency range 2.4–7.4 GHz) and the 3 dB axial-ratio (AR) bandwidth (close to 37.5% in the frequency range 4.9–6.9 GHz). The realised impedance and AR bandwidths cover the WLAN/Wi-MAX bands and Wi-Fi band (5.5 GHz).

In this study, a multi-polarised millimetre-wave (mm-Wave) base station operating at 28/38 GHz is introduced for future fifth generation (5G) mobile communication networks. The proposed station employs 32-element array antenna distributed in upside conical frustum configuration (UCFC) to synthesise multi-beam patterns adopting directivity and polarisation control. First, the design of dual-band circularly polarised antenna element at 28/38 GHz is discussed. The measured results show that the designed antenna element has a reflection coefficient less than −23 dB with a realised antenna gain of 8 dBi, on average, in the assigned frequency bands. Based on the designed antenna element, the antenna array performance is studied in terms of gain, radiation efficiency, reflection coefficient, and coverage efficiency. The results are compared with those obtained from 32-element structured in an octagonal prism configuration (OPC). For antenna design parameters and multi-beam pattern synthesis, a modified version of hybrid gravitational search algorithm and particle swarm optimisation is proposed. It is found that, the introduced UCFC outperformed the OPC by 19.8%. Moreover, distributing the antenna array in UCFC improved the convergence by 21.7% compared to OPC.

The authors present design of a printed log-periodic trapezoidal dipole array (PLPTDA) antenna with a balun-feed operating throughout the 2–18 GHz frequency range. For this purpose, the antenna is designed and simulated in the CST Microwave Studio environment. Thus, better directivity along the operation bandwidth is achieved via the iterative simulations with a 33.33% reduction in the dipole length at the lowest frequency 2 GHz as compared with the counterpart standard log-periodic antennas. Furthermore, they fabricated the PLPTDA antenna with and without the balun-feed. Thus, influence of the balun on the performance is investigated and the average gain of the PLPTDA antenna with the balun-feed is measured as 8.16 dBi provided voltage standing wave ratio (VSWR) < 2, which agrees with the simulation results. Furthermore, these measured gain/directivity results are compared with the counterpart antennas' measured performances in the literature considering their operation bandwidths and sizes. It can be concluded that their proposed PLPTDA antenna will prove to be useful in both wideband civilian and military applications in the S, C, X, and Ku bands within the 1:9 ultra-wideband bandwidth which is the largest achieved in this class antenna technology in the literature up to now.

A novel wideband microstrip magnetoelectric (ME) dipole antenna excited by a stripline aperture-coupled feeding structure is proposed and demonstrated. An antenna prototype is fabricated with low-cost standard printed circuit board process on three substrates and tested. Measured results show that an impedance bandwidth for voltage standing wave ratio (VSWR)  < 2 is ∼42% (10.2–15.6 GHz), and a measured boresight gain is ∼7.8 ± 0.7 dBi. Almost stable and symmetrical unidirectional radiation patterns with low in cross-polarization and backward radiation are obtained over the entire operating frequency band. By employing the proposed radiating element, two types of cavity (one is a square cavity, the other is a circular cavity) are introduced for enhancing the antenna gain and investigated. After analysis, a better one is chosen to be fabricated and measured. A measured impedance bandwidth for VSWR < 2 is ∼45% (10.1–15.9GHz), and a measured boresight gain is ∼8.9 ± 0.5 dBi. The measured average gain is ∼1.1 dBi larger than that of the radiating element. The measured radiation patterns, cross-polarization levels and half power beam width are almost the same as that of the radiating element except that the measured front-to-back ratios are improved several decibels.

The goal of this study is to propose a wideband circularly polarised (CP) psi-shaped antenna of high gain and wide bandwidth as necessitated by intelligent transportation system applications. A wide impedance bandwidth of 2 GHz (40%) centred at 5.3 GHz is achieved through the combination of modified TM₀₁, TM₂₀, and TM₂₁ modes with three slots on either radiating edge in the simple rectangular geometry of the antenna. The stacked patch configuration helps to achieve a minimum gain of 7 dBic in the entire bandwidth. The CP psi-shaped antenna has an impressive axial ratio bandwidth of 1 GHz, which is >19% of its centre frequency 5.3 GHz. The proposed antenna can be used for detection of the blind spots of a vehicle at 5.88 GHz (DSRC frequency band), vehicle-to-vehicle communication at 5.2 GHz (WLAN IEEE 802.11a frequency band), and vehicle to roadside communication at 5.5 GHz (WiMAX IEEE 802.16 frequency band).

A cold plasma-based re-configurable rectangular microstrip antenna has been computationally investigated. A power efficient frequency tunable patch antenna design is proposed wherein the substrate is replaced by a combination of air and plasma slots. The novel design consisting of plasma elements in localised regions achieves good tunability with minimal power requirements. Significant amount of frequency reconfigurability is achieved by varying the plasma density and slot configuration. A parametric study comparing the approximate power requirements and tunability of different designs has been presented. Finally the radiation properties of the proposed microplasma-based microstrip antenna has been evaluated.

Circularly polarised (CP) orbital angular momentum (OAM) antennas using aperture-coupled uniform circular array (UCA) are proposed to avoid the polarisation mismatch during the mobility and/or the space communications. A comprehensive derivation for ideal CP UCA is derived and numerical simulations are performed for further analysis. To validate the theoretical result and explore explicitly, the proposed prototype for ±1, ±2 mode OAM antennas with right-hand and left-hand circular polarisations are proposed and fabricated with measurements conducted, respectively. Measured radiation patterns, axial ratio versus elevation angle θ and the helical phase wavefronts are all favourable and exhibited. The transmission characteristics versus frequency f, relative azimuthal angel Δϕ and distance D are all measured and analysed, which shows high isolations between different channels.

A novel double-layer chessboard-like configuration is proposed to reduce radar cross-section (RCS) in microwave and mm-Wave bands. This structure is composed of two artificial magnetic conductor cells. One of these cells consists of four square patches on substrate and the other one is formed only by substrate without any copper on it. Despite the simple design of this structure, it has a big influence on significantly expanding the bandwidth of RCS reduction (RCSR). In comparison with the other works, the measured bandwidth of 10 dB RCSR is broadened to 113.33% (16.43–59.4 GHz). This wideband behaviour is achieved by effectively controlling the ratio between two dielectric constants and the ratio between their thicknesses and so the effective dielectric constant which adjusts the positions of all resonances. Although this structure contains two layers, its overall thickness is 1.4 mm, which is less than the ones that the recent works have. Measurements and simulations are performed for both monostatic and bistatic RCSs with the best number of the unit cells in the chessboard-like structure. The design, simulations, semi-analytical expressions and fabrication are presented and the measurements show an excellent agreement with the simulations.