The concept of source currents of a radiating source can be employed to express its directivity in some particular cases analytically. For an antenna array, this concept can be extended to the concept of a generalised directivity based on self- and mutual radiation intensities, self- and mutual radiated powers and excitation currents of array elements. These approaches were applied to examples of the array of two elementary dipoles and the array of two isotropic radiators. Particularly, the evaluation of the directivity of the end-fire arrays, which are of special attention due to superdirective properties, was treated. Novel closed-form expressions for the directivity of these arrays with out-of-phase excitation were derived. It was observed that the end-fire directivity can be further enhanced by optimising the excitation currents of the arrays. Their optimal phase difference and corresponding increased directivity were also found analytically. The results were validated by a full-wave simulator.

This study considers a wideband monopole antenna and its interactions with different separations from the human body phantom over a wide frequency. A meshed alternative version has also been developed which has similar performance to the circular wideband monopole antenna but requires less material. The specific absorption rate and efficiency of this antenna have been analysed as a function of separation distance and frequency. The mesh design has been fabricated by using embroidery and 3D printing techniques.

In this study, the authors propose a new idea to employ mantle cloaking in order to reduce the mutual coupling between electrically small antennas (ESAs) placed on man-pack radio systems. They determined and numerically optimised the parameters of the mantle cloak on loop and monopole antennas; these antennas are to provide near vertical incidence skywave (NVIS) and ground wave high frequency (HF) communication in man-pack radio systems. The authors’ designed cloak can effectively minimise the mutual coupling between the ESAs – in the man-pack radios – for only a very small impact on the antenna's radiation characteristics. The authors believe that these improvements pave the way toward effectively applying these compact radio structures in emergency HF communication in areas with limited access to conventional communication links.

A dual-polarised transmitarray antenna unit cell based on conventional parallel-plate with low loss and wide bandwidth is proposed. A conventional parallel-plate is developed into two new structures; the first structure consists of two sets of parallel-plates forming a four-wall transmission line, while in the second structure the plates of the first structure are replaced by sets of strips. To ease the fabrication, in both cases, the plates are etched on FR4 substrates. These two structures are evaluated as the unit cell of transmitarray antenna to improve the efficiency and gain bandwidth. There is a basic difference between these two unit cells: the first unit cell increases the phase velocity and the second unit cell reduces the phase velocity while passing through the unit cells. Therefore, the layout of the metal antenna plates designed based on the first unit cell becomes concave-like and the layout of the metal antenna plates designed based on the second unit cell becomes convex-like. The antenna designed based on the second unit cell results in higher efficiency as such this antenna is fabricated and measured. The measured results show that the antenna has a peak efficiency of 85.6% and 1 dB gain bandwidth of 22.6%.

A low-profile circularly polarised (CP) antenna with wide 3 dB axial ratio beamwidth (ARBW) is proposed in this study. The possibilities of widening the 3 dB ARBW of single diagonal slot-loaded conventional CP antenna have been explored successfully by incorporating a pair of rectangular slots along the diagonal line of a square patch antenna. The design parameters and the symmetrical arrangement of this pair of slots across the centre of the patch are crucial to adjust and achieve the wide 3 dB ARBW with high CP purity. Coupling between the two diagonal slots provides nearly equal and radiation patterns over a large angular range, and hence the 3 dB ARBW has been enhanced by 85.71 and 55.4% for  = 0° and  = 90° planes, respectively, in comparison to the conventional slot-loaded CP antenna. The minimum 3 dB ARBW of 208° and a maximum of 230° have been achieved across different planes of radiation  = 0°, 45°, 90°, and 135° with a minimum axial ratio of 0.20 dB only. The absolute planar antenna of size (0.29 × 0.29 × 0.013)λ₀ ³, where λ₀ is the free-space wavelength corresponding to the CP frequency, has been fabricated and tested. The measured and simulated results show fair consistency to each other.

A new, hybrid, nona-band antenna of the size 80 × 8 × 5 mm³ is proposed for recent metal-rimmed smartphone applications. The proposed design is composed of a metallic frame (external antenna) and an internal radiating structure. The proposed antenna is attached to the top part of the smartphone. It uses a microstrip feed as a direct coupling line for the internal radiating structure. The feed also represents a capacitive coupling feed for the overall metal-rim antenna structure. The metal rim with a small gap of 2 mm is connected to the system printed circuit board by two grounding points. The proposed antenna excites four coupled loop modes, three slot monopole modes, three dipole modes, and one monopole mode. By incorporating the excited modes, the proposed antenna covers all recent smartphone applications in the frequency range between 0.79 and 6.0 GHz. The proposed design is described with great details and both simulated and measured results are used to establish its validity. The antenna performance with mobile phone components is also investigated. The results demonstrate that the proposed antenna represents an excellent candidate for applications such as GSM850, GSM900, GSM1800, GSM1900, UMTS2100, LTE2300, LTE2500, LTE3500, and WiFi5500 bands.

In this work, a compact quad-element multiple-input-multiple-output (MIMO) antenna system is proposed. The design consists of four annular slot-line based MIMO elements, printed on a commercially available RO4350 substrate with a total size of . Each annular slot has a rectangular slot inside it to increase its electrical length to further bring down the resonance frequency. Furthermore, a T-shaped defected ground structure is utilised to increase the isolation between the closely spaced antenna elements. Each element is loaded with a unique split ring resonator (SRR), which results in size reduction as well as multi-band operation covering 1.7–2.28, 2.35–2.85 and 2.9–3.1 GHz. Moreover, by using only a single varactor diode per element, fine frequency tuning over these bands is achieved. A wide frequency sweep over multi-band and a peak gain of 2.95 dBi with 82% maximum radiation efficiency is achieved. Low envelope correlation coefficient values are observed throughout the operating band, thus offering good MIMO performance. The overall planar antenna design is compact, low-profile and can easily be integrated with radio frequency front ends in second generation cognitive radios among many other applications.

This work investigates the performance of an envelope detector under high peak-to-average power ratio (PAPR) waveform excitations. First, the use of multi-stage rectifiers is investigated in wireless power transfer systems, as a method to increase the DC output voltage. It is shown that at low input powers, a large number of stages can reduce the efficiency dramatically. Then, high PAPR waveforms are presented as an alternative method to multi-stage rectifiers that can increase the DC output voltage without decreasing the efficiency at low input powers. Simulations and experimental measurements with different multi-tone waveforms and modulation schemes are shown. The results demonstrate that the PAPR and complementary cumulative distribution functions (CCDF) of a waveform significantly influence the RF-to-DC conversion. Using a 5-tone input waveform with −10 dBm power and a 100 kΩ load, the DC output voltage was increased by 32.3%, while using a 256 QAM modulation the output voltage was increased by 12.6%. The results show similar behaviour to that of multi-stage topologies but without decreased efficiencies at low input powers. These results suggest that the use of high PAPR waveforms can decrease the required number of rectifier stages, and hence increase the rectifier efficiency at extremely low input powers.

In this paper, a novel annular ring antenna is proposed to achieve a multi-wideband antenna. The structure is annular ring. For systematic design of the antenna, fractal shape is employed. Each iteration of the structure acts as a matching load for the previous section. Consequently, by increasing the electrical current path, multi-band and wideband performances can be achieved. For tuning the frequency bands, three slots are cut from the ground plane. The first operating band is from 1.47 GHz to 1.86 GHz (fractional band width (FBW): 23.42%), the second band is from 2.44 GHz to 3.51 GHz (FBW: 35.96%), and the third one is from 4.49 GHz to 5.34 GHz (FBW: 17.29%). Furthermore, the antenna has the ability to cover the ultra-wideband (UWB) spectrum by inserting two vias in the substrate. Using vias, impedance matching of the antenna is improved in the frequency range from 2.8 GHz to 5 GHz (FBW: 56.4%) with criteria |S ₁₁|<−10 dB. Also, it has a noticeable impedance bandwidth (FBW: 117%) from 1.3 GHz to 5 GHz with criteria |S ₁₁|<−6 dB. A prototype of the proposed antenna is fabricated and tested. The measurement and simulation results are in good agreement.

This study describes a systematic approach for the stability analysis of RF and microwave non-linear circuits in the time-domain and that can be useful also for the verification of other non-linearities, like intermodulation. The time-domain analysis is the most reliable approach for the evaluation of complex non-linear phenomena but, in general, the transient behaviour of non-linear circuits is difficult to verify at high frequencies, where distributed elements are common. The solution here addressed overcomes this limitation and it may be applied, without restrictions, also to monolithic microwave integrated circuits and EM-based designs. Examples of application to hybrid prototypes are provided, and the comparison between simulations and measurements illustrates the accuracy and reliability of the proposed approach.

A self-balanced electromagnetic bandgap (EBG) integrated, conformal printed T-shaped folded dipole antenna for wireless body area network (WBAN) is proposed in this study. The folded dipole antenna designed using polyester film and copper tape operates at 2.4 GHz industrial, scientific and medical radio bands. Integrating antenna with a two-element EBG reduces the frequency shift and back scattering of radiation into the body. The higher impedance bandwidth of the folded dipole plays a key role in maintaining resonance while the antenna is wrapped on a human arm. When the antenna is conformed on a human arm, EBG surface minimises the specific absorption rate (SAR), increases the gain and front-to-back ratio (FBR). Additionally, full-wave simulations and measurements were done to validate the proposed antenna design even when conformed to the human arm of people from different age groups. The FBR and gain of the antenna are improved by 75 and 61% and SAR has been reduced by 99% when it is integrated with EBG. The good impedance matching, high gain and FBR and reduced SAR make it a perfect candidate for WBAN.

This study presents the design of a forward-wave directional coupler based on slow-wave coupled microstrip lines realised in a multilayered printed circuit board technology. Thanks to the concept of slow-wave, a high difference between the even- and odd-mode phase velocities can be inherently achieved. Hence, by choosing adequate even- and odd-mode characteristic impedances close to 50 Ω, reduced-length forward couplers can be designed. As a proof of concept, a 0 dB forward coupler operating at 4.5 GHz is proposed. It exhibits a very high coupling of 0.56 dB at the centre frequency and a very large bandwidth of 43%, corresponding to 10 dB of isolation. A length miniaturisation of 84% is obtained as compared to the standard microstrip, leading to a total surface area of 0.132, the smallest one reported so far.

In this study, the authors present the design and measurement results of a voltage-controlled oscillator (VCO) that is based on a Colpitts core topology and a cascade output buffer. The circuit has a centre frequency of 176 GHz and is implemented in a 130 nm SiGe BiCMOS technology provided by IHP foundry. The VCO is a differential fundamental wave tunable oscillator that makes use of a transistor-based LC-resonator. On-wafer measurements show a tuning range of about 5% from 171 to 179.5 GHz. The circuit achieves a maximum output power of 7.3 dBm when biased at 1.6 V and 6.5 dBm if biased at 1.4 V, both with an efficiency of ∼ 6.6%. DC power consumption is 82 and 52 mW, respectively. The measured phase-noise of the VCO is − 110 dBc/Hz at 10 MHz offset. The VCO demonstrates state-of-the-art performance at these frequencies with very good performance in term of output power, efficiency linearity, phase noise and compactness; in addition, thanks to the proposed architecture it shows high integrability at the system level.

This study proposes the design of broad-band microstrip loop antennas with less-dispersive group velocity of radiation patterns for accurate direction of arrival (DoA) estimation. The proposed structure consists of radiating and feeding microstrip loops, and the radiating loop is electromagnetically coupled to the feeding loop. This feeding mechanism helps to suppress phase variations of radiating patterns by avoiding dual resonances within a wide frequency band over 60 MHz, which allows to obtain the less-dispersive group velocity for accurate DoA estimation. The effectiveness of the less-dispersive behaviour is validated by implementing the proposed structure into a four-element array to estimate vertical and horizontal placements using dual-axis phase interferometry. The results confirm that the proposed antenna has less-dispersive group velocity compared to a conventional microstrip patch antenna and is more suitable for accurate DoA estimation.

This study presents an antenna design that operates at the three bands of the Global Positioning System (GPS) receiver. The optimised Minkowski island fractal structure combined with the central window is used to design the radiation surface to reduce the size and adjust the resonance frequencies of the antenna. The antenna was designed, and simulated by HFSS computer code, and was fabricated on microwave laminate FR4-epoxy by fast laser prototyping. The resulting fractal antenna has been operating at three GPS-bands: f _L2 = 1.227, f _L5 = 1.176, and f _L1 = 1.575 GHz, with return loss S ₁₁ < −15 dB, bandwidths BW > 21.5 MHz, for VSWR < 2, at 50-Ω input impedance, the power gains ∼2.4, 2.85, and 4.2 dBi, and axial ratio of right-handed circular polarisation 1.5, 03, and 06 dB at f _L5, f _L2, and f _L1, respectively. The simulated and experimental results agree favourable.

To achieve a uniform illumination on the ground in the context of an indoor millimetre-wave 5G base station, path loss compensation is necessary. The gains of the antenna radiating at ±45° require an additional gain of 3 dB to compensate for the additional path loss to maintain the same link budget at those angles. Thus, to enhance the end-fire gain with minimal electrical footprint, spatially modulated zero-index metamaterial is proposed and integrated with a tapered slot antenna. The antenna post integration is analysed for the phase correction of the wavefront leading to a gain of 8.5 dBi and aperture efficiency of 77% at 28 GHz. A co-polarised vertical stacking topology is proposed to achieve path loss compensated beam switchable antennas with an angular coverage of ±65° and mutual coupling <30 dB throughout the band. Detailed simulated and measured results are presented and are in good agreement.

This study describes substrate integrated waveguide (SIW) cavity models to control the bandwidth of frequency selective surfaces (FSSs). Two different SIW cavities were presented: one model to reduce (cavity-I) and another model to improve (cavity-II) bandwidth of FSS, respectively. The periodic structures were made of SIW cavies and an FSS slot on either side of the dielectric substrate at the centre of the cavity. To prove the concept, a well-known cross-slot FSS element has been studied in two different cases and compared their performance. The conformal behaviour of the unit cells has been estimated by considering the practical requirements. The addressed SIW cavities will improve the electromagnetic (EM) performance of conventional FSS without altering its basic shape. The high Q-factor of SIW cavity greatly improves the performance of FSS. A full-wave simulation is used to analyse the response of FSS. The cavity-I shows 12.61% lesser bandwidth and cavity-II shows 25.0% higher bandwidth compared to its conventional shape in planar form and 9.1, 26.17% in the conformal form at a bent radius of 50 mm. Besides, performance parameters were studied to evaluate the structural dependency on EM performance. Finally, two designed cavity models are fabricated and proved their performance experimentally.

A two-port network featuring programmable reflection and transmission coefficients is proposed. The network exhibits a simple structure and can operate over a wide frequency range. A detailed description of the presented system is given, including the theoretical analysis and derivation of design equations. To assess the performance of the manufactured system, the tuning range of reflection and transmission coefficients was measured in the frequency range from 0.5 to 2.0 GHz. The obtained results show good agreement with the theoretical results over the entire two-octave operational bandwidth. Moreover, the potential application of the developed two-port for the generation of various reflection coefficients in calibration of a multiport reflectometer is demonstrated.

It has been recently shown that random arrays paired with a proper excitation strategy can be used to shape the array factor. These random arrays were termed unequally excited totally random arrays (TRAs). This scheme allows to greatly enlarge the class of obtainable patterns, well beyond the classical single beam. However, its main disadvantage is a reduction of the achievable performance, the latter measured in terms of the deviation from the average array factor. In this contribution, a strategy for remedying (even only partially) to such a loss of performance by retaining the same flexibility in shaping the pattern is addressed. To this end, a new scheme for generating random arrays that merges unequal excitations and a suitable generalised binned procedure is introduced. It is shown that this new model of generalised binned arrays always outperforms unequally excited TRAs. In particular, the study is focused on symmetric arrays (for which the elements are deployed symmetrically with respect to the array aperture centre) which allows to get closed-form expressions without invoking some common assumptions that in general do not hold true in practice. The theoretical findings are assessed by several numerical examples for the case of multi-beam array factors.

Accounting for manufacturing tolerances is an essential part of a reliable microwave design process. Yet, quantification of geometry and/or material parameter uncertainties is challenging at the level of full-wave electromagnetic (EM) simulation models. This is due to inherently high cost of EM analysis and massive simulations necessary to conduct the statistical analysis. Here, a low-cost and accurate yield estimation procedure for compact microwave couplers is proposed. Authors’ technique involves variable-fidelity electromagnetic (EM) simulation models, as well as fast surrogates constructed using a response feature approach. In order to improve the computational efficiency of the analysis, the primary surrogate is obtained from the characteristic points of the low-fidelity model and, subsequently, corrected using a single evaluation of the high-fidelity model. Combination of both methods results in an extremely low cost of yield estimation being just a few high-fidelity EM analyses. For the sake of demonstration, a compact hybrid rat-race coupler operating at 1 GHz is considered. Yield estimation is carried out under several scenarios concerning various probability distributions of the geometry parameter deviations. Reliability of the approach is verified by comparing the results with direct Monte–Carlo analysis and single-fidelity feature-based yield estimation.

Design and development of coaxial line-based 10–40 dB variable loop type dual-directional coupler for the frequency range of 50–500 MHz for 2 kW power handling capability has been presented in this study. The directional coupler has to be used in the radio frequency plasma experiment to detect the reflected power, so as to safeguard the system from unwanted reflected power. It consists of a main coaxial transmission line with two similar movable loop type coupling arrangements. The movable loops provide variable coupling for forward and reflected power separately, where the magnitude of coupling can be selected independently both for forward and reflected wave. The developed coupler has been tested where test results show return loss and isolation better than −10 dB and coupling from − 10 to − 40 dB in the frequency range of 50–500 MHz.

Here, a novel geometry of dual-band polarisation reconfigurable patch antenna is presented. The proposed prototype design consists of two orthogonal arms as a patch, an inclined slotted asymmetric pentagonal-shaped ground plane and a PIN diode with a biasing circuit. The polarisation reconfiguration is controlled via ON/OFF states of the PIN diode. The unique feature of the proposed antenna design is that it provides polarisation reconfiguration without considerable deviation in −10 dB impedance bandwidth (IBW) frequency response. The proposed antenna has a measured −10 dB IBW of 49.28% (3.96–6.55 GHz) and 41.47% (4.3–6.55 GHz) in the ON-state and OFF-states of the PIN diode, respectively. In the ON-state of the PIN diode, the antenna exhibits a dual-band circular polarisation. The first 3 dB axial ratio (AR) band is 4.5% (4.3–4.5 GHz) and the second 3 dB AR band is 17.69% (5.1–6.09 GHz). The proposed antenna exhibits almost uniform gain over the entire operating axial-ratio bandwidth, with peak CP gain of 2.8 dBic. The suggested antenna has a wide −10 dB IBW, and it is suitable for C-band wireless applications.

A composite antenna structure consisting of a conducting cone loaded with a spherical cap is designed and analysed, considering all eigenmodes in the antenna structure. The configuration is used to enhance the impedance match to a loop antenna located above the cone tip. Matching characteristics to the loop are examined over a wide range of loop radius and loop location. It is found that the matching to the loop can be significantly enhanced over a wide range of parameter values, yielding a greater flexibility in design than the loop-around-the-cone structure presented earlier with the conducting cone body inside the loop. The structure also enables placement of the loop at a greater height than the loop-around-the-cone configuration, potentially improving the efficiency of signal transmission/reception or reducing scattering from the vehicular body. The mutual coupling effect of a parasitic loop placed adjacent to the primary loop is also analysed. The radiation characteristics are also evaluated.

A circularly polarised microstrip antenna with a wide beamwidth and wide axial ratio (AR) beamwidth is presented. The antenna is mainly composed of a dielectric substrate, a single-feed patch, a reduced ground, and six metal columns. The reduced ground and six metal columns are primarily used to broaden the radiation beamwidth and improve the AR performance of the wide angle. According to the measured results, the antenna achieves a bandwidth of 3.78% from 7.78 to 8.08 GHz with an AR <3 dB, and an impedance bandwidth of 10.3% from 7.65 to 8.43 GHz with a return loss (RL) >10 dB. In addition, half-power beamwidth of 211° × 209° and 3 dB AR beamwidth of 222° × 239° at 8.0 GHz are observed. The antenna is a single-feed type with a simple structure and no additional feed network, which can be extended to some wide beamwidth and wide AR beamwidth applications in the X-band, even higher than the Ku-band.

To investigate the electromagnetic scattering problem related to a foil in a turbulent hypersonic plasma environment, the scattering field formulas for hypersonic plasma turbulence were improved by applying the anisotropic power spectrum. Using the theory of electromagnetic scattering of a conducting object, the second moment of a conducting object's scattering field is obtained in the hypersonic plasma turbulence. Finally, the backscattering cross-section of the foil surrounded by the hypersonic plasma turbulence was calculated. The results showed that the fractal dimension led to an increase in the complexity of the hypersonic turbulent structure, and thus had a greater influence on the electromagnetic wave scattering. The scattering cross-section of the foil in the hypersonic turbulence decreases rapidly. Meanwhile, the anisotropic parameters in the model represent scale-dependent stretching of the turbulent eddy. As the anisotropic parameters of the turbulence increase, there will be less scattering of the hypersonic turbulence. At this point, the total scattering cross-section of the foil in hypersonic plasma turbulence increases gradually. By improving the aerodynamic shape of the aircraft, it can produce a large number of anisotropic turbulent eddies which can effectively improve the passive jamming capability when the foil is cast.