In this paper, a realization method for high-efficiency broadband power amplifier (PA) based on split-ring resonator (SRR) second harmonic output matching network (SHOMN) is presented. The SRR is composed of double rectangular rings, and it can flexibly match the second harmonic components to the high-efficiency impedance region in the Smith chart in a wide frequency band. Besides, the impedance transformation characteristic of SRR is derived, and the optimal output impedance is converted from the transistor drain to the output of SHOMN for accurate fundamental matching. For validation, a PA operating in 1.2–3.0 GHz is designed and fabricated for a 10-W GaN transistor. The measurement results show that the maximum efficiency can reach 78.3%; meanwhile, the average saturated output power is 41.0 dBm.

This paper presents the design of a novel lowpass–bandpass diplexer with compact size and good performance using microstrip cells. For this purpose, two microstrip lowpass and bandpass filters are separately designed and mathematically analyzed. The proposed microstrip diplexer has a novel and simple structure. It occupies a compact area of 0.037 λg2 (722 mm2), and its insertion loss and S11 at both channels are low. The insertion loss at the lower and upper channels is only 0.047 and 0.16 dB, respectively. Meanwhile, both channels are flat with the maximum group delay of 1.68 ns which is the lowest compared to the previously reported diplexers mentioned in this paper. To design the proposed diplexer, first, a lowpass diplexer with good performance is designed which has low losses and a good figure of merit. It has a flat passband with a sharp roll-off. Then, to achieve the proposed diplexer, a bandpass resonator is added to the lowpass filter without any extra matching circuit, which saves the overall size. The proposed diplexer is designed, simulated, and fabricated, where the simulation and measurement results are close.

In this work, three successive filter models based on defected ground structure (DGS) techniques are designed, fabricated, and tested to be utilized for different wireless applications. The filters are printed on a RO5880 substrate of an overall size of 22 × 11 × 0.8 mm3. Three DGSs are etched from the ground plane to achieve the suggested band stop features for all frequencies above 10 GHz. The first model is a low-pass filter that succeeded to pass all frequencies below 10 GHz, the second model is a bandpass filter that operated at 4.5 and 9 GHz after using a pair of interdigital capacitors, and the third model covered all ultra-wideband frequency range when a pair of lumped capacitors is utilized instead of the interdigital capacitors. Good consistency between simulated and measured outcomes is achieved for each of the three filters confirming their suitability to be embedded in small-sized wireless devices.

The ability of food products to store and dissipate electromagnetic energy is determined by the material's dielectric properties. In relation to this phenomenon, a non-destructive technique is presented for food evaluation based on the shift in resonant frequency and reflection coefficient magnitude value of the proposed slot-loaded microstrip line-fed antenna-based sensor caused by the change in dielectric properties of the food material. In this work, a miniaturized antenna sensor of 10 × 10 mm2 size comprised of a dielectric substrate FR-4 with permittivity (εr) = 4.4 having ground plane at the bottom and a radiating element at the top is designed to operate at 13.3 GHz. Three samples of spices, i.e. red chilli powder, black pepper powder, and turmeric powder, are considered for quality monitoring whose relationship in terms of reflection coefficient, resonant frequency, and dielectric permittivity at 13.17, 12.61, and 13.09 GHz respectively is analyzed. Further, second-order polynomial model is derived to predict dielectric permittivity of the material under test with high accuracy. The experimental procedure of this proposed sensor is based upon the interaction of the sample food materials with the electromagnetic field owing to shift in resonant frequency as a function of dielectric permittivity of the samples. The proposed antenna sensor has a Q-factor of 409, showing significantly high sensitivity of 280 MHz with 98% accuracy and standard deviation less than the difference between unadulterated and adulterated values, giving resolution high enough to distinguish adulteration with an acceptable statistical accuracy.

In this paper, we propose a novel method for obtaining wideband spectral information of the target in microwave imaging systems by using broadband frequency reconfigurable printed transceivers. The proposed transceiver is composed of two embedded stacked microstrip antenna configurations operating in C-band (5.5–8.5 GHz) and X-band (8.5–11.5 GHz) with each having 3 GHz bandwidth. The transceiver switches between the two configurations (and thus frequency bands) using PIN diodes and collects response of the antenna in the presence of the target, in both the operating bands. This information is then combined to obtain wideband spectral information of the target from 5.5 to 11.5 GHz to achieve improved image reconstruction. The proposed reconfigurable transceiver has advantage over traditional broadband transceivers (those have slots/meandering at ground plane to achieve wideband response) that it has unidirectional radiation patterns throughout its band of operation. Hence the imaging system is implemented without the absorbers. This keeps the system compact and inconspicuous when installed for security applications. Here, a bistatic system is employed with two-dimensional target scanning performed in a real outdoor environment.

Resonant frequency varies significantly due to temperature changes for microwave resonant cavities. Hence, temperature robustness enhancement is of great importance. In this paper, a resonant cavity with enhanced temperature robustness is proposed by applying the dual-material strategy to the middle cavity. Compared to the single-material cavity, the dual-material cavity can demonstrate better temperature robustness with a decrease of 72.7% in the frequency shift over the temperature range of −20 to 80°C. Moreover, the |S11| < −10 dB impedance bandwidth is 6.3% (3.39–3.61 GHz) and the gain is 20.4 dBi at 3.5 GHz for the manufactured dual-material cavity, which are much better than those of the manufactured single-material cavity. Finally, an experiment is conducted to measure the resonant frequencies with the sample solution tube of the dual-material cavity filled with nothing or 30 mg/dl CuSO4 solution, the measured values are consistent with the simulated ones. The influence of temperature drift is significantly reduced, and the feasibility of the dual-material strategy is verified.

In through-wall radar system, the wall parameters, including permittivity, and wall thickness are of crucial importance for locating targets precisely. Recently, to obtain a quick and accurate estimation of wall parameters, an approach based on machine learning was introduced. However, these approaches are less reliable as only simulations results are presented. One of the major concerns with machine learning-based approaches is the generation of training and testing data which require fabrication of wall with different permittivity, thickness, and conductivity. Creating walls with different permittivity, thickness, and conductivity can really be challenging and expensive. Therefore, an effort has been made in this paper to establish a cost-effective and robust machine learning-based wall parameter estimation process with the usage of transmission line method and artificial neural network. The implementation and efficacy of proposed approach have been demonstrated through simulation and experimental results. The proposed approach quickly and accurately predicted the wall relative permittivity and thickness of real building wall. The merit of proposed approach is that it is less complex and computational efficient as it can extract wall parameters from only one measurement and therefore can be used in conjunction with any commercial through-wall radar systems.

Extraction of reference signal is an indispensable step in the signal processing of polarization diversity passive radar (PDPR) based on a digital television signal. A conventional reference signal extraction method requires an additional reference antenna, which has a certain demand for space. Single dual-polarization antenna passive radar (SDPPR) systems do not require a reference antenna, and the radar station layout is flexible, which is suitable for a large-scale radar network. It is a main research direction of PDPR in future. However, its reference signal extraction needs to rely on the signal reconstruction method. When the signal to interference and noise ratio of the direct-path signal is relatively low, the signal reconstruction method will fail. In this paper, we propose a reference signal extraction method based on sub-carrier processing method, blind adaptive oblique projection technology, and extensive cancelation algorithm to solve the above problem. Experimental results show that the method proposed in this paper is a reasonable alternative after the failure of reference signal reconstruction, and it is an effective supplement to the reference signal extraction technology.

This paper presents a continuous frequency tuning Fabry–Perot Cavity (FPC) antenna of which the operating frequency can be varied from 4.87 to 5.84 GHz (18.1%). The cavity of the proposed antenna is excited by a double-layer antenna structure, which is made up of a main square patch and a parasitic one. The superstrate is a frequency-selective surface, consisting of 6 × 6 square-patch-type unit cells. The frequency tuning property is realized by employing 48 phase-changing elements placed around the main patch, which forms a reconfigurable high impedance surface (HIS). By controlling the biasing voltages of the varactors inserted in the HIS element, its capacitances and reflection phases can be varied continuously, which leads to a variation of the operating frequency. An antenna prototype has been fabricated and measured for validation. The measured results are in good agreement with the simulated ones. In the frequency tuning range, the measured realized gains have a smaller variation from 10.2 to 14.1 dBi compared with other reported frequency tuning FPC antennas.

In this communication, a compact flower shaped printed antenna for ultra-wideband communication is proposed. The “flower-shaped” structure is capable of transmitting UWB-band signals. The designed antenna exhibits a return loss ranging from 2.3 to 11 GHz with an 8.7 GHz central frequency and has 130% relative bandwidth. The suggested design includes a patch in the shape of a flower with several slots fed by a microstrip line. Multiple slots have been designed for better resonances at lower modes. The antenna is constructed with an FR4 substrate, and a 50 Ω A-type connector feeds it. The optimum dimensions of the designed antenna are 12 × 16 × 1.6 cubic-millimeters and 0.092λ × .12λ × 0.012λ in lambda. The proposed structure also demonstrates stable radiation patterns across the operating bandwidth. The proposed radiator has a high gain of 2.67 dBi, and an efficiency of 85%. It is compact, lightweight, and easy to make. Therefore, it can be used for UWB applications.

In this paper, filter and antenna integration is studied to produce a compact device for wireless front-end equipment. Filtering antennas are advanced due to their selectivity performance, represented by a flat gain response. Two filtering antennas are proposed to improve the selectivity using different orders. The first antenna is based on second-order filter and the other on third-order filter. Both antennas are designed to operate at 4.65 GHz for mid-band 5 G application with a bandwidth of 6.45%. The first antenna integrates a rectangular radiator and an interdigital resonator based on second-order filter. It obtained a bandwidth impedance of −10 dB for 300 MHz and a maximum gain of 6.48 dBi. Meanwhile, the second design consists of a rectangular radiator and two interdigital resonators based on third-order filter as the feedline. Having the same bandwidth as the first design, the second design achieved a flat gain of 6.37 dBi in the operational bandwidth. The second antenna design showed better selectivity with sharper gain than the first design. The two antennas were fabricated and measured for validation. The simulation and measurement results showed good agreement.

In this paper, a novel approach for simultaneously correcting multiple degraded patterns under the failure condition of time-modulated linear arrays is proposed. The approach is based on the use of trapezoidal pulse with non-zero rise/fall time to control the switching status of the radio frequency switches that enables ON-OFF keying modulation of the array elements. After deriving a closed form expression of harmonic power loss and through the in-depth analysis, it is explored that the proposed trapezoidal pulse, because of having non-zero rise/fall time, provides less undesired harmonic power loss as compared to the conventionally used rectangular pulse with ideally zero rise/fall time. With the aim of reconstructing the degraded patterns with improved directivity and suppressed higher sideband power, three pulse-switching strategies based on rectangular and trapezoidal pulse have been employed, and their comparative performances prove the superiority of the proposed approach.

A dual-band dual-linearly polarized waveguide slot array antenna is proposed in this paper. Two orthogonal slots share a short-circuit wall on broad side. The horizontal and vertical slots, which actualize dual-frequency bands and dual-linear polarization, generate vertically polarized electric fields in 9.39–9.78 GHz and horizontally polarized electric fields in 11.62–12.3 GHz, respectively. The equivalent permittivity of the waveguide is changed by two grooves at the bottom of the waveguide, which improves phase velocity. Two grooves can also reduce the distance between the elements while suppressing the side lobes. Compared to filling the medium in the waveguide, two grooves have less losses. Moreover, the antenna is simple with one input port. The array antenna also overcomes the problems of beam scanning compared with the traditional waveguide slot array antenna. A 1 × 8 antenna array is fabricated and measured to verify the design. The horizontal polarization is formed in 9.39–9.78 GHz, the vertical polarization is generated in 11.62–12.3 GHz, and the antenna gain can reach 15 dBi.

Main aim behind this study is to utilize 3D printing technique for designing and fabrication of a flexible wideband Vivaldi antenna for the upcoming 5 G systems. Frequency bands starting from 24 up to 65 GHz are receiving particular attention as they have potential for both high data rate communications and high-resolution radar applications. A Vivaldi antenna is used for its very wideband properties and planar design. The radiating structure covers a bandwidth of 25–65 GHz for a match better than −10 dB and demonstrates reasonably high gain and efficiency performance. The simulated radiation efficiency of the antenna remains above 90% for the entire bandwidth. Hence, the main advantage of this approach is that wideband or switched transceivers for future 5 G communications can be integrated using this concept without the need of complex matching networks.

A novel low-profile two-arm backfire tapered cone helical antenna for telemetry tracking and command (TT&C) is introduced theoretically and experimentally in this paper. Compared with the conventional helical antenna, this design has a substantial decrease in its height and an increase both in its half power beamwidth (HPBW) and 3-dB axial ratio beamwidth by replacing the uniform structure with two tapered cone structures. It provides a maximum gain of 5.1 dBi, an excellent circular polarization radiation over a wide angular range of more than 124°, a 3-dB axial ratio bandwidth of 18.8%, a wide HPBW of more than 125°, and nearly equal E- and H- plane far-field patterns with high degree of axial symmetry over S-band (2–2.3 GHz). These excellent radiation characteristics with an endurable and compact structure make it an attractive candidate for TT&C application in microsatellite system, where the installation envelope and space for antenna are extremely limited.

In this work, a compact, wideband, circularly polarized wearable antenna is proposed using asymmetric coplanar strip feeding for wireless health monitor applications in the ISM frequency band. The proposed antenna consists of a composite right/left transmission line (CRLH TL) inspired structure to achieve compactness and a rectangular ring slot is created in the metallic ground to enhance the CP radiation of the antenna. A 5 × 5 artificial magnetic conductor unit cell-based metasurface is designed which is operating at 2.4 GHz as a ground plane for the CRLH-based CP antenna. Implementation of AMC-based metasurface as a ground plane results in enhancement of radiation characteristics of CRLH TL antenna. The proposed antenna has a small footprint of 50 × 50 mm2 with measured impedance bandwidth of 2.3–3.2 GHz. The 3 dB axial ratio bandwidth of the proposed antenna is 2.36–2.78 GHz with peak gain of 5.89 dBi. Simulation and experimental results show that proposed AMC-based antenna is robust in terms of structural deformation. Further, to validate the performance of the antenna for biomedical applications, we have simulated the proposed antenna with human phantom model. The proposed antenna exhibits stable scattering and radiation characteristics with the phantom body environment and due to the inclusion of AMC structure, the proposed design satisfies maximum specific absorption rate value given as per the IEEE standard safety guidelines.

The study aimed to present a dual-band 5G multi-input multi-output monopole dual-antenna system, which was configured on the upper edge of the display ground plane for laptops. The dual-antenna unit is a 29 × 2 × 3 mm3 three-dimensional structure with two antennas of the same structure and size, consisting of a fed monopole antenna and a shorted monopole antenna. The antennas are arranged 4 mm apart, side by side in the same direction, to form a dual-antenna unit. Such a configuration allows the feeding points of the two antennas to be separated by the shorting point of the shorted monopole antenna, and the shorting point of the low-frequency resonant path (the shorted monopole antenna) of the two antennas to be larger than a quarter of the wavelength of the low frequency with appropriate bending. The strong current will be concentrated near the shorting point of the shorted monopole antenna when the low-frequency mode is excited, and the current flow from the ground to the feeding point of the other antenna will be reduced, achieving the isolation which is better than 10 dB in measurement between the two antennas without adopting any isolation element. The envelope correlation coefficients calculated from the measured complex E-field radiation patterns are all smaller than 0.12, which can cover the 5G dual-band operations of 3300–3600 and 4800–5000 MHz, and the measured antenna efficiencies can reach more than 40%, which are well suited for multiple antenna applications.

A compact microstrip to substrate integrated waveguide transition feed tapered slot antenna array (1 × 4) with the elliptical patch is proposed and analyzed. The new approach is presented, employing a parasitic patch in the flared aperture to generate more radiation in the end-fire direction and improve field coupling between the arms. It boosts gain while maintaining overall size and low-frequency performance without using electrically thin dielectric substrates. To validate the designed approach, the proposed structure is fabricated and measured. The measured results show that the array provides impedance bandwidth of 40%, peak measured gain of 9.14 dBi, and radiation efficiency is 90% over the 8.5–12.5 GHz frequency band. A four-way feed network is also fabricated with the insertion loss of −6 ± 0.9 dB; the reflection coefficient is below −15 dB over the desired frequency range and only ±4° phase imbalance. The overall dimension of antenna array is 50 × 54 × 0.813 mm3 or ~1.6λ × 1.8λ × 0.027λ at 10 GHz.

In this paper, a compact four-element quasi-self-complementary (QSC) ultra-wideband (UWB) multiple-input multiple-output antenna with triple band-notched characteristics is proposed. Coplanar waveguides fed QSC structure is designed as radiation element to achieve wideband and small size. Besides, regular hexagonal split-ring resonators with single negative properties are designed and added on the back of the substrate to obtain high isolation in the whole operating band. Moreover, a double-fork-shaped slit is etched in each radiating element to generate three notched bands covering 3.63–4.24, 5.15–5.98, and 7.17–8 GHz. The experiment results indicate that the isolation of the proposed antenna is higher than 22 dB and the peak gain varies from 2.5 to 6.7 dB in the entire band of 3–18 GHz, which demonstrates that it is suitable for UWB applications.

A 5G rotated frame radiator for multiple applications is presented in the following paper. The presented geometry is capable of radiating the large frequency band from 2.91 to 12.17 GHz, which covers the 5G-(I) sub-6 GHz band, X-band communication with high efficiency. The impedance bandwidth of the radiator is 128%, with an electrical size of 0.24 λ × 0.24 λ × 0.15 λ in lambda. The antenna is simulated with an FR4 substrate using CST Simulator. 06-stages evolution process is also investigated by simulations, and corresponding S-parameter results are presented. Antenna's design comprises a patch in a rotated square fractal-like frame fed by a microstrip line. The proposed structure also demonstrates stable radiation patterns across the operating bandwidth. The proposed radiator has a high gain of 3.8 dBi, and an efficiency of 85%, which claimed that UWB range of the designed antenna. Therefore, it is useful for 5G-(I) sub-6 GHz band, X-band applications, including mobile, radar, and satellite microwave communication.