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This paper presents a comprehensive review of research conducted on the Method of Auxiliary Sources (MAS) over a period of the last 22 years, i.e., since the last up-to-date survey was published. MAS is a very attractive numerical technique due to its simple algorithmic structure and the generally low computational cost it requires in terms of memory and CPU time; this is why it has been applied to a vast variety of cases, as concluded by the long citations list included. After a short introduction summarizing the fundamental concepts of the method, references since 2002 are categorized, briefly described, and commented on. This work is intended to assist every researcher who is involved in MAS computations, providing an exhaustive, to the best of the authors' knowledge, list of related publications. [ABSTRACT FROM AUTHOR]

Nowadays, additive manufacturing technologies have impacted different engineering sectors. Three- and four-dimensional printing techniques are increasingly used in soft and flexible electronics thanks to the possibility of working contemporarily with several materials on various substrates. The materials portfolio is wide, as well as printing processes. Shape memory polymers, together with composites, have gained great success in the electronic field and are becoming increasingly popular for fabricating pH, temperature, humidity, and stress sensors that are integrated into wearable, stretchable, and flexible devices, as well as for the fabrication of communication devices, such as antennas. Here, we report an overview of the state of the art about the application of 4D printing technologies and smart materials in electronics. [ABSTRACT FROM AUTHOR]

Additive manufacturing has emerged as a transformative methodology in numerous engineering domains, with the fabrication of antennas and electromagnetic devices being a promising application area. This study presents a comprehensive review of the application of these technologies for manufacturing electromagnetic devices, offering a categorized analysis based on different types of additive manufacturing techniques. Each category is examined, and its characteristics are briefly described, highlighting not only the most innovative and significant devices fabricated using specific technologies, but also identifying their limitations and strengths. Through a dual analysis, this paper provides a deep understanding of the potential of and challenges associated with using different additive manufacturing technologies in the design and crafting of electromagnetic components. Moreover, this review offers recommendations for future studies, suggesting how the unique features of this new manufacturing paradigm could be further leveraged for breakthroughs in the electromagnetic field. [ABSTRACT FROM AUTHOR]

Respiratory problems are common amongst older people. The rapid increase in the ageing population has led to a need for developing technologies that can monitor such conditions unobtrusively. This paper presents a novel study that investigates Wi-Fi and ultra-wideband (UWB) antenna sensors to simultaneously monitor two different breathing parameters: respiratory rate, and exhaled breath. Experiments were carried out with two subjects undergoing three breathing cases in breaths per minute (BPM): (1) slow breathing (12 BPM), (2) moderate breathing (20 BPM), and (3) fast breathing (28 BPM). Respiratory rates were captured by Wi-Fi sensors, and the data were processed to extract the respiration rates and compared with a metronome that controlled the subjects' breathing. On the other hand, exhaled breath data were captured by a UWB antenna using a vector network analyser (VNA). Corresponding reflection coefficient data (S11) were obtained from the subjects at the time of exhalation and compared with S11 in free space. The exhaled breath data from the UWB antenna were compared with relative humidity, which was measured with a digital psychrometer during the breathing exercises to determine whether a correlation existed between the exhaled breath's water vapour content and recorded S11 data. Finally, captured respiratory rate and exhaled breath data from the antenna sensors were compared to determine whether a correlation existed between the two parameters. The results showed that the antenna sensors were capable of capturing both parameters simultaneously. However, it was found that the two parameters were uncorrelated and independent of one another. [ABSTRACT FROM AUTHOR]

A review on planar printed structures that are based on Matryoshka-like geometries is presented. These structures use the well-known principle of Matryoshka dolls that are successively nested inside each other. The well-known advantages of the planar printed technology and of the meandered nested Matryoshka geometries are combined to generate miniaturized, multi-resonance, and/or wideband configurations. Both metal and complementary slot structures are considered. Closed and open configurations were analyzed. The working principles were explored in order to obtain physical insight into their behavior. Low-cost and single-layer applications as frequency-selective surfaces, filters, antennas, and sensors, in the microwave frequency region, were reviewed. Potential future research perspectives and new applications are then discussed. [ABSTRACT FROM AUTHOR]

Over the past few years, the Internet of Things paradigm has brought renewed significant interest to indoor positioning, tracking, and localization topics, principally since real-time locating technology allows a reference node to infer the position of tagged target nodes, creating the opportunity for millions of object-to-object awareness applications. This study first presents an overview of positioning localization techniques and discusses the use of ultra-wide bandwidth technology for complex environment monitoring, followed by consideration of the error sources that are present in line-of-sight (LOS) and non-line-of-sight (NLOS) scenarios between a reader and a tag. A technical review of the available industrial and commercial UWB real-time locating transceivers (RTLSs) is presented, with a focus on the frontend antennas that are integrated in these systems to establish the needed wireless communication for positioning. Then, the different characteristics of these antennas are summarized and discussed, along with their impact on the localization performance in terms of the reading range, position information accuracy, object-orientation-independent localization, and multipath mitigation. Solutions are suggested to achieve antenna-based improvements to the performance of RTLSs. [ABSTRACT FROM AUTHOR]

Continuous-wave tunable photonics-based THz sources present limited output power due to the restricted input optical power accepted by photomixers, along with reduced radiation resulting from low paraxial field amplitude. Here, we investigate multipolar antenna designs to increase the available continuous-wave THz output power by incorporating more photomixers. For this purpose, the spatial structures of the optical and THz E-fields are designed to enhance THz power and radiation in the far field. Simulations of 2 to 4 dipole antennas are conducted, demonstrating an improvement in antenna gain compared to standard dipole antennas. This is in addition to a potential increase in THz power and radiation for photomixing applications. Such work also paves the way for functionalizing the spatial structure of THz light for advanced applications. [ABSTRACT FROM AUTHOR]

Long-Range Wide-Area Networks (LoRaWAN) allow the transmission of data via radio link from sensors, which are potentially isolated or difficult to access, to gateways and servers that are connected to cellular networks for data processing, exchange, or relay, with low transmission power. This concept employs Long-Range (LoRa) modulation and has led to the emergence of many applications for the monitoring and tracking of objects. However, due to its characteristic of a low data rate for low-power communication, the transmission of information with LoRa technology is not suitable for the fast real-time monitoring of data. Additionally, due to its narrow bandwidth, an attempt to perform localization through the LoRa modulation technique will result in very limited accuracy because of its inability to resolve multipath problems. Thus, in this paper, we propose a multi-standard Ultra-Wide Bandwidth (UWB) and LoRa end-device that is capable of measuring location with high accuracy using UWB technology and then transmitting the location information through LoRa method to gateways and the Internet of Things Network. The results of measurements in indoor and outdoor scenarios show a UWB localization accuracy that is of sub-meter level, being between 10 and 33 cm, and a UWB range of 124 m in Line-of-Sight (LOS) and 55 m in Non-Line-of-Sight (NLOS) applications, respectively. [ABSTRACT FROM AUTHOR]

The growing interest in unmanned aerial vehicles (UAVs) from both the scientific and industrial sectors has attracted a wave of new researchers and substantial investments in this expansive field. However, due to the wide range of topics and subdomains within UAV research, newcomers may find themselves overwhelmed by the numerous options available. It is therefore crucial for those involved in UAV research to recognize its interdisciplinary nature and its connections with other disciplines. This paper presents a comprehensive overview of the UAV field, highlighting recent trends and advancements. Drawing on recent literature reviews and surveys, the review begins by classifying UAVs based on their flight characteristics. It then provides an overview of current research trends in UAVs, utilizing data from the Scopus database to quantify the number of scientific documents associated with each research direction and their interconnections. This paper also explores potential areas for further development in UAVs, including communication, artificial intelligence, remote sensing, miniaturization, swarming and cooperative control, and transformability. Additionally, it discusses the development of aircraft control, commonly used control techniques, and appropriate control algorithms in UAV research. Furthermore, this paper addresses the general hardware and software architecture of UAVs, their applications, and the key issues associated with them. It also provides an overview of current open source software and hardware projects in the UAV field. By presenting a comprehensive view of the UAV field, this paper aims to enhance our understanding of this rapidly evolving and highly interdisciplinary area of research. [ABSTRACT FROM AUTHOR]

In recent years, drones have been used in a wide range of fields, such as agriculture, transportation of goods, and security. Drones equipped with communication facilities are expected to play an active role as base stations in areas where ground base stations are unavailable, such as disaster areas. In addition, asynchronous operation is being considered for local 5G, in order to support all kinds of use cases. In asynchronous operation, cross-link interference between base stations is an issue. This paper attempts to reduce the interference caused by the drone network by introducing circularly polarized antennas against the conventional system using linearly polarized antennas. Numerical analyses are conducted to validate the effectiveness of the proposed system, where Signal-to-Interference Ratios (SIRs) are shown to be improved significantly as the numerical evaluation results. Specifically, for the scenario of only access links, in the region where conventional antenna architecture can only achieve an SIR of less than 20 dB, our proposed system applying circularly polarized antennas can almost realize an SIR of more than 40 dB. Significant improvement can be also observed in the scenario with the existence of backhaul links, where the conventional system had difficulty achieving our system design goal SIR of 16.8 dB, while the proposed antenna architecture could easily attain this goal in most regions of our evaluation ranges. [ABSTRACT FROM AUTHOR]

The capability of controlling and modifying wireless propagation channels is one of the prerogatives of beyond-5G systems. In this paper, we propose the use of a controllable local propagation environment surrounding the terminals, and analyze its positive effect on the multiplexing capability of massive MIMO systems. In particular, we focus on using a few switched passive elements surrounding each terminal. In this way, the modification of the propagation environment is not realized by means of a single structure, as in reconfigurable intelligent surfaces (RIS), but is achieved by the cooperative work of all the terminals. By employing numerical simulations, we show that the proposed system outperforms its non-reconfigurable counterpart in terms of the number of contemporary connected users. Moreover, the optimized system enables a substantial increase in the minimum received power by the terminals, thus guaranteeing superior channel fairness. [ABSTRACT FROM AUTHOR]

A low-profile, wideband, and high-gain antenna array, based on a novel double-H-shaped slot microstrip patch radiating element and robust against high temperature variations, is proposed in this work. The antenna element was designed to operate in the frequency range between 12 GHz and 18.25 GHz, with a 41.3% fractional bandwidth (FBW) and an obtained peak gain equal to 10.2 dBi. The planar array, characterized by a feed network with a flexible 1 to 16 power divider, comprised 4 × 4 antenna elements and generated a pattern with a peak gain of 19.1 dBi at 15.5 GHz. An antenna array prototype was fabricated, and the measurements showed good agreement with the numerical simulations as the manufactured antenna operated in the range of 11.4–17 GHz, with a 39.4% FBW, and the peak gain at 15.5 GHz was 18.7 dBi. The high-temperature simulated and experimental results, performed in a temperature chamber, demonstrated that the array performance was stable in a wide temperature range, from −50 °C to 150 °C. [ABSTRACT FROM AUTHOR]

In this contribution, a 25 GHz planar antenna, designed and realized in microstrip technology, is exploited as a lightweight and compact liquid sensor. The high working frequency allows minimization of the sensor dimension. Moreover, particular attention was paid to keeping the design cost low. Indeed, the frequency of 25 GHz is widely exploited for many applications, e.g., up to the last decade concerning radars and, recently, 5G technology. Available commercial antennas allowed minimization of the effort that is usually required to design the microstrip sensor. The antenna was in-house realized, and the microstrip Cu conductor was modified through controlled anodic oxidation in order to enhance the sensing features. The sensor capability of detecting the presence and concentration of ethanol in water was experimentally demonstrated. In detail, a sensitivity of 0.21 kHz/(mg/L) and an average quality factor of 117 were achieved in a very compact size, i.e., 18 mm × 19 mm, and in a cost-effective way. As a matter of fact, the availability of devices able to collect data and then to send the related information wirelessly to a remote receiver represents a key feature for the next generation of connected smart sensors. [ABSTRACT FROM AUTHOR]

This paper focuses on antenna modeling using wire-grid and surface triangulation as two of the most commonly used MoM-based approaches in this field. A comprehensive overview is provided for each of them, including their history, applications, and limitations. The mathematical background of these approaches is briefly presented. Two working algorithms were developed and described in detail, along with their implementations using acceleration techniques. The wire-grid-based algorithm enables modeling of arbitrary antenna solid structures using their equivalent grid of wires according to a specific modeling recommendation proposed in earlier work. On the other hand, the surface triangulation-based algorithm enables calculation of antenna characteristics using a novel excitation source model. Additionally, a new mesh generator based on the combined use of the considered algorithms is developed. These algorithms were used to estimate the characteristics of several antenna types with different levels of complexity. The algorithms computational complexities were also obtained. The results obtained using these algorithms were compared with those obtained using the finite difference time domain numerical method, as well as those calculated analytically and measured. The analysis and comparisons were performed on the example of a rectangle spiral, a spiral, rounded bow-tie planar antennas, biconical, and horn antennas. Furthermore, the validity of the proposed algorithms is verified using the Monte Carlo methodology. [ABSTRACT FROM AUTHOR]

Antennas that generate orbital angular momentum (OAM) have the potential to significantly enhance the channel capacity of upcoming wireless systems. This is because different OAM modes that are excited from a shared aperture are orthogonal, which means that each mode can carry a distinct stream of data. As a result, it is possible to transmit multiple data streams at the same time and frequency using a single OAM antenna system. To achieve this, there is a need to develop antennas that can create several OAM modes. This study employs an ultrathin dual-polarized Huygens' metasurface to design a transmit array (TA) that can generate mixed-OAM modes. Two concentrically-embedded TAs are used to excite the desired modes by achieving the required phase difference according to the coordinate position of each unit cell. The prototype of the TA, which operates at 28 GHz and has a size of 11 × 11 cm 2 , generates mixed OAM modes of −1 and −2 using dual-band Huygens' metasurfaces. To the best of the authors' knowledge, this is the first time that such a low-profile and dual-polarized OAM carrying mixed vortex beams has been designed using TAs. The maximum gain of the structure is 16 dBi. [ABSTRACT FROM AUTHOR]

This paper aims to analyse the state-of-the-art of graphene-based materials and devices designed for use in space. The goal is to summarise emerging research studies, contextualise promising findings, and discuss underway strategies to address some specific space-related problems. To complete our overview of graphene-based technology and address the relevance of graphene in the wide scenario of the space economy, we also provide an analysis of worldwide patents and the scientific literature for aerospace applications in the period 2010–2021. We analysed global trends, country distributions, top assignees, and funding sponsors, evidencing a general increase for the period considered. These indicators, integrated with market information, provide a clear evaluation of the related technology trends and readiness levels. [ABSTRACT FROM AUTHOR]

Since the end of the 19th century, radioelectronic devices (REDs) have actively penetrated into all modern community spheres. Achievements in the fields of radio engineering and electronics, as well as computing, information, telecommunications, and other technologies, have greatly contributed to this. The main elements of REDs are antennas and microwave devices. For example, linear (wire) antennas are the basis of long-distance communication agency networks of various law enforcement agencies and departments. The manufacturing of REDs requires the regular and rapid appearance of more and more advanced types with minimal costs. At the same time, the design complexity of REDs and the tightening of EMC requirements caused by the growth of upper frequencies of useful and interfering signals, the mounting density, as well as the capabilities of generators of intentional electromagnetic impacts, together with the need to take into account inter-element, inter-unit, and inter-system interference, require more and more accurate designs of REDs. However, this becomes impossible without computer modeling, which saves the time and financial resources required for their development, as well as to evaluate the correctness of the proposed technical solutions. During the design process, as a rule, a multivariate analysis or optimization of the product is performed. In this case, methods of computational electrodynamics (one of which is the method of moments) are used. They are based on the replacement of continuous functions with their discrete analogues (construction of a grid), which reduces the problem to the solution of a system of linear algebraic equations (SLAE). The problem's complexity depends on the complexity of the SLAE solution, which is determined by its order (which in turn is determined by the complexity of the simulated object and its surrounding area) and by the number of the required SLAE solutions for each problem (determined by the upper frequency of the signal, the number, and range of the optimized parameters). This dramatically increases the computational cost, which becomes the main constraint for the optimal design. Therefore, reducing the computational cost for the analysis and optimization of RED elements (in particular, linear antennas) is an important scientific problem. Meanwhile, finding new antenna structures that meet all the desired features (low price, required characteristics, manufacturable design with small dimensions and windage, etc.) is no less important today. One of the promise solutions for these problems is using a wire grid and sparse antennas for modeling and constructing antennas. Since the last century, a lot of research has been performed on them. The aim of this paper is to review their history and the main related aspects such as computational, acceleration, and optimization used methods, the fields of their application, and their evolution to this moment. In addition, this paper provides a possible future implementation of wire-grid and sparse antennas from the authors' point of view by presenting a new method that is under research to obtain effective wire sparse antennas. [ABSTRACT FROM AUTHOR]

Symmetries play an essential role in the field of physics. In this paper, we examine the relationship between the eigen-amplitudes of four (2 × 2) symmetrical antennas and the symmetry of the amplitudes of their sources (excitations) using mirroring effects. In our case, we find that changing mirrors using symmetry is identical to the point group theory. By exploiting the symmetry problem, we can show the advantage of reducing the size of the analysis domain, at least by a factor of two or more (2, 4, and 8...etc.) (depending on the problem). Several simulation examples have been developed by the MoM-GEC and HFSS to validate this approach. [ABSTRACT FROM AUTHOR]

A novel technique to enhance the bandwidth of mobile phone antennas using YIG ferrite is proposed. The technique is applied in two slot antennas which are the proposed antenna and the reference antenna. The two antennas have the same shaped slot consisting of a rectangle connected to a circle etched on the ground plane. A ferrite slab is attached at the region near the circular slot on the ground plane of each antenna. The measured bandwidth of the proposed antenna with ferrite slab is enhanced to 0.669–1.533 and 1.69–5 GHz compared with the bandwidth of 0.81–1.44 and 2.3–5 GHz for the antenna without ferrite. The bandwidth of the reference antenna with ferrite slab is enhanced to 0.715–5 GHz compared with the bandwidth of 0.813–1.01, 1.11–1.3 and 2.33–5 GHz for the antenna without ferrite loading. The technique has the virtues of easy fabrication and low cost. [ABSTRACT FROM AUTHOR]

The NATO SPS G-5014 project has shown the possibility of using a holographic RADAR for the detection of anti-personnel mines. To use the RADAR on a robotic scanning system, it must be portable, light, easily integrated with mechanical handling systems and configurable in its operating parameters for optimal performance on different terrains. The novel contribution is to use software programmable electronics to optimize performance and to use a time reference to obtain synchronization between the RADAR samples and the position in space, in order to make it easy to integrate the RADAR on robotic platforms. To achieve these goals we used the Analog Devices "ADALM Pluto" device based on Software Defined Radio technology and a time server. We have obtained a portable system, configurable via software in all its operating parameters and easily integrated on robotic scanning platforms. The paper will show experiments performed on a simulated minefield. The electronics project reported in this work makes holographic RADARs portable and easily reconfigurable, therefore adaptable to different applications from subsurface soil investigations to applications in the field of non-destructive testings. [ABSTRACT FROM AUTHOR]

This paper presents a novel approach to improving wireless communications in harsh propagation environments to achieve higher overall reliability and durability of wireless battery powered sensor systems in the context of in-vehicle communication. The goal is to investigate the physical layer and establish an antenna recommendation system for a specific harsh environment, i.e., an engine compartment of a vehicle. We propose the usage of electromagnetic (EM) and ray tracing simulations as a computationally cost-effective method to establish such a recommendation system, which we test by means of an experimental testbed—or test environment—that consists of both a physical, as well as its identical simulation, model. A pool of antennas is evaluated to identify and verify antenna behavior and properties at specified positions in the harsh environment. We use a vector network analyzer (VNA) for accurate measurements and a received signal strength indicator (RSSI) for a first estimation of system performance. Our analysis of the experimental measurements and its EM simulation counterparts shows that both types of data lead to equivalent antenna recommendations at each of the defined positions and experimental conditions. This evaluation and verification process by measurements on an experimental testbed is important to validate the antenna recommendation process. Our results indicate that—with properly characterized antennas—such measurements can be substituted with EM simulations on an accurate EM model, which can contribute to dramatically speeding up the antenna positioning and selection process. [ABSTRACT FROM AUTHOR]

Radio frequency identification (RFID) represents an emerging platform for passive RF-powered wireless sensing. Differential Multi-port RFID systems are widely used to enable multiple independent measurands to be gathered, or to overcome channel variations. This paper presents a dual-port/dual-integrated circuit (IC) RFID sensing tag based on a shared aperture dual-polarized microstrip antenna. The tag can be loaded with different sensors where the received signal strength indicator (RSSI) of one IC is modulated using a sensor, and the other acts as a measurand-insensitive reference, for differential sensing. The 868 MHz tag maintains a minimum unloaded read range of 14 m insensitive to deployment on metals or lossy objects, which represents the longest reported range of a multi-port RFID sensing tag. The tag is loaded with a light-dependent resistor (LDR) to demonstrate its functionality as a battery-less wireless RFID light sensor. Following detailed RF characterization of the LDR, it is shown that the impedance, and consequently the RSSI, of the sensing tag are modulated by changing the light intensity, whereas the reference port maintains a mostly unchanged response for a correlated channel. The proposed tag shows the potential for channel variations-tolerant differential RFID sensing platforms based on polarization-diversity antennas. [ABSTRACT FROM AUTHOR]

This paper focuses on the combination of the method of moments and the wire-grid approximation as an effective computational technique for modeling symmetrical antennas with low computational cost and accurate results. The criteria and conditions for the use of wire-grid surface approximation from various sources are presented together with new recommendations for modeling symmetrical antenna structures using the wire-grid approximation. These recommendations are used to calculate the characteristics of biconical and horn antennas at different frequencies. The results obtained using different grid and mesh settings are compared to those obtained analytically. Moreover, the results are compared to those obtained using the finite difference time domain numerical method, as well as the measured ones. All results are shown to be in a good agreement. The recommendations used for building a symmetrical wire-grid of those symmetrical antenna elements provided the most advantageous parameters of the grid and mesh settings and the wire radius, which are able to give quite accurate results with low computational cost. Additionally, the known equal area rule was modified for a rectangular grid form. The obtained radiation patterns of a conductive plate using both the original rule and the modified one are compared with the electrodynamic analysis results. It is shown that the use of the modified rule is more accurate when using a rectangle grid form. [ABSTRACT FROM AUTHOR]

In this paper, a compact SISO (Single-Input Single-Output) antenna with a novel slotted-decahedral geometry is presented. The design was performed using a full-wave simulator (FWS). A decahedral patch with an eight-pointed star-shaped slot and two rectangular grooves is the main characteristic of the proposed novel radiator. The decagon shape, eight-pointed star-shaped slot, rectangular grooves, and separate section give rise to radiation. The SISO antenna operates from 23.1 to 29.94 GHz covering the proposed frequency bands for 5G wireless communication systems. The radiation pattern stability, moderate to high gain values (6.5 dBi, average), good radiation efficiency (higher than 89.4%), wide impedance bandwidth (6.84 GHz), compactness (13 × 13 × 0.787 mm3) and ease of manufacture are the principles advantages of this novel geometry. The experimental validation of the SISO antenna was performed, and good agreement was obtained with simulation results, and an equivalent circuit model was proposed. The proposed SISO antenna can be easily integrated into wireless sensors, drones, backhaul, and 5G devices which support IoT, working in the n257, n258 and n261 frequency bands. [ABSTRACT FROM AUTHOR]

Substrate-integrated waveguide (SIW) is a modern day (21st century) transmission line that has recently been developed. This technology has introduced new possibilities to the design of efficient circuits and components operating in the radio frequency (RF) and microwave frequency spectrum. Microstrip components are very good for low frequency applications but are ineffective at extreme frequencies, and involve rigorous fabrication concessions in the implementation of RF, microwave, and millimeter-wave components. This is due to wavelengths being short at higher frequencies. Waveguide devices, on the other hand, are ideal for higher frequency systems, but are very costly, hard to fabricate, and challenging to integrate with planar components in the neighborhood. SIW connects the gap that existed between conventional air-filled rectangular waveguide and planar transmission line technologies including the microstrip. This study explores the current advancements and new opportunities in SIW implementation of RF and microwave devices including filters, multiplexers (diplexers and triplexers), power dividers/combiners, antennas, and sensors for modern communication systems. [ABSTRACT FROM AUTHOR]

The exigency for continuous use of electrical devices has created greater demands for electricity along with more efficient transmission techniques. Energy from natural resources can be solar, thermal, vibration, friction, or Radio Frequencies (RF) signals. This state-of-the-art work provides a summary of RF energy harvesting techniques and can be used as a guide for the manufacture of RF energy scavenging modules. The use of Radio Frequency (RF) Energy Harvesting (EH) technique contributes to the development of autonomous energy devices and sensors. A rectenna system includes three main units: the receiving antenna, the impedance matching network, and the rectifier. We thoroughly analyze how to design a rectenna system with special emphasis given on the design of the rectifier. At the same time many works of the last 10 years are presented. This review article categorizes the used topologies depending on the type of antennas, IMNs, and rectifiers and comparatively presents their advantages and disadvantages. [ABSTRACT FROM AUTHOR]

Wireless devices, such as smartphones, tablets, and laptops, are intended to be used in the vicinity of the human body. When an antenna is placed close to a lossy medium, near-field interactions may modify the electromagnetic field distribution. Here, we analyze analytically and numerically the impact of antenna/human body interactions on the transmitted power density (TPD) at 60 GHz using a skin-equivalent model. To this end, several scenarios of increasing complexity are considered: plane-wave illumination, equivalent source, and patch antenna arrays. Our results demonstrate that, for all considered scenarios, the presence of the body in the vicinity of a source results in an increase in the average TPD. The local TPD enhancement due to the body presence close to a patch antenna array reaches 95.5% for an adult (dry skin). The variations are higher for wet skin (up to 98.25%) and for children (up to 103.3%). Both absolute value and spatial distribution of TPD are altered by the antenna/body coupling. These results suggest that the exact distribution of TPD cannot be retrieved from measurements of the incident power density in free-space in absence of the body. Therefore, for accurate measurements of the absorbed and epithelial power density (metrics used as the main dosimetric quantities at frequencies > 6 GHz), it is important to perform measurements under conditions where the wireless device under test is perturbed in the same way as by the presence of the human body in realistic use case scenarios. [ABSTRACT FROM AUTHOR]

Featured Application: Experimental results prove that major improvement of THz radiation detection can be obtained by the optimal design of planar antenna with HTSC Josephson junction. Matching between the low impedance of the junction and the high impedance of the antenna is attainable, resulting in a high efficiency detection system, while enjoying the drastic reduction of system complexity of a THz radiation detection by a Josephson junction. Superconducting Josephson junctions have major advantages as detectors of millimeter wave radiation. Frequency of the radiation can be easily derived from the Shapiro steps of the current-voltage characteristics. However, system performance is highly sensitive to impedance mismatch between the antenna and the junction; therefore, optimization is essential. We analyzed and implemented an improved antenna structure, in which the junction is displaced from the antenna center and placed between the ends of two matching strips. Based on theoretical analysis and advanced electromagnetic simulations, we optimized strip dimensions, which affect both the detection magnitude and the frequency of the reflection coefficient dip. Accordingly, two Au bow-tie antennas with different matching strip widths were fabricated. Superconducting Yttrium Barium Copper Oxide (YBCO) thin films were deposited exactly at the bicrystal substrate misorientation points, forming Josephson junctions at the ends of two matching strips. We found a very high correlation between the simulations and the response to Radio Frequency (RF) radiation in the range of 145–165 GHz. Experimental results agree extremely well with the design, showing best performance of both antennas around the frequency for which impedance matching was derived. [ABSTRACT FROM AUTHOR]

In this paper, we present the design and fabrication of two dual circularly polarized (CP) patch antennas that can be profitably used as feeders for reflector systems normally adopted for satellite applications. In the first part of the manuscript, we propose the optimization of a dual-CP patch antenna, loaded by a fence of passive monopoles around it to increase antenna gain for high elevation angles. To achieve dual-CP operation mode, the circular patch has been sequentially-fed by three pins, whose mutual phase-shift is equal to 120 ° . The antenna feeding network was placed on the antenna back and designed using microstrip technology. Two different input ports provide both right-hand (RH) and left-hand (LH) circular polarizations. A prototype of such an antenna was fabricated and measured at f = 8.25 GHz. Furthermore, to test the versatility of the proposed single radiating patch, in the second part of the manuscript, we present the results of geometrically scaling at f = 2.2 GHz (higher UHF band) and this was used as a building block for the design of a dual-CP sequentially-fed 2-by-2 array antenna. The results for both the proposed antennas are satisfactory in terms of impedance bandwidth, broad radiation pattern, gain and cross-polarization rejection, thus they can be profitably used as feeders for reflectors at relatively low frequencies. [ABSTRACT FROM AUTHOR]

Rydberg atoms have been extensively utilized in microwave measurement with high sensitivity, which has great potential in the field of communication. In this study, we discuss the digital communication based on a Rydberg atomic receiver under simultaneously coupling by resonant and near detuning microwaves. In addition, we verify the feasibility of the Rydberg atom-based frequency division multiplexing (FDM) in microwave communication. We demonstrate the principle and performance of the atom-based FDM receiver by applying amplitude modulation (AM) and frequency modulation (FM), respectively. To demonstrate the actual communication performance at different data transfer rates, we consider monochromatic images as an example. The experimental results show that the maximum acceptable data transfer rate of both AM and FM is about 200 kbps, whereas their maximum bit error rates (BER) is less than 5%. When compared with the traditional electronic receiver, this atomic receiver, which is compatible with FDM, has numerous advantages, such as small size, low power consumption, and high sensitivity. Furthermore, this receiver has a strong ability of anti-electromagnetic interference, and the signals transmitted do not interfere with each other in different channels. [ABSTRACT FROM AUTHOR]

A review of substrate integrated waveguide (SIW) components designed for the use in beam-forming networks working in X-Band is presented. The proposed devices are four-port and six-port couplers and magic tees for the network. The devices take full advantage of the use of SIW technology in order to reduce size, weight, and cost. The design processes of all devices are exposed, and the experimental results of prototypes show the feasibility of these solutions. [ABSTRACT FROM AUTHOR]

Underwater Wireless Sensor Networks (UWSNs) using electromagnetic (EM) technology in marine shallow waters are examined, not just for environmental monitoring but for further interesting applications. Particularly, the use of EM waves is reconsidered in shallow waters due to the benefits offered in this context, where acoustic and optical technologies have serious disadvantages. Sea water scenario is a harsh environment for radio communications, and there is no standard model for the underwater EM channel. The high conductivity of sea water, the effect of seabed and the surface make the behaviour of the channel hard to predict. This justifies the need of link characterization as the first step to approach the development of EM underwater sensor networks. To obtain a reliable link model, measurements and simulations are required. The measuring setup for this purpose is explained and described, as well as the procedures used. Several antennas have been designed and tested in low frequency bands. Agreement between attenuation measurements and simulations at different distances was analysed and made possible the validation of simulation setups and the design of different communications layers of the system. This leads to the second step of this work, where data and routing protocols for the sensor network are examined. [ABSTRACT FROM AUTHOR]

In the 1990s, the Institute of Electrical and Electronics Engineers (IEEE) restricted its risk assessment for human exposure to radiofrequency radiation (RFR) in seven ways: (1) Inappropriate focus on heat, ignoring sub-thermal effects. (2) Reliance on exposure experiments performed over very short times. (3) Overlooking time/amplitude characteristics of RFR signals. (4) Ignoring carcinogenicity, hypersensitivity, and other health conditions connected with RFR. (5) Measuring cellphone Specific Absorption Rates (SAR) at arbitrary distances from the head. (6) Averaging SAR doses at volumetric/mass scales irrelevant to health. (7) Using unrealistic simulations for cell phone SAR estimations. Low-cost software and hardware modifications are proposed here for cellular phone RFR exposure mitigation: (1) inhibiting RFR emissions in contact with the body, (2) use of antenna patterns reducing the Percent of Power absorbed in the Head (PPHead) and body and increasing the Percent of Power Radiated for communications (PPR), and (3) automated protocol-based reductions of the number of RFR emissions, their duration, or integrated dose. These inexpensive measures do not fundamentally alter cell phone functions or communications quality. A health threat is scientifically documented at many levels and acknowledged by industries. Yet mitigation of RFR exposures to users does not appear as a priority with most cell phone manufacturers.

Partial discharges are ionization processes inside or on the surface of dielectrics that can unveil insulation problems in electrical equipment. The charge accumulated is released under certain environmental and voltage conditions attacking the insulation both physically and chemically. The final consequence of a continuous occurrence of these events is the breakdown of the dielectric. The electron avalanche provokes a derivative of the electric field with respect to time, creating an electromagnetic impulse that can be detected with antennas. The localization of the source helps in the identification of the piece of equipment that has to be decommissioned. This can be done by deploying antennas and calculating the time difference of arrival (TDOA) of the electromagnetic pulses. However, small errors in this parameter can lead to great displacements of the calculated position of the source. Usually, four antennas are used to find the source but the array geometry has to be correctly deployed to have minimal errors in the localization. This paper demonstrates, by an analysis based on simulation and also experimentally, that the most common layouts are not always the best options and proposes a simple antenna layout to reduce the systematic error in the TDOA calculation due to the positions of the antennas in the array [ABSTRACT FROM AUTHOR]

In this paper, we propose a high-durability X-band patch antenna with a coplanar waveguide (CPW) feeding network that can directly connect the upper antenna and the lower transmitting receiver modules without additional cabling. To improve the durability of the antenna, both the sub miniature push-on (SMP) connector and the feed-pin are separated through the CPW layer. This separated structure can minimize the deterioration of the antenna performance when the unwanted movement of the SMP connector occurs from external shocks. To verify the design feasibility, the reflection coefficient and radiation patterns of the proposed antenna are measured in a full anechoic chamber. The reflection coefficient of the proposed antenna is −27 dB at 10 GHz, and the operating frequency bandwidth (|Γ| < −10 dB) is 4.7% (9.82~10.29 GHz) in X-band. The proposed antenna has a maximum gain of 5.7 dBi at the bore-sight direction. In addition, the durability evaluations of the proposed design according to unwanted SMP connector movements provoked by external shocks are investigated using Monte Carlo simulations. The results demonstrate that the proposed high-durability antenna design is suitable for military ship applications that involve exposure to various external shocks. [ABSTRACT FROM AUTHOR]

This paper proposes a four-element ultrawideband (UWB) planar antenna array with elliptical-shaped radiators and a stripline excitation network designed for the 6–8.5 GHz UWB frequency band allowed in Europe by the European Commission. The designed antenna array has a symmetrical structure in which the radiators are placed along one line in the central conducting layer, arranged between two layers of a dielectric. Radiating elements are fed by the stripline excitation network that provides uniform power distribution. The dimensions of the elliptical radiators' axes are 14 mm × 16 mm. Two variants of array are proposed. The distance between the radiators' centers is L = 19 mm for a shorter variant and L = 24 mm for a longer one. The presented antenna array structures have a size of 81 mm × 41 mm and 96 mm × 41 mm. These arrays present a measured gain of 6.4–10.6 dBi for the shorter variant and 8.5–10.8 dBi for the longer one and a fair impedance matching. The measured |S11| is less than −8.7 dB and −9.7 dB for the shorter and longer corresponding variants. [ABSTRACT FROM AUTHOR]

Metamaterials are artificially engineered devices that go beyond the properties of conventional materials in nature. Metamaterials allow for the creation of negative refractive indexes; light trapping with epsilon-near-zero compounds; bandgap selection; superconductivity phenomena; non-Hermitian responses; and more generally, manipulation of the propagation of electromagnetic and acoustic waves. In the past, low computational resources and the lack of proper manufacturing techniques have limited attention towards 1-D and 2-D metamaterials. However, the true potential of metamaterials is ultimately reached in 3-D configurations, when the degrees of freedom associated with the propagating direction are fully exploited in design. This is expected to lead to a new era in the field of metamaterials, from which future high-speed and low-latency communication networks can benefit. Here, a comprehensive overview of the past, present, and future trends related to 3-D metamaterial devices is presented, focusing on efficient computational methods, innovative designs, and functional manufacturing techniques. [ABSTRACT FROM AUTHOR]

A high Q planar chipless RFID tag with high sensitivity is proposed for communication applications. In particular, the tag structure is composed of a complementary spiral structure (CSS) that is able to provide high sensitivity and compactness. A semi analytical formula for the design of a single bit tag is derived, and the behaviour of the CSS is analysed for different dielectric substrates. Different tags, composed of up to a set of eight resonators, have been numerically and experimentally assessed. In particular, a system prototype composed of a reader and a set of tags are fabricated and experimentally assessed as a proof of concept. The system provided an operative range of tens of centimetres (a maximum operative range of 65 cm). The obtained results are quite promising and the agreement between simulated and measured results are found to be good. [ABSTRACT FROM AUTHOR]

We demonstrate a concept for a large enhancement of the directivity and gain of readily available cm- and mm-wave antennas, i.e., without altering any property of the antenna design. Our concept exploits the high reflectivity of a Bragg reflector composed of three bilayers made of transparent materials. The cavity has a triangular aperture in order to resemble the idea of a horn-like, highly directive antenna. Importantly, we report gain enhancements of more than 400% in relation to the gain of the antenna without the Bragg structure, accompanied by a highly directive radiation pattern. The proposed structure is cost-effective and easy to fabricate with 3D-printing. Our results are presented for frequencies within the conventional WiFi frequencies, based on IEEE 802.11 standards, thus, enabling easily implementation by non-experts and needing only to be placed around the antenna to improve the directivity and gain of the signal. [ABSTRACT FROM AUTHOR]

Cognitive radio (CR) technology has the potential to detect and share the unutilized spectrum by enabling dynamic spectrum access. To detect the primary users' (PUs) activity, energy detection (ED) is widely exploited due to its applicability when it comes to sensing a large range of PU signals, low computation complexity, and implementation costs. As orthogonal frequency-division multiplexing (OFDM) transmission has been proven to have a high resistance to interference, the ED of OFDM signals has become an important local spectrum-sensing (SS) concept in cognitive radio networks (CRNs). In combination with multiple-input multiple-output (MIMO) transmissions, MIMO-OFDM-based transmissions have started to become a widely accepted air interface, which ensures a significant improvement in spectral efficiency. Taking into account the future massive implementation of MIMO-OFDM systems in the fifth and sixth generation of mobile networks, this work introduces a mathematical formulation of expressions that enable the analysis of ED performance based on the square-law combining (SLC) method in MIMO-OFDM systems. The analysis of the ED performance was done through simulations performed using the developed algorithms that enable the performance analysis of the ED process based on the SLC in the MIMO-OFDM systems having a different number of transmit (Tx) and receive (Rx) communication branches. The impact of the distinct factors including the PU Tx power, the false alarm probability, the number of Tx and Rx MIMO branches, the number of samples in the ED process, and the different modulation techniques on the ED performance in environments with different levels of signal-to-noise ratios are presented. A comprehensive analysis of the obtained results indicated how the appropriate selection of the analyzed factors can be used to enhance the ED performance of MIMO-OFDM-based CRNs. [ABSTRACT FROM AUTHOR]

Millimeter waves are increasingly used in modern telecommunication systems for wireless data transmission. However, higher path loss, especially caused by non-line-of-sight scenarios, remains challenging. The design of an electrically controllable reflector for the millimeter-wave range is elaborated and presented in this manuscript. The reflector design was based on distributed ferroelectric ceramic elements and could be used in a frequency range up to 100 GHz. The issue of the ferroelectric reflector impedance matching was analyzed in detail. Two possible implementations of the reflector for indoor and outdoor communication systems were considered and simulated. The prototype of the proposed reflector for an operating frequency of 60 GHz was manufactured. Both simulation and measurement results demonstrated the beam steering by the proposed ferroelectric reflector. [ABSTRACT FROM AUTHOR]

Several standards exist for testing the radiated "over the air" (OTA) performance of 5G User Equipment (UE) devices. All these standards are limited to a Single-Input Single-Output (SISO) evaluation of the 5G device. With 5G networks and devices already deployed and in operation, it becomes necessary to develop realistic Multiple-Input Multiple-Output (MIMO) and beamforming performance metrics for 5G UEs. Since the most important feature of 5G is beamforming, this paper reviews the main challenges to realistically evaluate 5G device performance and proposes several novel key performance indicators (KPIs). The results obtained by some measurements show that it is possible to derive figures of merit that address the complexity of beamforming and MIMO-based 5G performance of devices in a much better way compared with the existing SISO KPIs, and that academia should provide advanced future research on these more realistic KPIs for the industry to face the challenges ahead with a better analysis of the problem in hand. [ABSTRACT FROM AUTHOR]