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Space solar power (SSP), envisioned for decades as a solution for continuous, stable, and dynamically dispatchable clean energy, has seen tremendous interest and a number of experimental demonstrations in the last few years. A practical implementation has been elusive to date, owing to the high launch costs associated with heavy, rigid photovoltaic and wireless power transfer (WPT) arrays. Lightweight and flexible solutions for WPT have been demonstrated terrestrially but, to date, have not been deployed and tested in space. In this paper, we present an experimental space demonstration of a lightweight, flexible WPT array powered by custom radio-frequency integrated circuits. The transmit arrays, receive arrays, and the rest of the system were operated and tested for eight months in Low Earth Orbit (LEO). Results from these experiments, including pointing of the array's beam to Earth and its detection by a ground station, are presented and discussed in detail. Observations and results from this mission uncover existing strengths and weaknesses that inform future steps towards realizing SSP. • First demonstration of wireless power transfer in space. • Use of novel, custom, flexible phased array for wireless power transfer. • Pointing of in-space payload towards Earth and detection of power using custom ground station. • Presentation of short- and long-term performance degradation. [ABSTRACT FROM AUTHOR]

As operators and manufacturers eye the transition from 5G to 6G, spectrum availability continues to be a crucial factor in enabling faster, more reliable wireless connectivity. One key hurdle in this transition is the ever-increasing bandwidth demand due to higher performance and higher bitrates of 5G New Radio. Innovative solutions are, thus, required to optimize spectrum usage in traditional as well as new bands. We start our column by reporting on recent solutions for microwave spectrum usage for the backhaul of 5G networks as well as for fixed wireless access. Not only do these breakthroughs extend network reach and boost data transmission, but they also pave the way for a swift deployment of next-generation services. The importance of careful spectrum usage in general is further discussed as we examine the downsides of private spectrum allocations and their impact on public network performance. [ABSTRACT FROM AUTHOR]

This paper introduces an innovative tri-band metamaterial absorber designed for advanced sensing applications, demonstrating significant achievements in electromagnetic absorption and sensitivity across multiple frequencies. The proposed metamaterial absorber operates effectively at targeted resonant frequencies of 2.46 GHz, 6.25 GHz, and 12.45 GHz, exhibiting excellent reflection coefficients, indicating reduced energy loss and enhanced signal integrity at these frequencies. The compactness of the design is achieved through accurately calculated unit cell dimensions of 0.121λ0 × 0.121 λ0, where λ0 is the wavelength at a resonance frequency of 2.46 GHz, resulting in a total size of 18 × 18 mm2. One of the most notable features of this proposed design is the high absorption rate, achieving up to 99.99 % at its operational frequencies. The proposed design was validated numerically and experimentally, ensuring the theoretical models accurately predict practical behavior. The design also offers substantial flexibility for frequency-selective applications in sensing technology, enhanced by the sensor’s high sensitivity. This sensitivity was thoroughly verified using food color-water mixtures with concentrations up to 99.99 %, effectively demonstrating the metamaterial absorber’s ability to respond to the changes in dielectric properties with shifts in resonant frequencies that perfectly match theoretical expectations. Therefore, the proposed metamaterial absorber is an efficient solution for modern sensing challenges.

This paper presents the development of an innovative, tunable star-shaped microwave sensor operating at two distinct frequency bands C and X band, optimized for high-sensitivity sensing of both solid and liquid mediums. The sensor's innovative star-shaped geometry augments the surface interaction with the target substances, significantly enhancing the sensor's response and sensitivity. Furthermore, tunability allows the sensor to adapt to different sensing environments, maximizing the detection potential at both frequencies. This research delves into the sensor's performance when using four different substrates for solid sensing containing FR-4, RO-3035, RO-5880, and RO-6202. Each substrate, characterized by unique electromagnetic properties, contributes differently to the sensor's sensitivity and resolution. The extensive experimental results highlight the sensor's proficiency in detecting and differentiating between these substrates, revealing the impact of material selection on sensor performance. The sensor's ability to distinguish between various liquids, including water, milk, oil, and energy drinks, is evaluated in the liquid sensing scenario. The sensor successfully detects the subtle variances in the electromagnetic properties of these liquids, suggesting its capability for high-resolution liquid sensing. The proposed MTM-based sensor resonates at 4.46 GHz and 11.48 GHz, respectively. Numerical software CST was used to analyze all the parameters. A 3 × 3 array was used in the prototype design, and it was used to measure the C-band resonance frequency. On the other hand, a compact unit cell was used to measure the X-band resonance frequency. The outcomes suggest a substantial potential for this sensor design in various industries and scientific fields, including food quality control, the oil industry, biomedical applications, and environmental monitoring. The unique combination of star-shaped design, tunable frequencies, and various substrates offers unprecedented flexibility and sensitivity in solid and liquid sensing on microwave frequency ranges.

This paper presents the development of an innovative, tunable star-shaped microwave sensor operating at two distinct frequency bands C and X band, optimized for high-sensitivity sensing of both solid and liquid mediums. The sensor's innovative star-shaped geometry augments the surface interaction with the target substances, significantly enhancing the sensor's response and sensitivity. Furthermore, tunability allows the sensor to adapt to different sensing environments, maximizing the detection potential at both frequencies. This research delves into the sensor's performance when using four different substrates for solid sensing containing FR-4, RO-3035, RO-5880, and RO-6202. Each substrate, characterized by unique electromagnetic properties, contributes differently to the sensor's sensitivity and resolution. The extensive experimental results highlight the sensor's proficiency in detecting and differentiating between these substrates, revealing the impact of material selection on sensor performance. The sensor's ability to distinguish between various liquids, including water, milk, oil, and energy drinks, is evaluated in the liquid sensing scenario. The sensor successfully detects the subtle variances in the electromagnetic properties of these liquids, suggesting its capability for high-resolution liquid sensing. The proposed MTM-based sensor resonates at 4.46 GHz and 11.48 GHz, respectively. Numerical software CST was used to analyze all the parameters. A 3 × 3 array was used in the prototype design, and it was used to measure the C-band resonance frequency. On the other hand, a compact unit cell was used to measure the X-band resonance frequency. The outcomes suggest a substantial potential for this sensor design in various industries and scientific fields, including food quality control, the oil industry, biomedical applications, and environmental monitoring. The unique combination of star-shaped design, tunable frequencies, and various substrates offers unprecedented flexibility and sensitivity in solid and liquid sensing on microwave frequency ranges. [ABSTRACT FROM AUTHOR]

The paper presents a comprehensive explanation of the technology involved in the defected ground structure of a hairpin filter. Microwave communication systems rely heavily on microstrip hairpin bandpass filters, and this paper highlights their importance in these systems. The proposed multiband bandpass filter exhibits excellent performance, and the fabrication has been carried out on the material GaAs. This research work contributes significantly to the advancement of microwave communication systems, and the findings can be useful in developing more efficient and high-performance filters for future communication systems. [ABSTRACT FROM AUTHOR]

Abstract Optical beating is the usual approach to generation of microwave signals. However, the highest frequency achievable for microwave signals is limited by the bandwidths of optoelectronic devices. To maximize the microwave frequency with a limited bandwidth of a photodetector (PD) and relieve the bandwidth bottleneck, we propose to generate microwave signals with the single sideband (SSB) format by beating a continuous wave (CW) light with an optical SSB signal. By simply adjusting the frequency difference between the CW light and the carrier of the optical SSB signal, the frequency of the generated microwave SSB signal is changed correspondingly. In the experiment, amplitude shift keying (ASK) microwave signals with the SSB format are successfully generated with different carrier frequencies and coding bit rates, and the recovered coding information agrees well with the original pseudo random binary sequence (PRBS) of 27 − 1 bits. The proposed approach can significantly relieve the bandwidth restriction set by optoelectronic devices in high-speed microwave communication systems. Graphical Abstract

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. [ABSTRACT FROM AUTHOR]

Optical beating is the usual approach to generation of microwave signals. However, the highest frequency achievable for microwave signals is limited by the bandwidths of optoelectronic devices. To maximize the microwave frequency with a limited bandwidth of a photodetector (PD) and relieve the bandwidth bottleneck, we propose to generate microwave signals with the single sideband (SSB) format by beating a continuous wave (CW) light with an optical SSB signal. By simply adjusting the frequency difference between the CW light and the carrier of the optical SSB signal, the frequency of the generated microwave SSB signal is changed correspondingly. In the experiment, amplitude shift keying (ASK) microwave signals with the SSB format are successfully generated with different carrier frequencies and coding bit rates, and the recovered coding information agrees well with the original pseudo random binary sequence (PRBS) of 27 − 1 bits. The proposed approach can significantly relieve the bandwidth restriction set by optoelectronic devices in high-speed microwave communication systems. [ABSTRACT FROM AUTHOR]

The three-dimensional printed wideband prototype (WBP) was proposed, which is able to enhance the horn feed source by generating a more uniform phase distribution that is obtained after correcting aperture phase values. The noted phase variation obtained without the WBP was 163.65 ∘ for the horn source only, which was decreased to 19.68 ∘ , obtained after the placement of the WBP at a λ /2 distance above the feed horn aperture. The corrected phase value was observed at 6.25 mm (0.25 λ) above the top face of the WBP. The use of a five-layer cubic structure is able to generate the proposed WBP with dimensions of 105 mm × 105 mm × 37.5 mm (4.2 λ × 4.2 λ × 1.5 λ), which can improve directivity and gain by 2.5 dB throughout the operating frequency range with a lower side lobe level. The overall dimension of the 3D printed horn was 98.5 mm × 75.6 mm × 192.6 mm (3.94 λ × 3.02 λ × 7.71 λ), where the 100 % infill value was maintained. The horn was painted with a double layer of copper throughout its surface. In a design frequency of 12 GHz, the computed directivity, gain, side lobe level in H- and E- planes were 20.5 dB, 20.5 dB, −26.5 dB, and −12.4 dB with only a 3D printed horn case and, with the proposed prototype placed above this feed source, these values improved to 22.1 dB, 21.9 dB, −15.5 dB, and −17.5 dB, respectively. The realized WBP was 294 g and the overall system was 448 g in weight, which signifies a light weight condition. The measured return loss values were less than 2, which supports that the WBP has matching behavior over the operating frequency range. [ABSTRACT FROM AUTHOR]

Microwave filters are indispensable passive devices for modern wireless communication systems. Nowadays, electromagnetic (EM) simulation-based design process is a norm for filter designs. Many EM-based design methodologies for microwave filter design have emerged in recent years to achieve efficiency, automation, and customizability. The majority of EM-based design methods exploit low-cost models (i.e., surrogates) in various forms, and artificial intelligence techniques assist the surrogate modeling and optimization processes. Focusing on surrogate-assisted microwave filter designs, this article first analyzes the characteristic of filter design based on different design objective functions. Then, the state-of-the-art filter design methodologies are reviewed, including surrogate modeling (machine learning) methods and advanced optimization algorithms. Three essential techniques in filter designs are included: 1) smart data sampling techniques; 2) advanced surrogate modeling techniques; and 3) advanced optimization methods and frameworks. To achieve success and stability, they have to be tailored or combined together to achieve the specific characteristics of the microwave filters. Finally, new emerging design applications and future trends in the filter design are discussed. [ABSTRACT FROM AUTHOR]

In this article, a dual-functional microwave system with a single input port that can simultaneously be used for antenna and sensor applications is presented. The dual-functional ability of the proposed system is achieved by integrating a two-port microwave sensor and a Wi-Fi antenna with a novel frequency-selective multipath filter (FSMF). The FSMF ensures efficient system operation by not affecting the operational bandwidth of the communicating antenna in the presence of different sensed materials under test (MUTs) on top of the sensor. The proposed microwave sensor also has a unique capability of measuring one more sensing parameter in addition to the often used reflection and transmission coefficients. The additional parameter is the frequency distance of two closely spaced resonance frequencies of the microwave sensor in the presence of an MUT. Compared to existing state-of-the-art techniques, the additional sensing parameter (as done in this work) can be used as an alternative to transmission coefficient for characterization of different MUTs or as an additional parameter giving more degree of freedom. The entire system is designed in a single substrate with a common input source. The performance of the dual-functional system was tested using different MUTs and showed a good agreement with the measurement results. [ABSTRACT FROM AUTHOR]

The rapidly growing wireless communication market has resulted in tremendous changes in the requirements and capabilities of radios to support numerous multiband wireless connections. For example, the 850- and 1,900-MHz frequency spectrums are used to support the Global System for Mobile Communications (GSM) networks. New frequency bands of nearly 2,100 MHz have been adopted in 3G networks, and more frequency bands and spectrums are introduced in 4G systems, ranging from 600 MHz to 2.5 GHz ,. Wireless communications have resulted in radio that, by 2010, had evolved from a single-mode, triple-band 2G system, to a triple-mode, nine-band (4 × GSM, 5 × Universal Mobile Telecommunications System with High-Speed Packet Access+), high-speed data-capable system. The trend continues today with rapid deployment of 5G and future 6G systems, driving new bands and modes of operation. The frequency band of sub-6 GHz from 450 MHz to 6 GHz and the millimeter-wave spectrum from 24 to 52 GHz are employed in 5G systems to provide multifunctional services. To support the continuous evolution of high data rates, as shown in , 5G mobile networks have been widely developed as an ideal on-demand connection solution for devices and users in urban areas. [ABSTRACT FROM AUTHOR]

In next- and future-generation wireless communication systems, demand for antenna technologies has continued to grow. This demand has fueled the development of design techniques that pursue more desirable configurations with simplification, electrically small size, and low profiles as well as multiple functionalities and higher performance with multiband, broad bandwidth, single/multibeam scanning, polarization control, and dispersionless group delay. [ABSTRACT FROM AUTHOR]

Deep learning based solutions are being integrated into the physical and link layers of wireless networks. They often effect an improvement in transmission reliability and/or efficiency when there is a model or an algorithm deficit. In this letter, we propose a deep learning-aided soft demapper, consisting of a fully-connected deep neural network (DNN), to alleviate a channel model deficit. We apply it in microwave backhaul transmissions affected by impairments generated by the local oscillator and power amplifier. The proposed DNN soft demapper learns the best approximation for the log-likelihood ratios (LLRs). The learned LLRs show gains over model-based impairment-aware LLRs, as they capture the actual channel as observed through data. We implement weight pruning and periodical retraining to adapt to statistical changes and make our proposed approach fit for practical cost-aware applications. [ABSTRACT FROM AUTHOR]

Layer-stacked metasurfaces or frequency selective surfaces have been employed as a cost-effective approach to design reconfigurable lenses. Previous work showed that the intrinsic technical drawback, i.e., the transverse field interaction effect, significantly limits the beamforming performance of these electrically thick lenses. In this letter, an active metasurface lens loaded with parallel-plate waveguide (PPW) is experimentally validated. By inserting PPW into electrically thick lenses, the transversely propagating field inside the lenses can be completely blocked, based on which the beam-steering range and the sidelobe level (SLL) can be remarkably improved. A linear PPW lens array operating around 5.75 GHz is fabricated. Experimental results show that the scanning coverage of lens antenna can reach ±60°, and the SLL are below −15.7 dB within ±50°. [ABSTRACT FROM AUTHOR]

The papers in this special section focus on recent advancements in this field and provide an overview of the potential applications of this technology in the next generation antenna devices, with emphasis on metamaterial-based enhancements enabling real-time control of their radiation characteristics. [ABSTRACT FROM AUTHOR]

The three dimensional printing method is currently on stage of rapid development to realize complex nature of structure in simpler and feasible ways. In this paper, we proposed a 3D printed wideband antenna structure with high performance to improve the characteristics properties of feed source by eliminating the use of expensive superstates. A 60 mm $\times60$ mm $\times22.5$ mm wideband structure was first fabricated from VERO CMYK material through Multijet 3D printing method where the bottom layer are copper painted so as to realize the higher permittivity layer. The proposed structure is maintained at 16 mm ( $0.64\lambda $ ) above the ground plate of 60 mm $\times60$ mm $\times $ 1 mm, which is feed by WR-75 waveguide. The proposed antenna is designed to have a wide operating bandwidth from 10 to 15 GHz, which cover Ku-band frequency range. Directivity enhancement is obtained over a large bandwidth by using high permittivity material for Layer 1 and lower permittivity material for Layer 2 and Layer 3. The resulting antenna prototype demonstrates a directivity bandwidth product (DBP) of 3012.79 and DBP per unit area (DBP/A) of 523.06, where directivity and gain are found to be higher than 15 dBi over the entire operating frequency band. The total area of the new antenna prototype is $2.4\lambda \times 2.4\lambda \times 0.9\lambda $ ( $4.16\lambda _{g} \times 4.16\lambda _{g} \times 1.56\lambda _{g}$ ), and overall electrical height is $1.54\lambda $ at the design frequency of 12 GHz. The proposed prototype weight 36 g and overall system have weight of 118 g, which is light in weight. The measured VSWR is less than 2, which signifies wideband characteristics of overall system.

A highly accurate 3-D indoor passive localization and identification system is presented in this article. The system can monitor various commercial Wi-Fi devices through the proposed received signal strength indicator (RSSI)-based angle of arrival (AoA) estimation technique. RSSI-based localization systems conventionally use additional assistance techniques, such as distance–RSSI calibration, fingerprint analysis, machine learning, and a widespread setup to achieve high accuracy. On the contrary, this proposed system can operate in complex environments full of scattering objects and obstructions without requiring any additional assistance techniques. The proposed technique uses a six-port network to evaluate the phase difference in the carrier waves of Wi-Fi signals without the influence of modulated signals. The network also preserves the modulated signals without the influence of the phase difference of the carrier waves such that Wi-Fi devices can be identified. Regarding practical applications, the system is designed to be capable of detecting devices through walls and thus can be hidden outside the monitored room. The experimental results indicate that in single-source localization, the average error is 0.089 m; in multiple-source localization, it is 0.354 m; and for seeing through the wall, it is 0.24 m. In the worst case scenario, the error is still smaller than 0.63 m. The accuracy in all the experimental results was found to be at the decimeter level. In summary, this study experimentally validated a 3-D indoor localization and identification system for diverse Wi-Fi devices in various environments. [ABSTRACT FROM AUTHOR]

In today’s ever-developing electronic world, tremendous efforts are being made to develop highly efficient microwave absorbing materials with thin thickness and broad bandwidth while maintaining a simple manufacturing procedure. Since so much study is devoted to structural and component designing of microwave absorbers, the mechanism of microwave absorption still has potential for improvement. It is critical to have fundamental mechanistic knowledge of microwave absorption by absorbing materials with diverse types of geometries and components. It is also important to explore the influence of different physical/chemical characteristics of absorbing materials on microwave absorption performance. The methodical and thorough examination of these issues may provide internal information on the interaction of the microwave absorber’s components and subsequent structural design. Therefore, in this article, by taking cobalt (Co) as the base material, an attempt has been made to comprehend the microwave absorption mechanism of Co milled at various times, which results in changes in particle size and morphology. A significant change in the microwave absorption of Co has been observed with changed morphology after milling. The enhancement in microwave absorption characteristic of milled Co with the addition of different wt% of multiwall carbon nanotubes (MWCNTs) has also been analyzed. MWCNTs in milled Co increase interfacial polarization and multiple reflections in the sample, resulting in microwave attenuation. The prepared Co/MWCNTs composite samples yielded excellent bandwidth with low coating thickness. The effective bandwidth of 10 GHz (3.73–13.73 GHz) was obtained for the composite of 20 h wet-milled Co and 2 wt% MWCNTs, with a coating thickness of 2.5 mm. To further enhance the microwave absorption bandwidth and reduce coating thickness, a genetic algorithm (GA)-assisted multi-layering approach has been implemented for designing triple-layer microwave absorbers. The optimized triple-layer result for combination 5W4–10W1–20W2 shows an effective bandwidth of 13.65 GHz (4.35–18 GHz), with an overall coating thickness of 2.31 mm. [ABSTRACT FROM AUTHOR]

This letter presents a microwave gas sensor based on planar coupled open loop ring resonators (OLRRs) embedded with carbon nanotube sensing film for ammonia gas sensing. The sensing area adopts the interdigitated electrode patterns to provide a larger contact area for better sensitivity. By integrating carbon nanotube films as the local variable resistors into a microwave sensor platform, the platform can transduce the gas concentration to its electromagnetic properties. According to the step response of NH3 from 100 to 1000 ppm, the sensitivity is estimated as $- 11~\mu $ dB/ppm and −232 Hz/ppm for the insertion losses and resonant frequency variations, respectively. The sensor repeatability and reversibility are also measured by maintaining the concentration from 300 to 700 ppm and repeating ten cycles with the average value of each concentration consistent with the step response. The results show a linearly response and a high repeatability of OLRRs via the real-time detection for gas sensing at high frequency and room temperature. Microwave gas sensors can be further integrated in the IoT or RF front-end system, offering low-cost, lightweight, reliable, and power efficient system for remote monitoring. [ABSTRACT FROM AUTHOR]

Currently, the wireless communication industry is experiencing rapid advances in the smooth migration and upgrading of technology. Because of this, communication systems operating in multiple frequency bands are of interest, and multiband circuits play an important role in the design of compact and low-cost multiband communication systems. A multiband multimode communication system based on software-defined radio (SDR) is shown in. In this communication system, the multiband capability is primarily achieved by the hardware, (i.e., using multiband microwave circuits). Passive microwave circuits are essential to the design of any communication system; for example, quadrature branch line couplers (BLCs) are used in array beamforming circuit designs , while Wilkinson power dividers (WPDs) , baluns , and out-of-phase power dividers play a vital role in designing balanced/unbalanced circuits. [ABSTRACT FROM AUTHOR]

To keep up with recent developments in mobile communications, researchers urgently need to develop microwave and millimeter-wave (mm-wave) components that are economical, efficient, and compact in size, all while maintaining good performance. Thus, as an important part of any communication system, the filter, is attracting a great deal of research interest. Emerging applications in the field of wireless communications have engendered more advanced requirements for RF/microwave filters, such as smaller size, lighter weight, lower cost, multiband, and better performance. Taking the substrate integrated waveguide (SIW) filter as an example, cutting the equivalent magnetic wall is an effective miniaturization method –, but it can cause a small amount of radiation leakage because of the cut in the boundary. Multilayer technology can avoid radiation leakage –, but the manufacturing process is relatively complex. In addition, the available miniaturization methods involve one cavity multimode – and loading technologies –. Among the different options for these technologies, as a metamaterial structure, complementary split-ring resonators (CSRRs) have attracted the attention of many scholars for their unique characteristics. [ABSTRACT FROM AUTHOR]

A unified view of direct-integration (DI) and exponential-time-differencing (ETD) methods to incorporate Drude media, such as isotropic plasma and microwave graphene, into finite-difference time-domain (FDTD) simulators is provided. To this end, the Drude constitutive relation is expressed in integral form and the DI integrators are obtained by applying quadrature rules. Analogously, the ETD integrators are obtained by starting from the variation of constants formula and applying the same quadrature rules as in the DI case. This approach allows one to directly compare the two families of methods. In addition, the accuracy of each integrator is discussed and the stability condition of the resulting FDTD schemes is derived in exact closed form by applying the von Neumann method. [ABSTRACT FROM AUTHOR]

The occurrence of passive intermodulation (PIM) is inevitable in high-power microwave devices. It interferes with wireless communication and radar systems, resulting in severe deterioration of receiving performance. So far, narrowband PIM analyzers have been realized for different business bands. However, they are incapable of testing wideband systems. To solve this problem, rack-mounted analyzers piled with multiple narrowband analyzers were implemented. While such solutions significantly increase the volume, cost, and power consumption, they cannot provide a wide, continuous bandwidth for PIM localizations. Aiming to solve this problem, we propose a novel solution for realizing ultrawideband PIM analyzers. Based on the principle of cooperative radio frequency and baseband (RF-BB) nulling, we demonstrate an experimental prototype working within a continuous frequency band covering three octaves, exhibiting promising potential in PIM testing applications. [ABSTRACT FROM AUTHOR]

A broadband W-band microwave power module (MPM) is demonstrated. It provides a continuous wave (CW) output power of 65 W from 91 to 97 GHz with a constant driving power of 0 dBm. The MPM consists of a miniaturized folded-waveguide traveling-wave tube (TWT), an integrated power conditioner (IPC), and a gain equalizing solid-state amplifier (GE-SSA). The overall efficiency exceeds 20 % and the saturation gain fluctuation is lower than 0.8 dB. It has dimensions of 380 mm $\times$ 180 mm $\times$ 45 mm and a weight of 5.5 kg. [ABSTRACT FROM AUTHOR]

The three-dimensional printed wideband prototype (WBP) was proposed, which is able to enhance the horn feed source by generating a more uniform phase distribution that is obtained after correcting aperture phase values. The noted phase variation obtained without the WBP was 163.65∘ for the horn source only, which was decreased to 19.68∘, obtained after the placement of the WBP at a λ/2 distance above the feed horn aperture. The corrected phase value was observed at 6.25 mm (0.25λ) above the top face of the WBP. The use of a five-layer cubic structure is able to generate the proposed WBP with dimensions of 105 mm × 105 mm × 37.5 mm (4.2λ× 4.2λ× 1.5λ), which can improve directivity and gain by 2.5 dB throughout the operating frequency range with a lower side lobe level. The overall dimension of the 3D printed horn was 98.5 mm × 75.6 mm × 192.6 mm (3.94λ× 3.02λ× 7.71λ), where the 100 % infill value was maintained. The horn was painted with a double layer of copper throughout its surface. In a design frequency of 12 GHz, the computed directivity, gain, side lobe level in H- and E- planes were 20.5 dB, 20.5 dB, −26.5 dB, and −12.4 dB with only a 3D printed horn case and, with the proposed prototype placed above this feed source, these values improved to 22.1 dB, 21.9 dB, −15.5 dB, and −17.5 dB, respectively. The realized WBP was 294 g and the overall system was 448 g in weight, which signifies a light weight condition. The measured return loss values were less than 2, which supports that the WBP has matching behavior over the operating frequency range.

The introduction of new technology in the wireless communication industry increases the number of frequency bands in the communication environment. Therefore, a communication system capable of functioning over several frequency bands, known as a multiband communication system, is required. In addition, multiband communication systems support technology upgrades by smoothly bridging the migration from the existing technology to a newer technology. A multiband communication system can be developed with the help of two approaches. The first approach is a multiband communication system that contains multiple microwave circuits, with each circuit optimized for a single band; for example, a dual-band communication system has two low-pass filters and two power amplifiers. But this approach increases not only the size but also the cost of the system. In the second approach, rather than using a separate power amplifier and filter for each frequency band, a single power amplifier, filter, and other components are used that can support more than one frequency band. These circuits are known as multiband microwave circuits. Multiband circuits not only minimize size but also reduce the cost of designing the multiband communication system. [ABSTRACT FROM AUTHOR]

A novel flat beam steering prototype based on the specific arrangement of a cylindrical unit cell is designed, manufactured and tested. The wideband and broad scanning capability is demonstrated at the Ku-band. We have considered two configurations, first with circular rings (CR) of the defined permittivity values for respective radial distance from the center of the aperture, second with cylindrical rods that shall be placed on a respective CR, which mimics the defined permittivity. The structure is generated from Vero CMYK full color wax, which utilizes the Multijet 3D printing method. The proposed prototype is designed in the operating frequency of 12 GHz (λ = 25 mm) and separation distance between the two parallel plates are maintained at 12.5 mm (0.5 λ) for the TE 10 mode of operation. The diameter of the two parallel plates and the proposed structure is of 100 mm (4 λ) where the radius of cylinders varies from 0.5 to 3.5 mm and corresponding relative permittivity varies from 0.6687 to 2.4395. The overall height of the structure is 12.5 mm and is placed between two parallel plates. The minimum separation distance is maintained between the proposed structure and the feed WR-75 waveguide. Irises effect is performed to obtain impedance matching in the operating frequency bands. Beam steering of the radiated waves is observed for relative rotation angles of 0 ∘ , 30 ∘ , 45 ∘ , 75 ∘ and 90 ∘ of the WR-75 waveguide along the edge of parallel plates. The overall proposed system weighs 179 g, which signifies the light weight characteristics. Moreover, the proposed structure shows low return loss over 10 GHz to 15 GHz operating frequency band. [ABSTRACT FROM AUTHOR]

This article presents a simulated and experimental analysis of the effects of the evaporation duct on microwave propagation in the Irish Sea. The evaporation duct is a phenomenon that occurs almost permanently over all the world’s oceans and allows electromagnetic waves to travel beyond the horizon. Weather data logged over several years from four buoys off the east coast of Ireland has been analyzed to find the probability and strength of the evaporation duct. Signal propagation in the evaporation duct has been simulated using the parabolic equation model and compared to results obtained from an experimental setup in the Irish Sea. The best antenna heights and frequencies to maximize signal propagation at this location are also found. Results show that the evaporation duct can be used to provide high bandwidth communications beyond the horizon with an uptime of approximately 40%, and that weather data from buoys can be used to predict the performance of the communications link. While this method of communication is not one that can be relied on all the time, it could be useful to reduce dependency on expensive satellite links or provide nontime critical communications. [ABSTRACT FROM AUTHOR]

In this article, a compact distributed septuple-band negative group delay circuit (NGDC) is proposed and analyzed. It is mainly composed of a high–low–high step-impedance microstrip line with an E-shaped groove and two T-shaped open stubs. This NGDC was first simulated and then fabricated by using a conventional printed circuit board (PCB) process with the substrate of RO4350. To investigate the operational mechanism of the NGDC, an equivalent circuit, consisting of nine transmission lines, four series–parallel LC resonant circuits, and two T-type series LC resonant circuits, has been developed and analyzed. It is found that the theoretical group delay characteristic calculated from the equivalent circuit agrees well both with the simulated data by using ADS as well as HFSS and with the experimental results. The measured seven NGD resonant frequencies occur at 2.03, 2.84, 8.14, 8.74, 13.0, 16.02, and 18.46 GHz, and the corresponding NGD values read −3.65, −3.04, −1.73, −3.23, −1.16, −1.20, and −1.47 ns. The dimensions of the presented passive NGDC are only 25 mm $\times22$ mm without using any lumped components. [ABSTRACT FROM AUTHOR]

Security vulnerabilities demonstrated in implantable medical devices have opened the door for research into physically secure and low power communication methodologies. In this study, we perform a comparative analysis of commonly used ISM frequency bands and human body communication (HBC) for data transfer from in-body to out-of-body (IBOB). We develop a figure of merit (FoM) that comprises of the critical parameters to quantitatively compare the communication methodologies. We perform finite-element method (FEM)-based simulations and experiments to validate the FoM developed. [ABSTRACT FROM AUTHOR]

In this letter, a wideband and compact microwave rectifier using the conditions for flat frequency characteristics of RF–dc conversion efficiency is proposed. A lumped element design was used to minimize the size of the rectifier. The multistage matching method was effective in reducing the $Q$ value of the circuit, resulting in the wideband characteristic. Three capacitors and four inductors were used for matching. In a measurement result, RF–dc conversion efficiency exceeded 50% in the frequency range from 0.39 to 2.90 GHz, which corresponds with a fractional bandwidth of 152.6%. [ABSTRACT FROM AUTHOR]

This paper presents an angle-of-arrival (AOA) measurement system based on microwave photonic frequency down conversion. It has the novelty of no LO is needed for the frequency conversion process. Instead, a ramp waveform is used to shift the frequency of an optical carrier, which is phase modulated by a microwave signal in the subsequent stage. Two low-frequency IF signals are generated after photodetection. The AOA of the incoming microwave signal can be determined by measuring the phase difference of the two IF signals. Comparing with the previously reported frequency converter based AOA measurement systems, the proposed structure does not need a high-frequency tunable LO source and high-frequency electrical components. Expensive and bulky equipment such as an electrical spectrum analyser are not required in the proposed AOA measurement system. Experimental results show AOA measurement of different-frequency and different-power microwave signals, with small measurement errors. [ABSTRACT FROM AUTHOR]

Transmitted and received power must be controlled quickly and accurately to comply with government regulations, minimize interference, and optimize power consumption. Microwave power detectors (PDs) are critical building blocks employed in applications such as integrated circuit tests and measurements, wireless communication, and automatic-gain-control (AGC) circuits to monitor signal power level. PDs are utilized as received signal strength indicators (RSSIs) to adjust the gain of the intermediate frequency/RF signal via an AGC circuit. They are also used in mobile terminals to monitor the transmitted power, thereby optimizing power consumption. Two popular methods employed to measure the power signal level are root mean square (RMS) value and peak value. Peak detectors capture the input’s peak power to provide power information, whereas the RMS PD provides information about the average power. shows a conventional block diagram of a microwave PD employed in an RF transmitter. The directional coupler is used to couple the output signal from the power amplifier to a PD. The signal is then converted to a dc voltage that the power controller utilizes to change the power amplifier’s gain, thereby improving the transmitter’s linearity. [ABSTRACT FROM AUTHOR]

Since about one and half centuries ago, at the dawn of modern communications, the radio and the optics have been two separate electromagnetic spectrum regions to carry data. Differentiated by their generation/detection methods and propagation properties, the two paths have evolved almost independently until today. The optical technologies dominate the long-distance and high-speed terrestrial wireline communications through fiber-optic telecom systems, whereas the radio technologies have mainly dominated the short- to medium-range wireless scenarios. Now, these two separate counterparts are both facing a sign of saturation in their respective roadmap horizons, particularly in the segment of free-space communications. The optical technologies are extending into the mid-wave and long-wave infrared (MWIR and LWIR) regimes to achieve better propagation performance through the dynamic atmospheric channels. Radio technologies strive for higher frequencies like the millimeter-wave (MMW) and sub-terahertz (sub-THz) to gain broader bandwidth. The boundary between the two is becoming blurred and intercrossed. During the past few years, we witnessed technological breakthroughs in free-space transmission supporting very high data rates, many achieved with the assistance of photonics. This paper focuses on such photonics-assisted free-space communication technologies in both the lower and upper sides of the THz gap and provides a detailed review of recent research and development activities on some of the key enabling technologies. Our recent experimental demonstrations of high-speed free-space transmissions in both frequency regions are also presented as examples to show the system requirements for device characteristics and digital signal processing (DSP) performance. [ABSTRACT FROM AUTHOR]

The plasma limiter (PL) is a suitable method to protect electronic devices from high-power electromagnetic pulses. The $S$ -band PL has target specifications of 100 kW of power handling capability, < 100 ns of response time, and < 0.5 dB of insertion loss. Carbon nanotube (CNT) can be used to reduce power losses and instrument corrosion under frequent electromagnetic attacks. This article used CNT-coated copper needles in a PL to increase the electric field for the rapid breakdown process. Furthermore, the impact of dimensional parameters (needle’s length, needle’s radius, and axial displacement of the needles) were investigated. The experiment was conducted at 1800 W of peak power and 25 ns of the $S$ -band (2.45 GHz) microwave duration. In this case, the height of 19 mm and diameter of 1 mm were found to be the optimal needle parameters, in which the fastest breakdown of the electric field is available. The finite-element method was used by CST software to verify the experiment results. The transmission losses were < 5% in the normal state. The breakdown electric field threshold was ~431 kV/m near the atmospheric pressures. There was an error of < 1.28 in the case with the lowest compatibility. [ABSTRACT FROM AUTHOR]

A range of frequencies supporting low path loss ducting over microwave links at beyond-line-of-sight distances beyond 100 km have been demonstrated theoretically and experimentally. Three experiments were conducted in the South China Sea to confirm the operating frequency range of 8–12 GHz at link distances of 109 km, 63 km, and 87 km, respectively. [ABSTRACT FROM AUTHOR]

Microwave-induced thermoacoustic (TA) imaging, combining high microwave contrast with high ultrasonic resolution has the potential to revolutionize applications such as continuous healthcare monitoring, point-of-care imaging, and biometric authentication. However, the size, cost, and integration of a high-power microwave transmitter is a key bottleneck in making TA imaging truly portable, affordable, and ubiquitous. Toward that end, this work presents a compact 4.9-GHz pulsed power amplifier (PA) with a 4.87-mm2 active area implemented in a 55-nm BiCMOS technology, operating in a duty-cycled mode and achieving 37.3-dBm peak output power—the highest demonstrated peak power in PAs fabricated on a silicon substrate with deep submicron CMOS integration. We also reconstruct the first known high-fidelity TA images of tissue phantoms using an integrated silicon PA. [ABSTRACT FROM AUTHOR]

Over a hundred years later, the classic antenna, first invented by Heinrich Hertz, in 1888, , is still the dominant technology used for the measurement of RF fields. Just seven years after its invention, Guglielmo Marconi applied the antenna to long-distance radio communication and, in the process, transformed the global community into one connected civilization. It is clear that the antenna has been a powerful piece of technology. What reason would we have, then, to use anything different? [ABSTRACT FROM AUTHOR]

Abstract Real time monitoring and control of a modern power system has achieved significant development since the incorporation of the phasor measurement unit (PMU). Due to the time-synchronized capabilities, PMU has increased the situational awareness (SA) in a wide area measurement system (WAMS). Operator SA depends on the data pertaining to the real-time health of the grid. This is measured by PMUs and is accessible for data analytics at the data monitoring station referred to as the phasor data concentrator (PDC). Availability of the communication system and communication delay are two of the decisive factors governing the operator SA. This paper presents a pragmatic metric to assess the operator SA and ensure optimal locations for the placement of PMUs, PDC, and the underlying communication infrastructure to increase the efficacy of operator SA. The uses of digital elevation model (DEM) data of the surface topography to determine the optimal locations for the placement of the PMU, and the microwave technology for communicating synchrophasor data is another important contribution carried out in this paper. The practical power grid system of Bihar in India is considered as a case study, and extensive simulation results and analysis are presented for validating the proposed methodology.

A novel type of photonic background-free multiband arbitrarily phase-coded microwave signal generator with power fading elimination capability is reported. An $x$ -polarized phase modulated optical signal and a $y$ -polarized radio frequency (RF) modulated harmonic single-sideband (HSSB) optical signal are produced based on a dual-polarization quadrature phase shift-keying modulator (DP-QPSKM). After polarization modulation to intensity modulation conversion, a microwave signal with arbitrarily phase encoding in multiple frequency bands is generated. Benefiting from the phase modulation state of the driven electrical coded signal, the generated arbitrarily phase-coded signal has no baseband components. Moreover, the equivalent HSSB modulation ensures the system can realize antidispersion transmission of one-to-multi base stations (BSs). The main distinct highlights of our generator are multifunction. The proposed single-modulator-based photonic scheme can achieve phase encoding in arbitrary format over multiple frequency bands and transmit over long-distance single-mode fiber with the elimination of power fading simultaneously. This work is pretty promising in multiband radar systems for one-to-multi BSs transmission and can increase the detection range, range resolution, Doppler tolerance, and sensitivity of objective recognition of radars. [ABSTRACT FROM AUTHOR]

With the explosive growth of human society’s demand for information , microwaves have become a vital information carrier for applications such as wireless communication, military radar, and electronic countermeasures. However, the processing rates and bandwidth of traditional microwave processing circuits are limited, which can create an electronic bottleneck for high-frequency microwave signals. Relying solely on traditional electrical systems has not been enough to meet the emerging demand for high-frequency microwave signal generation, transmission, and processing ,. At the same time, broadband and low-loss photonics have led to an ever-increasing interest in generating, processing, controlling, and distributing microwave and millimeter-wave (mm-wave) signals for applications such as broadband wireless access networks, sensor networks, radar, satellite communication, instrumentation, and warfare systems. In response, beginning in the 1990s, these two technologies merged and produced a new interdisciplinary field known as microwave photonics ,. [ABSTRACT FROM AUTHOR]

There is a practical gap between conventional rigid electronics and bendable items from daily life, such as paper, tape, textiles, and the human body. This space can be bridged by flexible transistor technologies, which typically offer bendability, a light weight, ultrathin dimensions, transparency, some stretchability, suitability for large areas, and a low cost. Thanks to the continuous increase of the maximal operation frequency of flexible electronics, wireless communication is becoming one of the promising enablers for many new applications and is widely studied. For a long time, electronics have advanced in terms of speed, power consumption, integration density, and cost. In particular, reductions in feature sizes, which lead to improvements in integration density, are expected to keep slowing down, e.g. due to thermal noise constraints. This trend has long been predicted, and it has motivated the investigation of multiple alternative electronic technologies, including mechanically flexible ones. [ABSTRACT FROM AUTHOR]

Terahertz (THz)-band communications have been envisioned as a key technology to support ultra-high-data-rate applications in 5G-beyond (or 6G) wireless networks. Compared to the microwave and mmWave bands, the main challenges with the THz band are in its i) large path loss hence limited network coverage and ii) visible-light-like propagation characteristics hence poor support of mobility in blockage-rich environments. This paper studies quantitatively the applicability of THz-band communications in blockage-rich mobile environments, focusing on a new network scenario called FlyTera. In FlyTera, a set of hotspots mounted on flying drones collaboratively provide data streaming services to ground users, in the microwave, mmWave and THz bands. We first provide a mathematical formulation of the FlyTera control problem, where the objective is to maximize the network spectral efficiency by jointly controlling the flight of the drone hotspots, their association to the ground users, and the spectrum bands used by the users. To solve the resulting problem, which is shown to be a mixed integer nonlinear nonconvex programming (MINLP) problem, we design distributed solution algorithms based on a combination of echo state learning and reinforcement learning. An extensive simulation campaign is then conducted with SimBAG, a newly developed Simulator of Broadband Aerial-Ground wireless networks. It is shown that no single spectrum band can meet the requirements of high data rate and wide coverage simultaneously. Moreover, from the network-level point of view, THz-band communications can significantly benefit from the mobility of the flying drones, and on average $4 - 6$ times higher (rather than lower) throughput can be achieved in mobile than in static environments. [ABSTRACT FROM AUTHOR]

In this article, a tapered matching structure in a circulator junction is employed in order to achieve the purpose of high isolation and multipactor suppression in spacecraft electronic equipment. This matching structure consists of multiple ramped metal steps. Compared with the traditional flat steps, it changes the electric field distribution in the circulator junction, thus destroying the resonance condition of the secondary electrons. And it changes the trajectory of the secondary electrons. These particles tend to escape the multipactor sensitive region. So, the purpose of multipactor suppression is reached. Besides, a good match between the circulator junction and the waveguide is obtained by using multiple ramped metal steps. So, high isolation is achieved. Finally, an S-band ferrite waveguide circulator with these characteristics operating at frequencies in the range of 3.4–4.2 GHz is designed, fabricated, and measured. The isolation value of the circulator achieves a maximum value of 43 dB near 4 GHz. And the isolation exceeds 25 dB and the insertion loss is less than 0.3 dB over a bandwidth of 21% centered at 3.8 GHz. In addition, the experimental results show that the multipactor threshold of the circulator reaches 10000 W. [ABSTRACT FROM AUTHOR]

A wideband antenna is proposed based on three-dimensional printing technology. The antenna was designed using the PREPERM 10 material, with permittivity ϵ r = 10, where the overall height of the proposed prototype was maintained as 12.83 mm (0.51 λ), having a lateral dimension of 60 mm × 60 mm, at an operating frequency of 12 GHz (λ = 25 mm). The proposed antenna achieved a wide frequency bandwidth with a voltage standing-wave ratio (VSWR) of less than two, from 10 GHz to 15 GHz in the Ku-band, where the maximum directivity was 20 dBi over a reflection coefficient bandwidth of 50%. It showed a miniaturized non-uniform metasurface of 2.4 λ × 2.4 λ × 0.51 λ that was placed at 16.5 mm (0.66 λ) above the ground plane, which was 2.4 λ × 2.4 λ × 0.04 λ in dimension. Thus, the overall height of the proposed antenna system from the feed source was 29.33 mm (1.17 λ). The total weight of the system including the designed structures made of PREPERM 10 and ABS with copper-painted prototypes was 96 g and 79 g, respectively. The measured results were consistent with the simulated results, demonstrating the feasibility and effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

A photonics-assisted frequency conversion and self-interference cancellation approach is proposed and experimentally demonstrated for in-band full-duplex (IBFD) communication. Carrier suppressed single sidebands (CS-SSB) of the intermediate frequency (IF) signal and the local oscillator (LO) signal are simultaneously obtained through the X-DPMZM in a dual-polarization dual-parallel Mach-Zehnder (DP-DPMZM) modulator. A transmit signal with high purity is produced, and a shared LO optical sideband is also provided for the subsequent downconversion. The carrier suppressed double sideband (CS-DSB) modulation for the signal of interest (SOI) is achieved via the Y-DPMZM. The self-interference signal is suppressed by adjusting the bias voltage of its main MZM. An optical bandpass filter (OBPF) is combined to realize frequency downconversion. The experimental results show that the spurious-free dynamic range (SFDR3) is 102.6 dB/Hz2/3 for frequency upconversion, a cancellation depth of more than 50 dB for single-frequency self-interference cancellation (SIC), and more than 22 dB for wideband SIC. The proposed link has superiority in multifunctionality, including in-band transmission, reception, and self-interference cancellation, with a shared LO. [ABSTRACT FROM AUTHOR]