Radio Science

This work describes the instrumental error budget for space-based measurements of the absolute flux of the sky synchrotron spectrum at frequencies below the ionospheric cutoff (≲ $\lesssim $20 MHz). We focus on an architecture using electrically short dipoles onboard a small satellite. The error budget combines the contributions of the dipole dimensions, plasma noise, stray capacitance, and front-end amplifier input impedance. We treat the errors using both a Monte Carlo error propagation model and an analytical method. This error budget can be applied to a variety of experiments and used to ultimately improve the sensing capabilities of space-based electrically short dipole instruments. The impact of individual uncertainty components, particularly stray capacitance, is explored in more detail.

Detection of earthquake precursors has long been a controversial issue with regard to its possibility and realizability. Here we present the detection of electromagnetic anomalous signals before large earthquakes using an observation network of very high frequency radio wave receivers close to major tectonic lines in Japan. The receivers are equipped with specifically designed narrowband filters to suppress noises and to detect extremely weak signals. We detected different types of electromagnetic anomalies before earthquakes around mountainous and coastal regions, where presence of electric charges is anticipated on the surface located in the middle of the radio wave paths near major tectonic lines in Japan. We use numerical electromagnetic wave analysis to show that when electric charges are present on a ground surface as a consequence of tectonic activity, the surface charges interact strongly with radio waves and eventually cause strong diffraction of the radio waves. The analysis was performed using the three-dimensional finite-difference time-domain method with digital elevation models of the actual geographical landforms on a massively parallel supercomputer. The results confirm the consistent mechanisms of the electromagnetic precursors, which explains the anomalous electromagnetic signals observed by the authors before large earthquakes.

The growing demand for advanced wireless communication, high-resolution imaging, and innovative medical applications in the Terahertz (THz) frequency range has driven remarkable developments in meta-surface-based antennas. This comprehensive review delves into the cutting-edge advancements, novel designs, and practical applications of meta-surfaces in the THz spectrum. The review begins by exploring the materials employed in meta-surfaces and their crucial role in achieving efficient THz operation. It delves into the realm of polarization diversity, revealing innovative approaches to harnessing the potential of meta-surfaces for polarization control and conversion. A key area of focus is beam-steering technology, with a thorough investigation into beam-steering techniques that have significant implications for enhancing wireless communication, high-resolution imaging, and the internet of things. The paper highlights the potential of these techniques in addressing real-world challenges and advancing THz technology. Furthermore, this review provides an in-depth examination of the innovative antenna designs tailored for THz applications, shedding light on their characteristics and benefits. It also explores the exciting possibilities of THz technology within the medical field, including precise bio sensing and cancer cell detection.

No abstract is available for this article.

A rectenna designed for wireless power transfer at 900 MHz focuses on conjugate impedance matching and image impedance matching for improved efficiency. To do them, a voltage doubler rectifier circuit (VD) and a planar monopole antenna (PMA) were engineered with the same pure resistance value and integrated into the rectenna. The input impedance of the VD with 30 Ω load resistance indicated a pure resistance of approximately 73 Ω. This value closely matches the input impedance of a dipole antenna operating as a pure resistor. Since the prototype rectifier circuit is unbalanced, the authors constructed a PMA, an unbalanced antenna similar to a dipole antenna, on a double-sided circuit board. In this setup, a microstrip line was created by extending the radiating element, achieving the impedance matchings. Measurements indicated a voltage standing wave ratio of approximately 1.03. A rectenna efficiency of 37.4% was observed for a transmission distance of 50 cm. The rectification efficiency of the VD is nearly 0% when the input power is less than −20 dBm, and the received power of the PMA is less than −20 dBm when the transmission distance is 60 cm or more. It is predicted that the rectenna efficiency will be 0% when the transmission distance is 60 cm or more. However, the rectenna efficiency was 24.6% when the transmission distance was 60 cm. This over 20% improvement is due to the connection between the PMA and the VD using pure resistance.

In this study, a wideband circularly polarized (CP) H-plane horn antenna based on Gap Waveguide (GW) technology in K-band is presented. The proposed antenna consists of two unconnected metal planes. To produce broadband CP radiation, two main methods are utilized. First, two antipodal tapered plates (ATPs) are added in front of the horn. The ATPs are carefully designed for dissimilar polarization orientations. By this technique, the orthogonal electric fields can be prepared. Then, by embedding three metal square pins near the center of the aperture in both inner plates, the impedance bandwidth (BW) and BW of CP radiation of the proposed horn is entirely improved. Its BW for target |S11| < −10 dB is 18–28 GHz. Also, the peak gain fluctuates between 11.5 and 13 dB. This antenna can provide a 3 dB polarization axial-ratio BW of about 28.5% (20–26 GHz). Total radiation efficiency is higher than 94%. To verify the design, the proposed structure is manufactured and tested. The proposed horn antenna result has an appropriate agreement between measurement and simulation. Its miniaturized dimensions, easy and cheap fabrication, and broadband CP capability make it a proper volunteer for broadband communication systems.

Deep learning is applied to many complex tasks in the field of wireless communication, such as modulation recognition of spectrum waveforms, because of its convenience and efficiency. This leads to the problem of a malicious third party using a deep learning model to easily recognize the modulation format of the transmitted waveform. Some existing works address this problem directly using the concept of adversarial examples in the computer vision field without fully considering the characteristics of the waveform transmission in the physical world. Therefore, we propose two low-interception waveforms (LIWs) generation methods, the LIW and ULIW algorithms, which can reduce the probability of the modulation being recognized by a third party without affecting the reliable communication of the friendly party. Among them, ULIW improves LIW algorithm by simulating channel noise during training cycle, and substantially reduces the perturbation magnitude while maintaining low interception accuracy. Our LIW and ULIW exhibit significant low-interception performance in both numerical simulations and hardware experiments.

The Netherlands-China Low-Frequency Explorer (NCLE) (Boonstra et al., 2017, https://www.ursi.org/proceedings/procGA17/papers/Paper_J19-2(1603).pdf; Chen et al., 2020, https://ui.adsabs.harvard.edu/abs/2020AAS…23610203C/abstract) is a radio instrument for astrophysical studies in the low-frequency range (80 kHz–80 MHz). As a technology demonstrator, NCLE shall inform the design of future radio receivers that aim at low-frequency radio astronomy. NCLE can make observations at very high spectral resolution (<1 kHz) and generate radio sky maps at an angular resolution of ≈1.5 radians. NCLE uses three monopole antennas, each 5 m long, and three identical analog signal chains to process the signal from each antenna. A single digital receiver samples the signal and calculates the auto-correlated and cross-correlated spectra. The instrument's analog and digital signal chains are extensively configurable. They can be fine-tuned to produce broadband spectra covering the instrument's complete operating frequency range or sub-bands. NCLE was developed within a veryshort timescale of 2 years, and currently, it is on board Queqiao, the relay spacecraft of the Chang'e- 4 mission, in a halo orbit around the Earth-Moon L2 point. This paper outlines the science cases, instrument architecture with focus on the signal chain, and discusses the laboratory measurements during the pre-launch phase.

Today, wireless communication systems need an antenna with a high gain, efficiency and beamsteering, as well as broadband capability, especially important in radar communications. The array antenna is commonly used in many applications due to its advantages, such as high gain and wide bandwidth. This paper presents an advanced design of a horizontally steerable planar antenna array intended for significant radar applications. A novel structure is used to create a compact array antenna of 8 × 3 elements. The design features a comprehensive 60° steering sector alongside a notable 12% bandwidth (4.45─5.42 GHz), using 8 × 3 planar array in an novel configuration. A three-element series-fed vertical array is utilized, employing aperture feeds with precisely sized patches to maximize performance. A detailed description of the design and refinement process of this array is presented, with emphasis on its exceptional capabilities for horizontal steering and bandwidth efficiency. By employing series-fed vertical arrays with variable patch dimensions, we have successfully developed an antenna array that meets the stringent bandwidth requirements essential for radar technology, thereby enhancing the operational versatility of radar systems.

Utilizing multistatic specular meteor radar (MSMR) observations, this study delves into global aspects of wind perturbations in the mesosphere and lower thermosphere (MLT) from the unprecedented 2022 eruption of the Hunga Tonga-Hunga Ha'apai (HTHH) submarine volcano. The combination of MSMR observations from different viewing angles over South America and Europe, and the decomposition of the horizontal wind in components along and transversal to the HTHH eruption's epicenter direction allow an unambiguous detection and identification of MLT perturbations related to the eruption. The performance of this decomposition is evaluated using Whole Atmosphere Community Climate Model with thermosphere/ionosphere extension (WACCM-X) simulations of the event. The approach shows that indeed the HTHH eruption signals are clearly identified, and other signals can be easily discarded. The winds in this decomposition display dominant Eastward soliton-like perturbations observed as far as 25,000 km from HTHH, and propagating at 242 m/s. A weaker perturbation observed only over Europe propagates faster (but slower than 300 m/s) in the Westward direction. These results suggest that we might be observing the so-called Pekeris mode, also consistent with the L 1 pseudomode, reproduced by WACCM-X simulations at MLT altitudes. They also rule out the previous hypothesis connecting the observations in South America to the Tsunami associated with the eruption because these perturbations are observed over Europe as well. Despite the progress, the L 0 pseudomode in the MLT reproduced by WACCM-X remains elusive to observations.

This study reports coordinated observation of ionospheric irregularities from VHF Radar, GPS and IRNSS (Indian Regional Navigation Satellite System), from regions near the northern crest of the EIA (Equatorial Ionization Anomaly), which has not been explored earlier. Efforts have been made to study the signal-in-space environment for concurrent detection of ionospheric irregularities over a range of radio frequency, starting from 53 MHz of the Radar, to L-band of GPS at 1,575.42 MHz and S band signal of IRNSS at 2,492.5 MHz. The radar is operational at Ionosphere Field Station, Haringhata (geographic latitude 22.93°N; geographic longitude 88.51°E; magnetic dip angle 36.2°N) of University of Calcutta. The GPS and IRNSS data are recorded at Calcutta (22.58°N, 88.38°E geographic; magnetic dip: 36°N), separated from Haringhata by 50 km. The spatial as well as temporal variations of irregularities affecting different radio frequencies have been presented. Coordinated observations have been made during period of March–April 2023. Results of the study reveal the common zone of impact of the different radio frequency links spanning from 53 to 2,592.5 MHz and was identified within 16°–25°N, 85°–90°E. During coordinated observations made over several days, irregularity structures have been observed with radar, having backscatter SNR (Signal to Noise ratio) intensity within −5 to 15 dB. During this time, while intense L band scintillation was recorded on multiple satellites of GPS, scintillation recorded at S band signal was moderate to intense.

This study provides the design and demonstration of a Ku/K band horn-fed linear polarization (LP) to circular polarization (CP) converter using a reflectarray antenna based on the holographic technique and the generalized law of total reflection without any iterative algorithms. The proposed hologram performs wide-range frequency beam scanning with minimum gain losses and cross-polarization levels. It comprises 2,500 diagonal slotted octagonal subwavelength metasurfaces with a periodicity of 0.266λ 0 = 4 mm at 20 GHz as the center frequency. Two equations are defined to compute Y 11 of the proposed unit cell regarding its dimensions for TE(0,0) and TM(0,0) Floquet modes. They significantly simplify the coding procedure and reduce the computational time for synthesizing the hologram. The antenna is simulated using the CST software from 14 to 25 GHz. As a confirmation, a prototype is manufactured and measured at 16, 18, 20, 22, and 24 GHz to verify its performance. The simulated and measured results are well-matched. The presented hologram achieves 40% 1.8-dB axial ratio (AR) bandwidth (16–25 GHz), 40% 3.3-dB gain bandwidth (16–24 GHz), and above 30% 2-dB gain bandwidth (16–22 GHz). Moreover, the antenna can perform beam scanning from 42° to 24° by changing the frequency from 16 to 24 GHz with peak gain values greater than 20.33 dBi. The LHCP pencil beams are at least 24° off-broadside, so the proposed hologram avoids the feed blockage. These achievements make the hologram one of the best candidates for satellite communications, radar applications, short-range communication, and point-to-point communication.

No abstract is available for this article.

Numerous studies have reported anomalous ultralow frequency (ULF) electromagnetic fields preceding earthquakes. In this paper, we estimate the current intensity responsible for generating the earthquake-related ULF fields under the assumption that the origin is a current flowing at the hypocenter and that it has the same frequency dependence for all cases. To estimate current intensity, we perform ULF electromagnetic field simulations with an absorbing boundary condition developed in this study, taking into account the conductivity distribution of the Earth's crust. We analyze 11 earthquakes, including those that occurred in Loma Prieta, Spitak, Guam, Biak, Kagoshima, Iwateken Nairiku Hokubu, Izu swarm, Jammu and Kashmir, Alum Rock, Wenchuan, and L’Aquila. Our results show that, for nine out of the 11 events, there is a positive correlation between current intensity and earthquake magnitude, suggesting that the measured ULF fields originate from seismic activity and supporting our assumptions.

Metasurface arrays can achieve beam control at low cost and high quality by providing different phase compensations for each unit, effectively focusing microwave energy on target locations. With the development of short-range communication technology or microwave power transmission technology, the demand for focusing has also increased. Using metasurface arrays to achieve multi-target focusing has wide application value. However, as the number of focal points increases, the superposition of electromagnetic wave propagation paths leads to significant interference phenomena, which can impact potential applications. Existing solutions are unable to solve such complex problems involving a large number of targets with conflicts between them. Multi-objective algorithms, by iteratively obtaining a set of optimal solutions, provide decision support for designers in complex multi-objective problems. This paper alters the phase of cells in a reflective array, calculates the near-field electric field model using the Fresnel diffraction formula, and employs various solutions using the Non-dominated Sorting Genetic Algorithm III (NSGA-III) combined with different constraints. Finally, we select the balanced solution to establish the array. After simulation, three adjacent focal points with normalized central values of 1, 0.86, and 0.88 were obtained, with the maximum electric field value outside the focal points being only 0.58, demonstrating the feasibility of multi-objective algorithms in solving complex multi-focal problems.

The urban radio-frequency (RF) noise generated by our cities continues to change with time. Although models exist to describe the RF noise as functions of frequency and urban land use types, very few models describe the spatial character or structure of the noise on the scales of city blocks (50–150 m). The goal of this work is to investigate the connection between urban morphology and the higher-order spatial statistics of the noise field. To achieve this goal, a large measurement campaign was conducted in Boston, Massachusetts. Many spatial measurements allowed for calculation of spatial correlation functions of noise power in three different neighborhoods, which were used to quantify the spatial structure of the fields. A statistical point source model is then developed, with adjustable parameters relating to urban morphology. Good agreement between the model and the experimental correlation functions suggests the 25 MHz urban noise field is well described by a random network of fixed point sources, radiating with a 1/r power law behavior.

This paper presents an alternative approach to improve the achievable beam scan angle of a traditional multi-beam waveguide lens antenna. Due to the focusing mechanism by manipulating the geometrical curvatures of the waveguide lens, the angular scan range is limited in the conventional waveguide lens design using dual-focal points of excitations because the geometrical curvatures of the waveguide lens only provide two design freedoms. To overcome this limitation, a solution of treating the waveguide lens as a transmit array consisting of non-identical elements is proposed so that each element of the antenna array can be well calibrated to improve the maximum scan angular range, where a third focus point of excitation can be created by adding another design freedom from the differentiation between non-identical elements. Each element of this new transmitting array can be well calibrated with the help of a mathematical expression to improve the maximum angular scan range. Numerical simulations show that the proposed antenna architecture exhibits better radiation characteristics than the traditional waveguide lens antenna. Radiation characteristics are studied and compared for both types of lens antennas to validate the design concept. The proposed triple-focal point provides a higher gain than the traditional lens antenna with fewer antenna elements. The gains of the beams at ±10°, ±20°, ±30°, and ±40° are found to be 27.78, 26.94, 26.19, and 24.04 dBi, respectively. From the comparison, it is seen that the variation in gain by the proposed triple-focal array design is more stable than the conventional dual-focal point design.

The ground-based microwave radiometer (MWR) retrieves atmospheric profiles with a high temporal resolution for temperature and relative humidity up to a height of 10 km. These profiles have been widely used in the field of meteorological observation. Due to the inherent fragility of neural networks, one of the important issues in this field is to improve the reliability and stability of MWR profiles based on neural network inversion. We propose a deep learning method that adds noise to the BP neural network inversion (NBPNN) process. Comparison of the radiosonde data and NBPNN results shows that if the error of MWR brightness temperature is in the range of −2–2 K, the root-mean-square error (RMSE) of the temperature profile is 2.15 K, and the RMSE of the relative humidity profile is 19.46 % inverted by NBPNN. The results are much less than the errors of the temperature profile and relative humidity profile inverted by the traditional backpropagation neural network inverse method. From the comparison, we demonstrated that NBPNN significantly increases the inversion accuracy and robustness under the condition of errors in brightness temperature, which can reduce requirements for BT accuracy of MWR and achieve MWR long-term stability.

The ground-based, high-frequency radars of the Super Dual Auroral Radar Network (SuperDARN) observe backscatter from ionospheric field-aligned plasma irregularities and features on the Earth's surface out to ranges of several thousand kilometers via over-the-horizon propagation of transmitted radio waves. Interferometric techniques can be applied to the received signals at the primary and secondary antenna arrays to measure the vertical angle of arrival, or elevation angle, for more accurate geolocation of SuperDARN observations. However, the calibration of SuperDARN interferometer measurements remains challenging for several reasons, including a 2π phase ambiguity when solving for the time delay correction factor needed to account for differences in the electrical path lengths between signals received at the two antenna arrays. We present a new technique using multi-frequency ionospheric and ground backscatter observations for the calibration of SuperDARN interferometer data, and demonstrate its application to both historical and recent data.

Ionospheric delay, as one of the largest error sources in radio propagation, can only be corrected for this error using the ionospheric delay correction model for Global Navigation Satellite System (GNSS) single-frequency users. In this paper, the 2021 geomagnetic storm event is selected, and based on the measured ionospheric data from the GNSS observatory, the perturbation of the ionosphere by the geomagnetic storm event is analyzed, and it is found that the response of the ionosphere to the geomagnetic storm has obvious differences in the response characteristics and response time in different latitude regions. The performance of the global ionospheric map (GIM), the empirical model, and the broadcast ionospheric model during the geomagnetic storm-induced ionospheric perturbation is analyzed and the change in the accuracy of each ionospheric model during the geomagnetic storm-induced ionospheric perturbation is investigated, using the measured electron content of the GNSS as a benchmark. The results show that there is good agreement between the GIM products and the measured electron content during the period of ionospheric calm and the period of ionospheric perturbation. It is worth noting that geomagnetic storms do not necessarily lead to a decrease in the accuracy of ionospheric delay-correction models, and in some cases, the models that were originally under-accurate show a tendency to improve their accuracy during the period of perturbation instead. Neither the broadcast ionospheric model nor the electron content of the empirical model output responds to geomagnetic storm-induced ionospheric perturbations.