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This paper presents the design and simulation of radio frequency (RF) filters to mitigate colocation interference between high-frequency (HF) and very high-frequency (VHF) tactical radios used in military ground vehicles with minimal insertion loss. The interference occurs due to the proximity of the HF and VHF radio antennas, with HF signals distorting the VHF radio signals and vice versa. In order to minimise the interference, low- and high-pass filters are designed and simulated to achieve steep roll-off, low passband ripple and complete rejection in the stop-band region. The Advanced Design System (ADS) simulator is used in the filter design, which starts with preliminary studies to choose the best method for designing the RF filter based on military communication system specifications. This includes selecting the insertion loss method, filter topology and passband ripple. Based on this, the RF filter implements a Chebyshev type 1 design with passband ripple of 0.15 dB and passive lumped elements. Therefore, 19th-order Chebyshev low- and high-pass filters are designed and simulated using ADS. From the simulation, a low-pass filter with a cut-off frequency of 29.73 MHz at -3.00 dB was designed. This filter exhibits a sharp suppression of about -114.83 dB in the stop-band at 40 MHz and a return loss of -14.7 dB. Similarly, a high-pass filter with cut-off frequency of 29.27 MHz at -3.00 dB was designed. This filter also shows a sharp suppression of about -114.82 dB in the stop-band at 21.76 MHz and has return loss of -14.7 dB, making it effective for eliminating co-location interference.

The inverted tripole charge structure in thunderstorm over the central Tibetan Plateau was discovered for the first time, primarily through observations from lightning very high frequency interferometer capable of high‐precision lightning channel mapping. The dominant cell exhibited an inverted tripole charge structure initially, characterized by a negative charge region at temperatures near 0°C, a main positive charge region between −30°C and −5°C, and an upper negative charge region at T < −20°C. The cell's rear portion exhibited a normal tripole before detaching, leaving a pure inverted tripole. Dissipation of the lower negative charge transitioned the structure to an inverted dipole, consisting of an upper negative (T < −20°C) and lower positive (T > −20°C). Throughout this thunderstorm, no positive cloud‐to‐ground (+CG) flashes were detected, while five −CG flashes were recorded. Among 109 intracloud (IC) flashes detected, 90% occurred between the upper inverted dipole. Radar reflectivity showed that this thunderstorm was more intense than conventional plateau thunderstorms.

Geosynchronous (GEO) spaceborne–airborne very high-frequency ultra-wideband bistatic synthetic aperture radar (VHF UWB BiSAR) can conduct high-resolution and wide-swath imaging for ocean scenes. However, GEO spaceborne–airborne VHF UWB BiSAR imaging faces some challenges such as the geometric configuration, huge amount of echo data, serious range–azimuth coupling, large spatial variance, and complex motion error, which increases the difficulty of the high-efficiency and high-precision imaging. In this paper, we present an improved bistatic fast factorization backprojection (FFBP) algorithm for ocean scene imaging using the GEO satellite-unmanned aerial vehicle (GEO-UAV) VHF UWB BiSAR, which can solve the above issues with high efficiency and high precision. This method reconstructs the subimages in the orthogonal elliptical polar (OEP) coordinate system based on the GEO satellite and UAV trajectories as well as the location of the imaged scene, which can further reduce the computational burden. First, the imaging geometry and signal model of the GEO-UAV VHF UWB BiSAR are established, and the construction of the OEP coordinate system and the subaperture imaging method are proposed. Moreover, the Nyquist sampling requirements for the subimages in the OEP coordinate system are derived from the range error perspective, which can offer a near-optimum tradeoff between precision and efficiency. In addition, the superiority of the OEP coordinate system is analyzed, which demonstrates that the angular dimensional sampling rate of the subimages is significantly reduced. Finally, the implementation processes and computational burden of the proposed algorithm are provided, and the speed-up factor of the proposed FFBP algorithm compared with the BP algorithm is derived and discussed. Experimental results of ideal point targets and natural ocean scenes demonstrate the correctness and effectiveness of the proposed algorithm, which can achieve near-optimal imaging performance with a low computational burden.

The continuous‐wave free‐electron laser (CW‐FEL), based on superconducting radiofrequency (SRF) technology with an electron bunch repetition rate of up to MHz levels, is one of the most advanced light sources, providing exceptionally high average and peak‐brightness FEL pulses. Among the new CW‐FEL facilities worldwide, the recently proposed Dalian Advanced Light Source (DALS) occupies a unique position as an extreme ultraviolet (EUV) facility primarily designed for chemical physics research. Since the beam emittance requirement for DALS is not as stringent as that for X‐ray CW‐FEL facilities, a direct current (DC) gun is considered as the primary electron source, with a very high frequency (VHF) gun also planned. To demonstrate key technologies and characterize the electron beam performance, a superconducting CW injector testbed, named the Electron Source Test Facility (ESTF), has been designed and is currently under construction. The testbed is uniquely designed to accommodate both guns with minimal switching effort, where the rest of the beamline layout remains unchanged except for the swapped guns. The two‐gun switching scheme for the testbed is shown to be a feasible and cost‐effective approach. Furthermore, the injector performance with both guns has been evaluated through a start‐to‐end simulation based on the DALS configuration, including the production of electron beam in the ESTF injector, the following beam acceleration and compression in a superconducting linear accelerator, and finally the beam lasing performance in the undulator section. The evaluation confirms that the DC gun is a promising electron source for CW‐FEL facilities, especially for EUV applications, even though all currently constructed CW facilities have employed the VHF gun. This paper provides a comprehensive description of the injector design and the corresponding performance evaluation. An injector testbed for continuous‐wave free‐electron lasers based on a direct current gun and an interchangeable very high frequency gun is under construction. Its physical design and performance have been studied carefully.

Monitoring lightning and its location is important for understanding thunderstorm activity and revealing lightning discharge mechanisms. This is often realized based on very low-frequency/low-frequency (VLF/LF) signals, very high-frequency (VHF) signals, and optical radiation signals generated during the lightning discharge process. The development of lightning monitoring and location technology worldwide has largely evolved from a single station to multiple stations, from the return strokes (RSs) of cloud-to-ground (CG) lightning flashes to total lightning flashes, from total lightning flashes to lightning discharge channels, and from ground-based lightning observations to satellite-based lightning observations, all of which have aided our understanding of atmospheric electricity. Lightning monitoring and positioning technology in China has kept up with international advances. In terms of lightning monitoring based on VLF/LF signals, single-station positioning technology has been developed, and a nationwide CG lightning detection network has been built since the end of the twentieth century. Research on total lightning flash positioning technology began at the beginning of the 21st century, and precision total lightning flash positioning technology has improved significantly over the last 10 years. In terms of positioning technology based on VHF signals, narrowband interferometers and wideband interferometers have been developed, and long-baseline radiation source positioning technology and continuous interferometers have been developed over the last ten years, significantly improving the channel characterization ability of lightning locations. In terms of lightning monitoring based on optical signals, China has for the first time developed lightning mapping imagers loaded by geosynchronous satellites, providing an important means for large-scale and all-weather lightning monitoring.

Use of Global Positioning System (GPS) collars on free-ranging ungulates overcomes many limitations of conventional very high frequency (VHF) telemetry and offers a practical means of studying space use and home range estimation. To better understand winter home ranges of white-tailed deer (Odocoileus virginianus), we evaluated GPS collar performance, and we compared GPS- and VHF-derived diurnal home ranges (for the same animals) and GPS-derived home range estimates for diurnal and nocturnal locations. Overall, the mean fix success rate of our GPS collars was 85% (range = 14–99%). Kernel density estimates of home range (using the 95% probability contour) derived from GPS and VHF locations were generally similar, as were GPS-derived diurnal and nocturnal home ranges. Overlap indices between GPS and VHF utilization distributions (UDs) ranged from 0.49 to 0.78 for the volume of intersection (VI) index and from 0.67 to 0.94 for Bhattacharyya's affinity (BA); overlap indices for GPS-diurnal and nocturnal UDs ranged from 0.29 to 0.81 for VI and from 0.56 to 0.94 for BA. Despite similarities of home ranges estimated from GPS versus VHF locations and GPS-diurnal versus nocturnal locations, our data also indicate that differences may have important implications for studies focused on deer use of space, habitat, and resources at a finer scale.

E-region field-aligned irregularities (FAIs) are a hot topic in space research, since electromagnetic signal propagation through ionospheric irregularities can undergo sporadic enhancements and fading known as ionospheric scintillation, which could severely affect communication, navigation, and radar systems. However, the range resolution of very-high-frequency (VHF) radars, which is widely used to observe E-region FAIs, is limited due to its bandwidth. As a technology that is widely used in atmosphere radars to improve the range resolution of pulsed radars by transmitting multiple frequencies, this paper employed the multifrequency radar imaging (RIM) technique in a Wuhan VHF radar. The results showed that the range resolution of E-region FAIs greatly improved when compared with the results in traditional single-frequency mode, and that finer structures of E-region FAIs can be obtained. Specifically, the imaging results in multifrequency mode show that E-region FAIs demonstrate an overall descending trend at night, and it could be related to the tides or gravity waves due to their downward phase velocities or even driven by downwind shear. In addition, typical quasi-periodic (QP) echoes with a time period of around 10 min could be clearly seen using the RIM technique, and the features of the echoes suggest that they could be modulated by gravity waves. Furthermore, the RIM technique can be used to obtain the fine structure of irregularities within a short time period, and the hierarchical structure of E-region FAIs can be easily found. Therefore, the multifrequency imaging RIM technique is suitable for observing E-region FAIs and their evolution, as well as for identifying the different layers of E-region FAIs. Combined with the RIM technique, a VHF radar provides an effective and promising way to observe the structure of E-region FAIs in more detail to study the physical mechanism behind the formation and evolution of ionospheric E-region irregularities.

\"Extremely High Frequency (EHF)\" and \"Very high frequency (VHF)\" bands are mainly utilized with \"Underwater Wireless Sensor Networks (UWSNs)\" for communication purposes. However, due to the mobility of underwater sensors in water because of the water tide, the EHF/VHF signals may attenuate, lose or fade depending on the condition of the water. Therefore, it is a challenging stint of finding the optimal parameters of UWSN topology planning. In this paper, three \"Multi-Objective Optimization Algorithms (MOOAs)\" have been utilized to mitigate this problem, namely MOSFP, SPEA2 and NSGA-II. This work also intends to minimize path loss. On the other hand, it intends to maximize the power density of the network. Various network configurations, such as distance between sender and receiver, water conductivity and water permeability, are considered to evaluate the proposed objective models. Qualitative and quantitative tests have been conducted to analyze the results. From the analysis of the intersection point of Pareto-front of the objective functions, it is shown that all the algorithms find the optimal distance between transmitter and receiver, which balances the aforementioned maximization and minimization objective functions. This value is 36 m.

The study of lightning radiation spectra is of great importance forthe scientific study of lightning physics and lightning protection systems of the electromagnetic threat of lightning. The frequency spectra characteristics of very high frequency (VHF) electric field radiation with the corresponding low frequency (LF) electric field changes during various events of the physical process of the seven positive ground flash events have been analyzed using the short-time Fourier transform (STFT) method. It was found that the frequency range of initial breakdown (IB), leader (L), and continuing current (CC) was 24-52 MHz, 30-50 MHz, and 30-50 MHz, respectively, with the arithmetic mean (AM) initial breakdown (IB), leader (L), and continuing current (CC) of 39 MHz, 39 MHz, and 38 MHz, respectively. The differences in the frequency spectral pattern range of 24-52 MHz between different positive ground flash events may indicate that these events have different discharge mechanisms and that frequency domain information is required to identify these various discharge events. We also found that time delay between wideband and narrowband return stroke (RS) radiation is about 2-17 lis.

As an important non-traditional machining technology, micro-electrical discharge machining (micro-EDM) has become one of the ideal methods for conducting material parts in the mesoscopic scale. However, the current micro-EDM pulse generator has low discharge frequency and wide pulse width, and its nano-scale high-efficiency erosion ability is still difficult to meet the increasing practical requirement. For this purpose, a micro-energy pulse source with narrow pulse width and high-voltage amplitude, which is different from the current typical micro-energy pulse generator, is designed for a pulse generator with more fine-etching ability. Using the principle of circuit resonance, the very high-frequency (VHF) resonant discharge pulse generator can produce an open-circuit voltage waveform with a discharge frequency of 90 MHz and a peak value of 70 V. The single-pulse discharge energy can be continuously processed as low as 6nJ. Based on the VHF pulse generator and the transistor-type pulse generator, the discharge erosion experiments are carried out, and the surface roughness and recast layer are analyzed and compared. The experimental results show that the former processing quality is obviously better than the latter. The surface roughness after processing at a discharge frequency of 90 MHz can reach Ra 42 nm, the average diameter of craters is 0.38 μm, and the hole edge processed at a discharge frequency of 55 MHz has almost no recast layer which achieves similar cold processing effects.