拉曼激光相位噪声对原子干涉重力仪测量分辨力的影响

为了保证相控阵雷达的性能,它的天线现场校准越来越受到重视。本文首先介绍相控阵雷达天线现场校准的现状, 提出目前存在的问题。然后分析光学电磁场探测技术优势,阐述基于光子技术的相控阵雷达天线现场校准实现的可行性,建立了现场校准装置。通过实验验证,该技术能实现对相控阵雷达天线现场校准,具有高精度,对天线的辐射场的影响较小,能满足相控阵雷达天线现场校准要求。     

精密水槽收缩段流场模拟与收缩曲线选型评估

通过求解RANS方程,数值模拟了精密水槽收缩段的流场特性。针对两种不同出口尺寸不同收缩比的收缩段,选取了三种不同的收缩曲线来进行选型评估。根据壁面是否分离、逆压梯度变化缓和程度、出口速度均匀度,来判断不同收缩曲线的优劣。研究表明:对于出口尺寸为250 mm×20 mm、收缩比为30的收缩段,双三次收缩曲线是最优的;对于出口尺寸为250 mm×10 mm、收缩比为60的收缩段,维氏收缩曲线是最优的。本次数值模拟计算的工作可为精密水槽收缩段优化设计工作提供借鉴。     

Non-destructive evaluation of concrete moisture by GPR: Experimental study and direct modeling

This paper reports research into the application of Ground Penetrating Radar (GPR) technology to the physical characterization of concrete and gives the details and results of an experimental study on the effect of concrete moisture on radar waves propagating through concrete laboratory slabs. Radar measurements were performed using a commercial radar system with 1.5 GHz ground-coupled antennas. The concrete slabs were conditioned so as to possess different degrees of saturation and a homogeneous moisture distribution. Electrical resistivity measurements were also carried out on each concrete sample to provide complementary information regarding the moisture content of the concrete. Interesting results were found concerning the ability of the radar technique to characterize the moisture state of concrete. Attention was especially focused on the amplitude, velocity and frequency spectrum of the waveforms recorded. In particular, the behavior of the transmitter-receiver direct wave was found to be greatly influenced by the moisture in the concrete. Finally, some experimental results were simulated using a simple dielectric model of heterogeneous mixtures.     

Scattering properties of patch antennas loaded with inhomogeneous substrates via a combined spectral domainmoment method

Abstract

In this paper a combined spectral domain-moment method is proposed in order to investigate the scattering properties of microstrip patch antennas with inhomogeneous substrates. The closed analytical expressions for the spectral dyadic Green's functions for different inhomogeneity profiles both in the rigorous and in the asymptotic form are obtained. A method of moments (MoM) formulation involving these expressions and using roof-top subdomain expansions is developed in order to calculate the radar cross-section for the antennas under analysis. Finally, different rectangular geometry patch shapes are considered (rectangular itself, L-patch, cross-patch, rectangular ring, etc.) and, for all of them, the monostatic radar cross-sections, compared to those obtained by using homogeneous substrates, are improved.     

Breaking Reciprocity with Space‐Time‐Coding Digital Metasurfaces

Metasurfaces are artificially engineered ultrathin structures that can finely tailor and control electromagnetic wavefronts. There is currently a strong interest in exploring their capability to lift some fundamental limitations dictated by Lorentz reciprocity, which have strong implications in communication, heat management, and energy harvesting. Time‐varying approaches have emerged as attractive alternatives to conventional schemes relying on magnetic or nonlinear materials, but experimental evidence is currently limited to devices such as circulators and antennas. Here, the recently proposed concept of space‐time‐coding digital metasurfaces is leveraged to break reciprocity. Moreover, it is shown that such nonreciprocal effects can be controlled dynamically. This approach relies on inducing suitable spatiotemporal phase gradients in a programmable way via digital modulation of the metasurface‐elements' phase repsonse, which enable anomalous reflections accompanied by frequency conversions. A prototype operating at microwave frequencies is designed and fabricated for proof‐of‐concept validation. Measured results are in good agreement with theory, hence providing the first experimental evidence of nonreciprocal reflection effects enabled by space‐time‐modulated digital metasurfaces. The proposed concept and platform set the stage for “on‐demand” realization of nonreciprocal effects, in programmable or reconfigurable fashions, which may find several promising applications, including frequency conversion, Doppler frequency illusion, optical isolation, and unidirectional transmission.     

Solution of Maxwell's Equations for the Simulation and Optimization of the Radar Assessment of Concrete Structures

Recent developments in ground-penetrating radar (GPR) models have brought about complex electromagnetic fields due to the fine variations of structures. Therefore, it has become necessary to use more accurate analysis including environmental characteristics, antennas, and interpretation of the raw data in three-dimensional space. Such analyses require numerical techniques by which problems having complex scattering properties can be solved as accurately and as fast as possible. Some methods had been developed for resolution of specific problems involving differential or integral equations. However, each technique has limitations and trade-offs. The research work presented in this article aims to develop electromagnetic wave-propagation models in concrete structures using different numerical techniques to investigate the detection and location of buried objects.     

Amplitude calibration of a digital radio antenna array for measuring cosmic ray air showers

Radio pulses are emitted during the development of air showers, where air showers are generated by ultra-high energy cosmic rays entering the Earth's atmosphere. These nano-second short pulses are presently investigated by various experiments for the purpose of using them as a new detection technique for cosmic particles. For an array of 30 digital radio antennas (LOPES experiment) an absolute amplitude calibration of the radio antennas including the full electronic chain of the data acquisition system is performed, in order to estimate absolute values of the electric field strength for these short radio pulses. This is mandatory, because the measured radio signals in the MHz frequency range have to be compared with theoretical estimates and with predictions from Monte Carlo simulations to reconstruct features of the primary cosmic particle. A commercial reference radio emitter is used to estimate frequency dependent correction factors for each single antenna of the radio antenna array. The expected received power is related to the power recorded by the full electronic chain. Systematic uncertainties due to different environmental conditions and the described calibration procedure are of order 20%.     

Superresolution for high-frequency radar

High-frequency (HF) radars require large-array antennas to achieve narrow beamwidths and are required to discriminate targets with low Doppler shifts compared with microwave radar. Superresolution processing can discriminate signals at separations much less than the beamwidth or conventional Doppler bin width (or conversely, achieve a given minimum resolvable separation with a smaller array or less time-on-target). Simulation results and analysis of real radar data presented here show at least a 2-fold improvement compared with conventional processing, assuming simple theoretical gain manifolds. A novel polarisation-sensitive superresolution algorithm applicable to skywave HF radar is also discussed.     

Method for obtaining gas concentration with a phase-based metrology system

A new technique is presented for obtaining gas concentration by measuring the slope of the anomalous dispersion at a resonance. We describe the equations that govern this process using a Lorentz model and show that the slope of the anomalous dispersion is directly related to the absorption coefficient. The slope is obtained from an interferometric setup and a frequency modulation spectroscopy technique. Experimental data are presented that illustrate this technique for two different sample cells containing water vapor.     

Microwave diversity imaging system ‐ principle,methodology and experimental results

Abstract

The basic principle, methodology and experimental results of the automated microwave diversity imaging system developed in the Electromagnetic Research Laboratory of National Taiwan University are presented. The microwave image formulation is based on Bojarski's identity. In practice, accessing the object backscattering information using the frequency, angular and polarization diversity techniques is conducted by the developed microwave measurement system with major parts including dual polarization antennas, network analyzer and a MicroVax III. It is shown that images of a scale model of the space shuttle reconstructed from the measured data can be identified visually. The implications for high‐resolution microwave imaging radar networks are also discussed.     

An ultrawide-band microwave radar sensor for nondestructive evaluation of pavement subsurface

A new ultrawide-band (UWB) microwave radar sensor operating from 0.6 to 5.6 GHz has been developed using microwave integrated circuits for pavement subsurface characterization. UWB antennas operating from 0.5-10-GHz have been designed and tested for use in the sensor. A new simple, yet effective, accurate procedure was also developed to compensate for the common amplitude deviations and nonlinear phase errors produced by the inherent imperfection of the system. The developed compensation method is applicable to other systems and effectively reduces the potential masking of adjacent targets as well as facilitating and increasing the accuracy for target identification of the sensor. The sensor has been used to assess a pavement sample with less than 0.1 in of error in the pavement's layer thickness. The developed system represents the first UWB stepped-frequency radar sensor completely realized using microwave integrated circuits over the frequency range of 0.6-5.6 GHz for subsurface sensing applications.     

Ka-band bistatic ground-based noise waveform SAR for short-range applications

The designing and testing of a new Ka-band ground-based noise waveform synthetic aperture radar (GB NW-SAR) are presented. Its design is based upon new synthetic aperture antennas and noise radar technology. The authors present preliminary results of its indoor trials. The GB NW-SAR designed may be reconfigured for operation in mono-, bi- and multistatic (or MIMO) modes in both CW and pulse regimes using single- and dual-frequency modes to enhance range resolution via expanding the frequency bandwidth of the signal. Potential resolution of the GB NW-SAR is 15 cm both in range and azimuth. Results of indoor trials are presented for both SAR imaging and differential interferometry measurements. Designed and tested Ka-band GB NW-SAR is applicable for precise remote monitoring of various manmade or natural objects, such as ceilings and roofs of big halls and hangars, dams, bridges, TV towers etc.     

Deinterleaving of radar signals and PRF identification algorithms

Electronic warfare (EW) receivers are passive receivers which receive emissions from other platforms, and do certain analysis on these emissions. Some EW receivers receive radar pulses, measure the parameter of each pulse received and group the pulses that belongs to the same emitter together to determine the radar parameters for each emitter. These parameters are then compared with values stored for known radar types, to identify the emitter type. Two parts are focused, emitters deinterleaving and PRF-type identification. The deinterleaving is done through parameters clustering. Two parameters are selected for clustering direction of arrival and radio frequency. A self-organising neural network called Fuzzy ART is proposed for clustering. This algorithm has a very good clustering quality and can run in real-time applications.The PRF-type identification is done through time-of-arrival (TOA) analysis. Three previously presented algorithms are combined in new scheme to do the TOA analysis (or PRF-type identification). These algorithms are difference TOA histogram, TOA folding histogram and sequence search algorithm. The complete proposed system has been tested using three different tests. These tests are simple PRI test, jittered PRI test and staggered PRI test. The proposed system identifies up to 90 simple emitters, 20 jittered emitters and 20 staggered emitters. In all tests, the data were simulated and generated using software.     

Measurements of the Doppler spectra of breaking waves

A number of experiments have been carried out in two large wave tanks with three different radar systems. The radar frequency, grazing angle, azimuth angle, water wavelength, wave steepness and the breaking wave strength were all varied systematically. The velocity of the peak Doppler power spectral density was found to depend on the phase velocity of the breaking wave in the radar line of sight, but was independent of the radar frequency. The spectral width depended on the phase velocity of the wave, but not on the grazing angle used. The peak Doppler power and radar cross-section of the breaking waves was found to scale with the radar wavelength (proplambda_r ^1.5)     

Photocurrent mapping of near-field optical antenna resonances

An increasing number of photonics applications make use of nanoscale optical antennas that exhibit a strong, resonant interaction with photons of a specific frequency. The resonant properties of such antennas are conventionally characterized by far-field light-scattering techniques. However, many applications require quantitative knowledge of the near-field behaviour, and existing local field measurement techniques provide only relative, rather than absolute, data. Here, we demonstrate a photodetector platform that uses a silicon-on-insulator substrate to spectrally and spatially map the absolute values of enhanced fields near any type of optical antenna by transducing local electric fields into photocurrent. We are able to quantify the resonant optical and materials properties of nanoscale (～50?nm) and wavelength-scale (～1?μm) metallic antennas as well as high-refractive-index semiconductor antennas. The data agree well with light-scattering measurements, full-field simulations and intuitive resonator models.     

Application of graphene-based flexible antennas in consumer electronic devices

We describe the fabrication and characterization of Near-Field Communication (NFC) devices based on highly flexible, carbon-based antennas composed of stacked graphene multilayers. This material features a high value of conductivity (4.20?*?10⁵?S/m) comparable to monocrystalline graphite, but is much more flexible and processable. We first studied the replacement of metal with carbon antennas using computer modeling, to select the best design. Then we manufactured several devices to be used according to the communication protocol ISO/IEC 15693. The inductance of the G-paper antennas was tested before and after hundreds of thousands of bending cycles at bending radii of 45 and 90?mm. During bending the self-resonance frequency and inductance peak showed minimal variation and the resistance at 1?MHz changed from 33.09?Ω to 34.18?Ω, outperforming standard, commercial metallic antennas. The devices were successfully tested by exchanging data with a smartphone and other commercial NFC readers, matching the performance of standard, commercial metallic antennas. The graphene antennas could be deposited on different standard polymeric substrates or on textiles. Smart cards, flexible NFC tags and wearable NFC bracelets were prepared in this way to be used in electronic keys, business cards and other typical NFC applications.     

雷达调频编码脉冲信号的设计与处理

现代雷达为获得较高的距离分辨力通常采用编码频率脉冲串信号和步进频率脉冲串信号,但都存在数据率低和较为严重的距离-多普勒耦合问题。在研究这两种信号特点的基础上,提出了调频编码脉冲信号形式并给出了相应的信号处理方法。经仿真对比可见,该信号形式及其处理方法能同时解决步进调频信号高距离-多普勒耦合、低数据率两大问题,具有较好的联合分辨力。     

Space-time constant modulus algorithm for multipath removal on the reference signal exploited by passive bistatic radar

The authors address the problem of multipath cancellation in the reference signal used in passive bistatic radar (PBR), which exploits an existing transmitter as emitter of opportunity. The presence of multipath echoes in the reference signal is demonstrated to strongly affect the detection performance of passive radar. Based on the well-known constant modulus algorithm (CMA) approach in the time-domain, new multi-dimensional techniques are considered for the adaptive equalisation of the reference signal based on an array of antennas and multiple receiving channels. The effectiveness of these techniques for PBR purposes is demonstrated with reference to typical simulated scenarios. The considered strategies, addressed as space-CMA and space-time-CMA, are shown to be able to recover the performance loss because of the multipath contribution in the reference signal so that they appear as very appealing solutions for PBR equipped with an array of antennas and multiple receiving channels.     

Effect of soil moisture on microwave scattering for remote sensing

The main objective of remote sensing is to design space borne microwave sensors to sense a target and derive useful geophysical parameters. For this purpose, the knowledge of the target characteristics must be obtained through ground based remote sensing. In this paper, soil has been taken as the target. Various percentages of gravimetric soil moisture (m _g) have been taken for establishing its relation with the scattering coefficient (σ ₀) for both like polarizations at X-band frequencies. A linear model has been developed for correlating these two variables (i.e.m _g andσ ₀). Using this model, regression analysis has been done for obtaining different regression parameters and predicted values. Treating measurements from bare smooth soil fields with different soil moistures, the data were analysed to examine the effect of soil moisture on scattering co-efficient (σ ₀) at 9.50 GHz frequency in X-band. The scattering coefficient increases with increase in soil moisture content. Different regression parameters have been obtained, which show that the best look-angle is at 25° for HH-pol and 60° for VV-pol for observingσ ₀ from bare moist soil. Data analysis indicated that the basic cause and effect relationship between the sensor measurements and soil moisture can be extrapolated from theory and small-scale tests to larger resolution elements observed by the sensing aircraft. These results indirectly provide reference data for a satellite-borne remote sensor. From the results, the look angles suitable for operation with radar antennas can be suggested.     

An experimental GPR detection study of environmentally-influenced structural defects in hydraulic engineering

Structural defects such as separation between concrete slab and foundation, and structural voids often occur in hydraulic engineering, which threatens the safety of hydraulic engineering. As the size and internal material of the hydraulic engineering can be queried, ground penetration radar (GPR) detection has the advantage to detect these defects when compared with other nondestructive detection methods. At the same time, when GPR detection is applied to defect these structural defects in hydraulic engineering, complex environmental factors including the uneven structural surface, clutter interference, water reflection, etc. have to be taken into account. In this work, two experimental models are designed to represent two different types of hydraulic structures and the structural defects including separation between concrete slab and its bottom material and void or hole defects are simulated on the two test models, respectively. Through the GPR detection on the two experimental models, the effects of the three environmental factors on the radar images and the signatures of the radar images under the influence of the three environmental factors are studied. Then, different image processing methods are adopted to reduce the influence of the three different environmental factors, and the effects of these methods are verified using the radar images obtained from the experiments. Finally, the GPR detection on a practical hydraulic engineering influenced by the environmental factors and the image processing methods are investigated, which successfully verify the experimental investigation results. It is expected that this study would provide significant technology support for structural defects detection in hydraulic engineering.