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This paper presents a theory of the time-domain radar cross section (RCS) of objects in discrete random media. The time-domain formula is obtained by applying the inverse Fourier transform of the t...

EM coupling in Mars soils is important for water-extraction on Mars. We present new measurement data for thermal rise in Mars soil simulants across a range of microwave frequencies. Results show an inverse relationship between frequency and thermal rise. Data is collected at low temperature to simulate the Mars environment.

The technology of “through-the-wall radar imaging” enables seeing through walls by reconstructing the image based on backscattered waves passing through an optically opaque dielectric wall. However, when the electromagnetic waves cannot penetrate the shielding walls (such as metallic walls), a problem called “hard-wall radar imaging” (HWRI) must be investigated. As the objects are located in the shadow region, where only the diffracted field can be reflected back via edges, conventional imaging techniques are not applicable. The HWRI problem is an imaging scenario with an intrinsic multipath of signal propagation. Therefore, it has been studied using a model-based time-reversal (TR) decomposition of TR operator imaging method. In this paper, an imaging technique with a distributed transmitting and receiving array is proposed as a robust way resolving the problem with a low signal-to-clutter ratio in TR imaging. In this method, the interelement response matrix is collected for the construction of the covariance matrix, from which the geometry of the hard wall is determined from the direction of arrival by the multiple signal classification techniques. Next, the adaptive interference cancellation technique is applied to suppress the interference terms in the image function due to the multipath effects. The experimental results were obtained from a distributed multiple-input multiple-output radar system and show that this method is useful not only for the detection of a weak object echo but also for the localization of multiple targets hidden behind the wall.

One of the important current problems is imaging and detection of objects hidden by obscuring obstacles. There have been extensive works reported on “Through-Wall Imaging” in recent years. This paper extends the previous works to the case where the wall is non-penetrating and the diffraction around the edge becomes important. This hard-wall imaging is related to the historical problem of “Poisson Spot” and “Anti-Podal point.” This paper makes use of the modified time-reversal technique and GTD, and shows the formulations of the multistatic data matrix, eigenvectors and steering vectors to form the imaging function. Signal processing includes the multiplicative modified time reversal technique to eliminate the unwanted extraneous images. Numerical examples include comparisons between the modified and the conventional time-reversal imaging to show the effectiveness of the hard-wall imaging method.

A space-fed X-band phased array antenna (PAA) is presented. The space-fed PAA uses physically autonomous individually addressable unit cells as the building blocks of an arbitrarily large array antenna. The X-band PAA unit cell consists of both a feed facing printed antenna element and a target facing printed antenna element. These elements are related by an RF phase shifter placed in the RF signal path between them which introduces relative phase delay and enables analog beam forming. Unit cell autonomy is realized by wirelessly transferring power and command signals required to operate a microcontroller (MCU) and the RF phase shifter at each unit cell. Each unit cell includes RF to DC conversion circuitry as well as an MCU designed to interpret a common control signal and command unit cell phase delay. The wireless phased array unit cell can be replicated in configurations of size and shape that are independent of RF, power, and array module control distribution networks. To demonstrate feasibility a four-element wireless PAA is constructed and antenna patterns for various control states are shown.

This paper considers the imaging of objects located close to rough surfaces such as ocean or terrain. If transmitters and receivers are also located close to rough surfaces, incident wave is no longer a plane wave nor a spherical wave in free space and it is necessary to consider Green’s functions with the point source located close to the surface, similar to the Sommerfeld dipole problem. This paper considers the near-surface imaging by making use of time-reversal imaging and surface flattening transform. Surface flattening transform converts the rough surface problem into flat surface with inhomogeneous random medium. Mutual coherence function is obtained and used to obtain imaging of point target near rough surface, making use of the multi-static data matrix, time-reversal matrix, the eigenvectors, and the steering matrix. Numerical examples are given. An important point is that integration of stochastic wave propagation and signal processing is necessary to obtain imaging through complex clutter envir...

Statistical wave theories have been developed over many years and have been applied to imaging, remote sensing, and communications for geophysical and biological media. There have been increasing interest in integrating statistical wave theories with other disciplines. For example, communication and signal processing through random media have largely studied without using the results of extensive studies on statistical wave theories even though physical problems that are considered are closely related. It is therefore important to integrate the results of statistical wave theories with signal processing. If we include stochastic Green's function and mutual coherence function (MCF) in the formulations of communication, the formulation of channel capacity can show clearly physical characteristics of random media, and the eigenvalues that include Statistical Green's Function and MCF.

Wireless power transfer (WPT) is a popular research field in recent years and can be categorized into three approaches: inductive coupling, laser beaming, and microwave power transmission (MPT). MPT system operates at the microwave and transfers the energy over more than a few wavelengths. It has its unique advantages of supplying power to non-accessible and mobile receivers. The overall efficiency, which is the ratio between available DC power at the receiver and supplied DC power at the transmitter, depends on both circuit design and wave propagation. As a comprehensive theory of MPT system is not available, this chapter starts with the study of MPT system from the perspectives of mathematical formulation and the experiment in the indoor environment, in Sect. 3.1. The preliminary study leads to the conclusion that highly directional wireless transmitter is very useful in the MPT system for achieving high transmission efficiency. For this reason, phased array antennas with beamforming functionality are usually used to direct the electromagnetic wave toward mobile receivers, and adaptive array algorithms are implemented to enable wireless power focusing in the complex environment. Section 3.3 presents a novel beamforming algorithm, which is proven to give the optimal transmission efficiency and applies to the arbitrarily positioned unequal array based on our problem formulation. To verify this algorithm, Sect. 3.4 validates it with numerical electromagnetic simulation in different cases. The numerical comparison in these examples shows that this algorithm gives higher transmission efficiency over other optimal beamforming algorithms discussed in Sect. 3.2.

Imaging of objects through complex environment is important in several applications, including imaging of hidden objects in obscuring media such as atmospheric and ocean turbulence, rough ocean surfaces, rain, fog, snow, and biological tissues. These media are often randomly varying in space and time, and statistical treatments are necessary to obtain images with useful spatial and temporal resolutions. In recent years, there has been increasing interest in using signal processing and correlation techniques to improve resolutions and to distinguish images from clutter. This paper presents several imaging techniques for objects in the presence of random media. Time-reversal MUSIC (multiple signal classification) imaging has excellent resolution when multiple scattering is small or moderate. Modified beamformer imaging has moderately high resolution even at large multiple scattering. We also include time reversal (TR) and synthetic aperture radar (SAR) imaging for comparison. The technique involves stochastic Green's function and mutual coherence function (MCF), eigenvectors of time-reversal matrix and pseudo spectrum. Numerical examples are given to show the effectiveness of these imaging techniques.

Multiple input-multiple output systems have received considerable attention because of their potential to achieve high channel capacity. This paper presents a study of the effects of a random scattering medium on channel capacity. Formulations are given including stochastic Green's functions and mutual coherence functions. Transmitter and receiver characteristics are included and analytical formulation for eigenvalues and channel capacities are given in terms of the medium scattering characteristics, optical depth, frequency, number of transmitter and receiver elements, transmitting power, and noise spectral power. As an example, we show 500 m link at 60 GHz through rain. The eigenvalues and the channel capacity are calculated in terms of SNR and the rain rate representing the optical depth. It is shown that as the rain rate increases, the correlation of waves at antennas decreases and the capacity increases. However, at high rain rate, the capacity tends to decrease due to the absorption and scattering.

In order to improve the performance of terrestrial free-space optical communication systems in adverse visibility conditions, we present a method for estimation of the atmospheric channel impulse response function which governs the optical intensity propagation. This method reduces run-time computational demands and system complexity in comparison to our previously proposed dual-wavelength channel estimation technique. We consider propagation of optical wavelengths in fog, where the droplet diameters are close to the wavelength and thus scattering and absorption effects are significant. A method for rapid calculation of a channel response function based on estimating the effective optical depth of the channel and curve-fitting is described. The channel response estimate can then be used to design a receiver-side equalizer (minimum mean-squared error linear equalizer) to correct the signal distortion due to propagation through the dispersive channel. The channel estimates are based on parametric curve-fitting functions which have been developed using the modified-vector radiative transfer theory to model the channel response. The optimal fit parameters are found using particle-swarm optimization to minimize the simulated bit-error rate of the received signal.

The time-reversal effects on super-resolution in random scattering media are analysed using numerical finite-difference time-domain (FDTD) simulations. The analytical solutions and results have been presented previously in the literature, which provide confirmation of spot-size reduction and also explanations of the shower curtain effects and backscattering enhancement. However, the analytical solutions are based on several approximations. Thus, validation of the analytical results against realistic scattering events is necessary. Two-dimensional FDTD Monte Carlo simulations have been employed for this investigation to simulate wave propagation and scattering in a random medium. The scattering environments are created by randomly locating cylindrical rods in the background medium. The simulation process involves a point source emitting a Gaussian pulse wave that propagates through the scattering medium, gets time-reversed, and then back-propagated into the same scattering medium. The focusing behaviours i...

A general formulation of a theory of imaging through random obscuring layers is described. Previously we presented a theory for the temporal behavior of a short pulse scattered from a random medium and from a point target. In this paper, we generalize our study to include the imaging of objects of finite size and the actual imaging pattern at the receiver. This involves the study of two-frequency fourth order moments. Numerical examples are given to illustrate several important features, including the optical depth, backscattering enhancement, shower curtain effects, aperture size, bandwidth and target size.

A perfluorinated carboxylic acid membrane, i.e. Flemion, shows improved performance as actuator material compared with Nafion (perfluorinated sulfonic acid). Flemion has a higher ion exchange capacity and good mechanical strength. In particular, Flemion will deform with no back relaxation under applied electrical stimulus. However, with water as solvent, the operation of Flemion in air has serious problems, since water would evaporate quickly in air. Moreover, the electrochemical stability for use in water is around 1 V at room temperature. In previous work, investigations on Nafion with ionic liquid as solvents have been carried out by some researchers and good results have been obtained. In this work, we explore the use of highly stable ionic liquid instead of water as solvent in Flemion. Experimental results indicate that Flemion-based actuators with ionic liquid as solvent have improved stability as compared to the water samples. Although the forces exhibited by Flemion-based actuators with the use of ionic liquid decreased dramatically compared to water, these preliminary results suggest good potential for the use of Flemion with ionic liquid in future applications.

A simple steerable array antenna is designed and developed using a movable dielectric phase shifter. The change of effective dielectric constant at different dielectric slab positions on a coplanar waveguide is used as the phase shifter. The impedance matching and desired phase shift conditions are satisfied at two slab heights, and the reflection is designed to be minimized at these slab positions. The low-loss dielectric material is used as the dielectric slab and is placed close to a coplanar transmission line with airgap. The 4times4 steerable array antenna with the phase shifters is designed and fabricated at 20 GHz. The H-plane radiation patterns are measured at different phase shift positions and compared with the expected results

A steerable antenna using a movable dielectric phase shifter is designed and developed. The change of the effective dielectric constant at different dielectric slab positions on a CPW is used as the phase shifter. The impedance matching and desired phase shift conditions are satisfied at two slab heights. Additionally, the reflection is designed to be minimized at these slab positions and at the designed frequency. A low-loss dielectric material is used as the dielectric slab and is placed close to a coplanar transmission line with an airgap. A tapered 8 × 7 steerable array antenna with phase shifters is designed and fabricated at 24 GHz. The E- and H-plane radiation patterns are measured at different phase shift positions and compared with the expected results. © 2007 Wiley Periodicals, Inc. Microwave Opt Technol Lett 49: 3118–3122, 2007; Published online in Wiley Inter-Science (www.interscience.wiley.com). DOI 10.1002/mop.22906

It has been noted that angular and frequency correlations of the scattered wave from multiple scattering or rough surfaces exhibit the characteristics indicating that the scattered wave remembers the angular and frequency characteristics of incident wave under certain conditions. This is called the “Memory Effects” and was discussed by Feng in 1988. In July 2014 at URSI meeting in Memphis, we presented a preliminary study of the use of Memory Effects to reduce the clutter from rough surface and to improve the resolution of images of objects located near rough surface.

“Through-Wall Radar Imaging” (TWRI) has been studied extensively to reconstruct the image from the backscattered waves through the opaque dielectric wall. When the objects are located in the shadow region of the metallic wall (hard wall) where the waves can only bypass the wall via diffraction, a new problem arises as “Hard-Wall Radar Imaging” (HWRI), which has been proposed and discussed in our previous papers.

Statistical Electromagnetic Theories have been developed over many years and applied to a wide range of practical problems in remote sensing of geophysical media, biological media, medical optics, ultrasound imaging and object detection and communication in clutter. This paper gives a review of recent developments in applications of statistical wave theories. The super resolution of images occurs in random media due to the multiple scattering and the increase of apparent aperture size of the transmitter. Another interesting effect is that the scattered wave from multiple scattering remembers the direction of the incident wave and strong correlations can be observed under certain conditions. This is called the “Memory Effect”. Recent study shows that under certain conditions, the angular and frequency correlations of the scattered wave can be enhanced or reduced and this effect can be used to reduce the clutter from the rough surface. It is also noted that the coherence in multiple scattering causes the increase of radar cross sections in turbulence due to the interference between the forward and backward waves, called the “double passage effect”. This is related to the Anderson localization and the coherent backscattering.

This paper presents a theory of imaging objects behind layers of scattering media. The transmitter is a focused array or an aperture emitting a short pulse. The scattered pulse is received by a focused array or aperture. The received signal consists of two components: the pulse scattered from a random medium and from the target, and these two components can be distinguished by the use of ultra wide band (UWB) pulse. The second moment of the received signal includes the fourth-order moments of stochastic Green's functions, which are reduced to the second moments by the use of the circular complex Gaussian assumption, and of the generalized two-frequency mutual coherence function. This imaging theory is a generalization of optical coherence tomography (OCT), SAR and confocal imaging. It clarifies the relationships among resolution, coherence length, shower curtain effects and backscattering enhancement.

Ionic polymer metal composite (IPMC) is a promising candidate for artificial muscles and other bio-related applications. Compared with the widely reported Nafion, Flemion-based IPMC has some more attractive advantages. To improve its mechanical stability in dry environments, the organic solvent glycerol was applied. The mechanical characteristics such as tip displacement, tip force and durability were measured and analyzed. The results showed that Flemion-based IPMC with glycerol as solvent has optimum mechanical stability in dry conditions and will be a good candidate for artificial muscles working in air.

The active flow control (AFC) technology has been studied and shown that it can help aircraft improve aerodynamic performance and jet noise reduction. AFC can be achieved by a synthetic jet actuator injecting high momentum air into the airflow at the appropriate locations on aircraft wings. To produce strong synthetic jet flow at high frequency, a new membrane actuator based on ferromagnetic shape memory alloy (FSMA) composite and hybrid mechanism was designed and constructed. The hybrid mechanism is the stress-induced martensitic phase transformation caused by large force due to large magnetic field gradient, thus enhancing the displacement, as the stiffness of shape memory alloy reduces due to the martensitic transformation. This sequential event can take place within milliseconds. The high momentum airflow will be produced by the oscillation of the circular FSMA composite diaphragm close to its resonance frequency driven by electromagnets. Due to large force and martensitic transformation on the FSMA composite diaphragm, the membrane actuator that we designed can produce 190 m/s synthetic jets at 220 Hz.

A simple steerable array antenna is designed using a movable dielectric phase shifter. The change of the effective dielectric constant at different dielectric slab positions on a coplanar waveguide is used as the phase shifter. The impedance matching and desired phase shift conditions are satisfied at two slab heights, and reflection is designed to be minimized at all slab positions. A four-element steerable array antenna at 5.8 GHz is fabricated and measured for testing purposes. © 2006 Wiley Periodicals, Inc. Microwave Opt Technol Lett 48: 2222–2227, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.21920

The conductor loading method has been used for creating a lossy dielectric material with a desired loss tangent. This method may also be applicable for developing high dielectric constant materials. In this paper, we will present the experimental results of the dielectric properties of BaTiO/sub 3/ and BaTiO/sub 3/ -platinum (BaTiO/sub 3/-Pt) composites. These composite materials were designed to increase the real part of the effective dielectric constant at high frequency. Three different platinum volume fractions were used, 3, 5 and 10%, to make BaTiO/sub 3/-Pt composites, in addition to a pure BaTiO/sub 3/ material. To characterize the BaTiO/sub 3/-Pt composites, microwave frequency measurements were conducted using the waveguide transmission technique. The experimental and numerical results verify that it is possible to increase the dielectric constant using the conductor loading method.

Optical surface plasmon resonance sensors have been known for a long time. In this paper, we discuss the use of metamaterials to construct a surface plasmon sensor which can be used at microwave frequencies. We review the conditions for the existence of surface plasmon and the use of the forward and backward surface waves. A sharp dip in the reflection coefficient occurs when the propagation constant of the incident wave along the surface is nearly equal to the propagation constant of the plasmon surface wave and may be used to probe bulk material characteristics or to determine metamaterial characteristics. Numerical examples are given to illustrate the basic characteristics.

This paper presents a quasi-static analysis of a metama- terial consisting of a three-dimensional array of small tunable stacked split ring resonators (SSRRs). The resonance frequency of the pro- posed resonator structure can be controlled by adjusting the auxiliary lumped elements which are inserted between the split of each ring. In addition, the size of the ring resonator can be reduced to an order of 0.01λ that is one tenth that of the split ring resonator (SRR) by choosing the proper lumped elements. The analysis is based on the quasi-static Lorentz theory, and the generalized matrix representation of the macroscopic constitutive relations of the composite medium is calculated.

This paper presents a theory of the radar cross section (RCS) of objects in multiple scattering random media. The general formulation includes the fourthorder moments including the correlation between the forward and the backward waves. The fourth moments are reduced to the second-order moments by using the circular complex Gaussian assumption. The stochastic Green’s functions are expressed in parabolic approximation, and the objects are assumed to be large in terms of wavelength; therefore, Kirchhoff approximations are applicable. This theory includes the backscattering enhancement and the shower curtain effects, which are not normally considered in conventional theory. Numerical examples of a conducting object in a random medium characterized by the Gaussian and Henyey–Greenstein phase functions are shown to highlight the difference between the multiple scattering RCS and the conventional RCS in terms of optical depth, medium location and angular dependence. It shows the enhanced backscattering due to multiple scattering and the increased RCS if a random medium is closer to the transmitter.

This paper presents a method of calculating the elements of the generalized matrix representation of the macroscopic constitutive relations for a three-dimensional (3-D) array of non-magnetic inclusions with arbitrary shape. The derivation is based on the quasi-static Lorentz theory and the inclusions are represented by electric and magnetic dipole moments. The 6/spl times/6 constitutive relation matrix is expressed in terms of the interaction matrix and the polarizability matrix, which can be numerically calculated using the sum and the difference of opposing plane wave excitations. Numerical examples are given for split ring resonators and a chiral medium consisting of an array of helices to illustrate the usefulness of the formula and to verify the consistency constraint and reciprocity relations for a bianisotropic medium.

There has been an increasing interest in the use of spaceborne very high frequency ultra high frequency (VHF-UHF) synthetic aperture radar (SAR) for measuring forest biomass and for detecting underground facilities. The propagation characteristics of the low-frequency electromagnetic wave are severely affected by the ionosphere. Recently, Faraday rotation effects and SAR image degradation have been studied using an analytical model and a homogeneous ionosphere. In this paper, a numerical model is developed to investigate the SAR image degradation caused by an inhomogeneous ionosphere. Both horizontal and vertical structures of the ionosphere are considered in this model. Three different cases are studied. The first is a vertically homogenous ionosphere, where the simulation condition is the same as in the analytical study by Ishimaru and others. The second is a vertical profile, which is introduced using the Chapman formula. The ray-bending effect is added for the ionosphere with a layered structure. Finally, both the vertical profile in electron density and the horizontal gradient in total electron content are considered in the simulation. Simulation results show good agreement with the theoretical analysis under the same conditions of the ionosphere. When both horizontal and vertical structures and the inhomogeneity of the ionosphere are considered in the model, the simulation result shows further image degradation and shift caused by the ray-bending effect. The simulation results also show the strong frequency dependence of the SAR image resolution.

The problem of polarized light propagating through scattering media can be explained using the vector radiative transfer equation. This equation is an integro-differential equation and is well-known to be unsolvable analytically. One of the approximate solutions is discrete ordinates method which is based on the discretization of the Stokes parameters and the Mueller matrix. Although it produces accurate results, it requires a lot of computational resources. In addition, there are limitations on the calculation for angles that are very close to the optical axis. The solutions at these angles are necessary for some applications such as atmospheric imaging. First-order scattering approximation has been applied to mitigate the computational resource situation. It can also be used to calculate the solution at the angles that are very close to optical axis. However, it lacks information about the cross-polarization and it is inaccurate when light encounters more scattering events. Second-order scattering approximation provides more accurate solutions and offers some information about cross-polarization. We develop the first-order and second-order scattering approximations and their solutions for the pulse wave case. We investigate the second-order approximation solutions and compare them to the solution from the complete vector radiative transfer equation in several cases.