Active Radar Cross Section Enhancement: Principles, Developments and Applications

Critical analysis of thermodynamic cycle modeling of adsorption cooling systems for light-duty vehicle air conditioning applications

This paper presents the design and simulation of a jumping mechanism as a means of flight initiation in Flapping Wing Micro Air Vehicles (FWMAV) inspired by birds. Most flying birds take off for flight from standing or perching position at low air speed. A lightweight 4-bar leg mechanism has been designed using dimensional synthesis by motion generation. Graphical and analytical methods are established to determine the kinematic dimensions that fulfill some prescribed take-off maneuvers. Link cross sections have been optimized to reduce weight. The jumping leg has been tested with an efficient aerodynamic model that accurately predicts unsteady flapping wing aerodynamic characteristics just after launching. Transition of forces from leg to wings has been analyzed. For a jumping angle of about 50 degrees at 10m/s the 33.3 g ornithopter model gains momentum and reaches steady state flight at a velocity close to the initial value.

: Currently, Unmanned Air Vehicles (UAVs) are too large to penetrate buildings for situational awareness and reconnaissance, emplace important sensors, and sample and return material. While a variety of Micro Air Vehicles have been built and flown that use propellers and flapping wings for lift generation, Nano Air Vehicles (NAVs), defined as weighing less than 10 grams with wingspans less than 7.5 cm, have yet to be flown. NAV-scale actuators and wings with the requisite range of motion, power, and efficiency do not exist. In this project, we used a newly developed micromachining process based on Inductively Coupled Plasma Reactive Ion Etching (ICP-RIE), for PZT chips to fabricate novel, high performance actuators for NAVs. A novel T-beam actuator was fabricated using ICP-RIE etching from the front of a bulk PZT chip. Masked electrode deposition created active and passive regions in the PZT structure. With a T-shaped cross section, and bottom and top flange and web electrodes, a cantilevered beam can bend in-plane and out-of-plane with bimorph actuation in both directions. One of these T-beam actuators was fabricated and experimentally tested. An experimentally validated model predicted that the cross-section geometry can be optimized to produce higher displacement and blocking force.

Abstract: Managing and retrieving event photos efficiently has become increasingly important in today's data-driven world, especially at large-scale gatherings like weddings, corporate functions, and sports meets. Traditional methods of photo retrieval are time-consuming and often frustrating for both attendees and photographers. To overcome these limitations, the Smart Multi Event Image Locator and Retrieval Assistant project introduces an intelligent, face recognition–based web application designed to automate and streamline the image search process. The system enables users to upload or capture their photo to instantly retrieve all matching images from a centralized event database, eliminating the need for manual searching. Leveraging technologies such as Python Flask, React.js, and MySQL, the platform ensures high accuracy, rapid image processing, and intuitive interaction. Core components include the User Retrieval Module, Admin Photo Management Panel, and the Face Recognition Engine, which work in unison to deliver a seamless experience for both end-users and administrators. Built with scalability and usability in mind, the system supports real-time face matching, responsive design for various devices, and secure image handling. This project presents a modern, intelligent, and user-focused solution for efficient event photo management and retrieval, with future potential for expansion into video indexing, cloud integration, and multi-face tagging capabilities. Objectives: To design a web-based application that enables event attendees to retrieve their photos using face recognition by uploading or capturing an image. To build a robust backend that securely stores and manages images using MySQL and performs real-time face matching with high accuracy. To implement an intuitive user interface that simplifies photo search for users while providing photographers with efficient upload and management tools. To ensure seamless integration of frontend and backend components using React.js and Flask, while maintaining scalability, performance, and security. Keywords: Face Recognition, Event Photo Retrieval, Python Flask, React.js, MySQL, Image Matching, Web Application, Photo Management, User Experience, Smart Search, AI-based Retrieval, Face Matching Engine, Event Assistant.

The fact that millimeter-wave radars can be designed to be compact in size and light in weight, while at the same time be broadband and narrow beam systems, have led to a proliferation of these radars over a wide range of applications. These applications include high resolution synthetic aperture radar imaging, target surveillance and tracking, vehicle collision avoidance, autonomous navigation of robotic ground and air vehicles, and assistive landing of airborne vehicles. Recently, there has been an increased interest in use of radar systems operating at frequencies in the upper millimeter-wave and sub-terahertz portion of the spectrum (90 GHz < f < 300 GHz). At these frequencies, wave propagation through the atmosphere is of concern, especially during inclement weather, such as brownouts due to sandstorms, whiteouts due to snow storms, and reduced visibility due to heavy rain. Distortions introduced by rain, in the form of extinction, depolarization, and increased backscatter, are of particular concern due to the fact that water droplets have a higher permittivity and larger size than other airborne particles. The size of a water droplet may extend anywhere between few microns and 8 mm in diameter. For example, at a moderate rain fall rate of 4 mm/hr, the median diameter of water droplets is 1.6 mm, which is larger than the wavelength of a radar operating at 200 GHz (wavelength = 1.5 mm). These relatively large particle sizes, coupled with high permittivity of liquid water, are expected to result in strong scattering at these frequencies. Furthermore, the shape of a falling water droplet depends on its size. When the droplet size is small (i.e. diameter < 1 mm) then its shape is essentially spherical. However, as the droplet size increases beyond 1 mm, its shape becomes increasingly distorted, resembling a deformed oblate spheroid. The radar cross section of large droplets can be quite different compared to spherical particles of the same volume. In addition, the distorted water droplets will result in cross-polarized return of the reflected signal and depolarization of the forwardly-scattered signal as well. Hence, a polarized radar signal propagating through rain may suffer from significant attenuation as well as from polarimetric distortions in the form of depolarization and dispersion. As a result, radar signatures of targets may appear significantly different from reality, and detection features may not work efficiently. It should be noted that the degree of severity of such effects increases with the increase in the rain fall rate. No thorough study of wave propagation through rain has been reported to date at sub-terahertz frequencies.

The rapid advancements in optical communication technologies have highlighted traditional optical components' limitations, particularly in size, adaptability, and integration capabilities, underscoring the need for more compact and versatile solutions. Metalenses offer a promising pathway to address these challenges, with their ability to provide high-functionality, miniaturized optical components. We developed a varifocal metalens with a polarization separation function designed for the wavelength of 1550 nm for potential application for next-generation communication technologies. To integrate the varifocal and polarization separation functions, polarization-dependent phase profiles for an off-axis Alvarez lens were derived and encoded by amorphous silicon pillar meta-atoms with rectangular cross sections to provide independent 0-2π phase delays for both orthogonal linear polarization components. The fabricated metalens achieved a varifocal range of 0.75 mm to 10.65 mm and a polarization extinction ratio of 18.5 dB.

Effects of microfluid in the veins of the deployable hindwings of the Asian ladybeetle on flight performance

This paper reports on the comparison of the radar cross-section (RCS) of a 4-element retrodirective array (RDA) calculated from its measured gain with the predicted RCS of a flat plate, and a 90/spl deg/ dihedral corner reflector. The comparison showed that the boresight RCS of the 4-element RDA is 2.33 dB higher than the boresight RCS of the 90/spl deg/ dihedral passive corner reflector, but 3.66 dB lower than the flat plate at broadside Incidence of the same sizes. In this paper, the different scattering field effects that cause the RCS of the 4-element RDA to be lower that the RCS of the flat plate at angles less than /spl plusmn/10/spl deg/ was explained. In addition, the paper shows for the first time that retrodirective arrays can be made to have better or worse RCS performance than passive reflectors depending on the gain of the antenna elements and the performance of the conjugate circuit.

In recent years, the use of microelectromechanical systems (MEMS) devices has led to high performing actuators for various applications, including unmanned air vehicles (UAVs) for defense applications. The incorporation of MEMS technology in this field has resulted in miniaturized UAVs with the capability of carrying out sophisticated reconnaissance and relaying real time information remotely; however, maneuverability of these devices around obstacles is still a challenge. This paper presents the design and fabrication of a functionally modified bimorph actuator with enhanced UAV aerodynamics and maneuverability. The actuator is a metal-based MEMS device consisting of stainless steel, lead zirconate titanate (PZT), and titanium/platinum electrodes. COMSOL analysis was performed to examine optimal device design parameters and is presented in this paper. The design consists of off-axis PZT segments on a bimorph PZT layer which results in bend twist coupling. A detailed description of the fabrication process flow developed based on the optimization of the device design is also given. MEMS processing technology was incorporated to produce a torsional cantilever beam that produces angular and linear displacement for superior UAV maneuverability and its performance is also presented in this paper.

The increasing complexity of microwave and mm-wave devices and components that are required to meet the demands for 5G/6G communication, biomedical, security, and intelligent wireless systems necessitates the development of new methodologies that combine theoretical knowledge with computational power. Classic electromagnetic simulations, although accurate, are computationally intensive. Therefore, their application to high-scale optimization processes is limited. Exploiting machine learning techniques could provide an attractive alternative by providing fast and efficient design of microwave and mm-wave devices and components. Herein, we provide a thorough review of the research efforts made so far on this topic by presenting the current state-of-the-art techniques, methodologies, and systems, including surrogate modeling approaches, inverse design strategies, physics-aware learning schemes, and issues related to model generalization and reproducibility. Additionally, we address specific challenges and issues. Finally, we discuss emerging directions and future trends.

Liquid Crystal (LC) is an anisotropic liquid material which flows like a liquid, but at the same time its molecules have an orientational order like in the solid state [1]. Thus, LC is a promising dielectric material for designing reconfigurable devices at microwave frequencies. In order to optimize the design of reconfigurable microwave devices, accurate values of the dielectric permittivity and the loss tangent of LCs are needed. However, new LCs are not well characterized at these frequencies because of its recent use for microwave applications. Therefore, the characterization in this frequency range is required for its practical use within microwave components and devices [2]. In this work, the split-cylinder resonator method has been used for the characterization of LCs at two frequency points, i.e. 5 and 11 GHz. The method is based on the measurement of the resonance frequency and quality factor of the two states of the LC molecules for extracting the complex dielectric permittivity [3]. For achieving these two states, no electric or magnetic fields are needed, just the cell must be turned 90º inside the cavity. The dielectric properties (permittivity and loss tangent) of four different LC samples, GT3-23002 from Merck and QYPD193, QYPD142, and QYPD036 from Qingdao QY Liquid Crystal Co, have been obtained. The highest values of the dielectric anisotropy are presented for the samples QYPD036 and QYPD193, together with the highest values of the corresponding loss tangent parameters. Furthermore, it is observed for all the LCs that the loss tangent decreases and the dielectric anisotropy increases at higher frequencies, which must be taken into account in the development of reconfigurable microwave devices.

The far-field radar cross section (RCS) measurement of a large scatterer is generally difficult due to the limitation of available measurement range. One way to avoid this difficulty is to adopt a scale model by compact range measurement. However, there are some problems in this measurement: it is labour intensive and expensive to construct the scale model, and the frequency characteristics of the surface impedance of the scatterer may not be reproduced at shorter wavelength measurements, so real target measurement is necessary to obtain accurate data. There exists a near-field antenna measurement technique, however a direct application of this technique for RCS measurement fails because the RCS depends on the locations of both the source and the observation points. Falconer (1988) has overcome the above difficulty and successfully estimated the far-field RCS by the near-field RCS measurement. We have also proposed a far-field RCS prediction algorithm which uses cylindrical or planar near-field RCS data. However these algorithms are applicable only for the monostatic RCS. We report novel prediction algorithms of the far-field bistatic scattering cross section (SCS) by extending the proposed algorithms. These are presented in cases of spherical, cylindrical and planar scanning. We also show simulation results of the far-field SCS prediction in order to validate our methods.

[1] Two orbital synthetic aperture radars (SARs), the Chandrayaan-1 Mini-SAR (13 cm wavelength) and the Lunar Reconnaissance Orbiter (LRO) Mini-RF (13 and 4.2 cm wavelengths), have been imaging the lunar surface searching for ice deposits in the polar permanently shadowed areas. To understand the radar signatures of lunar polar ices, an empirical two-component model with parametric variations of the specular and diffuse components was developed and validated. This model estimates scattering differences associated with slopes, surface roughness, thin regolith over ice, and patches of ice. Lunar radar backscatter cross sections for the average surface for the Chandrayaan-1 and LRO instruments are estimated from the radar cross sections from the Moon at 3.8, 23, and 68 cm wavelengths measured in the 1960s at the Massachusetts Institute of Technology. This modeling predicts that enhanced diffuse scattering from near-surface ice can be separated from rocks if the scattering is characterized by both the high reflectivity and circular polarization ratios (CPRs) like those observed on Mercury, Mars, and the Galilean satellites. Scattering from near-surface ices covered by a thin regolith can be separated from rocks if the enhancement is twice the average or more. If, however, the lunar ice is dispersed throughout the regolith as ice-filling pores, then scattering differences might be too small to detect. Preliminary validation using LRO radar data for a few polar and midlatitude craters indicate that the observed CPRs are consistent with our models for different regolith ice and roughness conditions.

Electron–ion collision and polarization of X-ray fluorescence radiation under hot quantum plasma conditions

The signature of radar target is an important aspect in military war. The demands for low signature targets have been increased to achieve the ‘critical mission successes’ in defence. Worldwide scientists and engineers are incessant working on it. This paper presents the data analysis of radar absorbing material (RAM) suitable for attaining of low signature applications for small air vehicles such as sUAV, μUAV, drones, hexcopter, spy birds etc. In this paper, the polyethylene (PE) and their composites characteristics and data analysis are presented, for applications in small air vehicles to attain the lower value of their target signatures. This is achieved in terms of EM measurement parameters of RAM material such as reflection loss, transmission loss, absorption loss, shielding effectiveness (SE), radar cross section (RCS) etc. The obtained values of reflection loss are – 29.49 dB, - 36.91 dB and – 23.94 dB; then, the transmission loss are – 133.18 dB, - 128.70 dB and – 0.19 dB for PE, polyethylene terephthalate (PET) and PET + conductive paint (PETCP) respectively for materials by waveguide measurement (WGM) method. The SE range are – 0.009 dB to – 0.19 dB, - 0.005 dB to - 0.29 dB, and - 0.022 dB to – 1.91 dB in WGM; then, - 9.44E-07 dB to – 1.13E-06 dB, - 9.57E-07 dB to – 1.01 E-06 dB and – 9. 31 E-07 dB to – 1.08E-06 dB in OATS experiments for PE, PET and PETCP respectively for the materials are obtained. The lowest value of RCS in OATS method are 1.63E-08 , 6.46E-08 and 5.63E-08 for PE, PET and PETCP respectively for material are obtained. The contact angle &amp; material thickness (t) are 78.50 &amp; 0.08 mm; 66.18 &amp; 0.09 mm and 93.45 &amp; 0.12 mm for PE, PET and PETCP respectively, are proposed in this paper. There is good agreement between the material controlling parameters design and the tabular analysis presented in this paper. This materials are used as RAM in small aircrafts, sUAV, μUAV, drones, hexcopter, spy birds, ships etc, to attain their low signature value, low RCS, stealth and shielding applications in X – band of 8 to 12 GHz frequency range.