Shape optimization for aerodynamic efficiency and low observability

The design of two high-gain Fabry–Perot antennas with wideband low monostatic radar cross section (RCS) is presented in this paper. First, a phase gradient metasurface (PGM) without lossy layer is used to realize in-band RCS reduction and high aperture efficiency, simultaneously. The simulated results show that its peak gain is 19.2 dBi, and a high aperture efficiency of 58.6% is achieved. By utilizing the PGM structure, the monostatic RCS is reduced from 7 to 11 GHz, and the in-band co-polarization RCS is reduced by 13.1 dB. After that, a PGM with metamaterial absorber is designed to broaden the bandwidth of RCS reduction. A prototype antenna is fabricated and measured. The peak gain of 17.9 dBi is obtained, and the monostatic RCS is reduced in a wider frequency band of 7–15 GHz.

The Georgia Tech Research Institute (GTRI), under contract to the U.S. Air Force 46 Test Group, Radar Target ScatteringDivision (RATSCAT), at Holloman AFB, NM, has designed and developed a fttlly polarimetric, bistatic coherent radarmeasurement system (BICOMS). It will be used to measure both the monostatic and bistatic radar cross section (RCS) cftargets, as well as create two-dimensional, extremely high-resolution images of monostatic and bistatic signature data.BICOMS consists of a fixed radar unit (FRU) and a mobile radar unit (MRU), each of which is capable of independent monostatic operation as well as simultaneous coherent monostatic and bistatic operation. The two radar systems are coherently locked via a microwave fiber optic link (FOL). This paper discusses the system design requirements and keysystem features ofthe BICOMS and its major radio frequency (RF) subsystems.Keywords: bistatic, RCS, imaging, polarimetric, low observable. 1. INTRODUCTION AND BACKGROUND The use of low-observable technology has significantly reduced the monostatic RCS of aircraft, making their detection byconventional surveillance systems extremely challenging. The principal signature reduction technique employed in low-observable design is vehicle shaping. This technique relies on redirecting the incident radar energy in a direction away fromthe transmitting source, thereby reducing the monostatic signature ofthe vehicle. The laws of physics maintain that energymust be conserved. Hence, if the monostatic RCS is reduced by shaping, the incident energy must be distributed elsewheresuch that the signature is increased at some or all bistatic angles. It is therefore conceivable that a bistatic radar will havebetter performance than a monostatic radar for detection of low-observable targets where shaping is the principal method usedin RCS reduction.' While bistatic surveillance techniques have been proposed for several decades, only recently has thenecessary technology been available to perform both the difficult phase locking needed between the transmitter and receiverand the large volume of data processing required for the coordinate translation and matching between the transmit and receiveradar beams. These new advances in phase locking and data processing have made the development of bistatic surveillancesystems possible. In addition, a number of radar-directed anti-aircraft missiles in use today operate in a bistatic mode duringsome or all oftheir engagement envelope. Thus, there must be concern for the bistatic signatures of even non-low-observableair vehicles. The Department ofDefense (DoD) has therefore undertaken a program to upgrade and expand its bistatic RCSmeasurement capability at RATSCAT. BICOMS will provide the DoD with the capability to conduct monostatic andbistatic exploitation measurements of threat targets and evaluation of the radar signatures of blue targets.

We present scattering computations performed with a newly developed stable hybridization of the finite element method (FEM) and the finite-difference time-domain (FDTD) scheme, which is based on Nitsche's method. This hybrid has not been tested on scattering problems previously, and here we compute the radar cross-section (RCS) for three different targets: (i) the perfect electric conducting (PEC) sphere, (ii) the NASA almond, and (iii) a generic aircraft called RUND. In order to assess the discretization errors associated with the hybrid, we provide comparisons with established results and techniques: (i) the Mie-series for the PEC sphere, (ii) the method of moments (MoM) implemented in the commercial code FEKO, and (iii) a stable FEM–FDTD hybrid that exploits pyramids and a curl-conforming representation of the electric field. The bistatic RCS for the PEC sphere shows second order convergence towards the analytical solution and a relative error of 2% is achieved for about 20 points per wavelength. The NASA almond has a low monostatic RCS in its horizontal plane, which makes it a good benchmark problem for scattering computations. It features a sharp tip and, as a consequence, the order of convergence for the RCS is lowered. Given a careful convergence study for the NASA almond, we achieve a highly accurate monostatic RCS by means of extrapolation and this result is state-of-the-art in the open literature.

This letter presents a compact dual-polarized over-the-air testing antenna with low radar cross section (RCS) for the fifth-generation and emerging wireless communication systems. The proposed antenna design is achieved by slotting two orthogonal Vivaldi antennas in a cross-shaped form into a two-part three-dimensional printed radome and loading microwave-absorbing materials (MAMs) at the antenna's outer edges. The proposed antenna has a rocket-nest shape with a diameter of 63.4 and 113.68 mm length. The -10 dB impedance bandwidth (S11) is obtained from a range of 2.1 up to 6 GHz. The two-part radome is formed by polylactic acid and acrylonitrile butadiene styrene dielectric materials, respectively. Their respective dielectric constants are approximately 2.6 and 4.5 within the -10 dB impedance bandwidth. The MAMs geometry and distribution location are optimized to suppress induced current over the antenna. An antenna prototype has been fabricated and measured. The measured results agree reasonably well with the simulated results. The results show that the use of the two-part radome enables the achievement of low RCS with compact form factor. Furthermore, a low bistatic RCS of between 2 and 6 GHz is observed in simulation for angular ranges of θ ∈ [-30°, +30°] and Φ ∈ [-90°, +90°].

The reflection variation property of Electromagnetic Band-Gap (EBG) structures can be utilized to reduce the Radar Cross Section (RCS) of ground planes. Combining PEC and EBG structures on the same ground plane forms checkerboard surfaces which can achieve a RCS reduction in a narrow frequency band. However, a wider band RCS reduction can be realized by combining two different EBG structures. It can realize 60% frequency bandwidth for the 10-dB RCS reduction. The scattered fields for square checkerboard surfaces are re-directed toward four quadrants, while the scattering for hexagonal checkerboard surfaces are toward six directions. The maxima of the RCS for checkerboard surfaces in the four quadrants are 10 dB less than those of the PEC ground plane, and more than 19 dB RCS reduction is obtained along the principal planes. Both bistatic and monostatic RCS are compared with those of the equal-sized PEC ground planes.

F-117 Nighthawk Stealth Fighter Jet is considered as the world's first true ‘Stealth’ aircraft and its considerably low Radar Cross Section (RCS) has been measured in this paper. RCS is an important parameter for radar that improvises on the detectability the target accurately. Varying two of the most important parameters of RCS such as frequency of the radar transceiver and aspect angle, the best outputs are recorded as the results. Using COMSOL Multiphysics software a virtual chamber has been created and the CAD model of F-117A is applied in it. With a planar electric field as the incident radar signal, the RCS has been recorded. In this paper, the bi-static RCS calculation method is considered for simulation. The results clearly depicted that better RCS can be obtained at the upper order frequency of S band wave range (above 3.5 GHz) and at some specific aspect angles the results exhibit higher detectability.

In this paper, an integrated structure design is introduced by loading an ultrathin wideband metamaterial absorber to a 2×2 patch antenna array. The radiation character and radar cross section (RCS) property are analyzed using the finite element method. The result shows that there is little difference of the working frequency, matching performance, pattern and gain between the reference antenna and the designed antenna. However, the monostatic RCS has been efficiently reduced in a wide frequency range, with the maximum reduction reaching as much as 13dB.

In this article, a planar wideband circularly polarized (CP) slot antenna array based on metasurface (MS) with low radar cross section (RCS) is proposed. The array is comprised of CP MS‐based antennas fed with a sequentially rotated feeding network. For each CP antenna, the CP radiation is achieved due to polarization conversion of the MS. Simultaneously, the total MS patches of the array can be viewed as checkerboard composed of MS array and mirror array. A phase difference between the MS units and the mirror units ensures RCS reduction. Simulated results demonstrate that about 34.6% operating bandwidth in the frequency of 4.3–6.1 GHz (both S11 –10 dB and AR < 3 dB). In addition, over 5 dB RCS reduction is obtained in the entire operation band under normal incidence with respect to the array without MS for both x‐ and y‐polarizations. The simulated monostatic RCS less than −30.9 dBsm is observed at 5.0 GHz. A prototype of the array is fabricated and measured, which are in good agreement with simulated results.

In this communication, a low radar cross section (RCS) antenna array with reconfigurable scattering patterns based on digital antenna units is proposed. The digital antenna unit has two scattering states that are controlled by p-i-n diodes. For $y$ -polarized normally incident waves, the phases of the two reflected states exhibit a 180° phase difference at approximately 5 GHz. For $x$ -polarized incident waves, the antenna units act as meta-absorbers, and the incident power is absorbed by the antenna units. Moreover, the radiation properties of these two states are approximately the same at 5 GHz. Thus, the scattering patterns of the antenna array can be tuned by controlling the scattering states of the digital antenna units without affecting the radiation performance. By tuning the directions of the scattering pattern nulls to dynamically align to the direction of the incident wave, both the monostatic and bistatic RCS can be significantly reduced. An $8\,\, {\times }\,\,8$ antenna array was fabricated and measured to validate the proposed concept. The measured results are in good agreement with the simulated ones. This indicates that the proposed antenna array is a promising solution to address the challenges of bistatic radar systems.

In this article, a novel design of compact frequency-selective rasorber exhibiting absorption–transmission–absorption characteristics and having high passband selectivity has been proposed. A square-loop resonator with mounted lossy elements serves as the elemental broadband absorber from 4 to 12 GHz. The transmission frequency at 8.4 GHz is realized by printing a Minkowski fractal-shaped resonator with a high-Q factor on the resistive layer, which provides a transmission pole at the frequency corresponding to the passband of a double-cross-slot-shaped bandpass layer. The functioning of the proposed frequency-selective rasorbers (FSR) is explained by a corresponding equivalent circuit model. The proposed FSR exhibits higher selectivity at the passband, thus making it a suitable candidate for shielding the narrowband radiating system. Measurements performed on the fabricated prototype of a 17 × 17 unit cell array provides experimental validation. Further, a low radar cross-section antenna is realized by integrating a patch antenna with the proposed FSR, which achieves an average out-of-band monostatic radar cross-section (RCS) reduction of 11.92 and 5.04 dB in the lower (4–7.5 GHz) and upper (9.2–10.8 GHz) absorption bands, respectively, while maintaining the other antenna parameters. Furthermore, the bistatic total RCS reduction of 77% and 60% are achieved in the lower and upper frequency bands, respectively.

This communication presents a physical optics based method developed by CELAR to calculate radar cross section (RCS) in function of the distance separating the target and the radar system. Initially this method has been developed to calculate RCS in the same geometrical configurations than the measurements performed in the CELAR facilities. So, after a short presentation of RCS measurement facilities of CELAR, this paper will expose the method developed to calculate near field RCS. Comparisons between measurements and calculations will be shown to validate the method. Then, some applications will be given on canonical and complex targets. For flat circular and square plates, particular phenomena will be detailed and some analytical approximated formula will be proposed to express their monostatic specular RCS

The radar corner reflector stands as a typical passive jamming decoy in electronic countermeasures, which presents distinctive Radar Cross Section (RCS) features in monostatic or bistatic radar owing to its particular geometric structure. In this paper, the monostatic and bistatic RCS characteristics of a single octahedral corner reflector and octahedral corner reflector array are analyzed. The impact of different bistatic base angles on the RCS area of the corner reflector is also studied by contrasting the main lobe width and statistical parameters of the RCS, which provides theoretical analysis and data support for the effective identification of targets and corner reflector decoys.

ABSTRACTUnder atmospheric pressure, dielectric barrier discharge (DBD) with a plasma layer is utilized to produce large-volume plasma without a vessel. When a 16-kV bias at 1 kHz is applied to 85 copper strips, the DBD actuator generates plasma with the dimensions 170 × 170 × 1 mm. When the incident wave with an electric wave vector perpendicular to the strip array is illuminated, the monostatic radar cross section (RCS) is reduced by a maximum of 3.5 dB. The measured results are validated using full-wave simulation based on homogeneous plasma, with estimated plasma parameters. By changing the number of copper strips connected to the feed line, the relative volume of plasma can be changed. If the relative volume decreases by 20%, the maximum RCS reduction is reduced by nearly 20%. This demonstrates the possibility of using plasma to control monostatic RCS in a wide bandwidth.

This communication presents an approach to design a beam-steerable dipole array with low radar cross section (RCS). A 2-D periodical array of dipole antennas is colocated with absorptive frequency-selective reflection (AFSR) structures to effectively reduce the out-of-band RCS and maintain high gain at the in-band frequencies. The AFSR structures consist of 3-D bandstop frequency-selective structure (FSS) and pyramidal-shaped absorbers to provide a reflection band for the dipole array with enhanced front-to-back ratio. Compared with the reference array, the radiation performance of the proposed array is maintained with an aperture efficiency of 70% and a measured peak gain of 21.5 dBi. The beam-steering angle is up to ±45°. Meanwhile, more than 10 dB two-sided out-of-band RCS reduction is achieved with a bandwidth of 96.5% and 30% for copolarization and 142% for cross polarization, and the in-band RCS is unchanged.

Detecting and tracking small drones is significant for security requirements, but is challenging since small drones typically have a low radar cross section (RCS), small size, and hovering ability. Polarimetric analysis of returned radio wave is of great significance in detection and track of small drones. In this letter, the polarimetric analysis based on RCS and micro-Doppler signature of small drone rotor blade is first studied in VHF/UHF band. Both the simulated and measured RCS results confirm that the horizontal–horizontal polarization is preferred for drone detection in low-altitude angles at this frequency band. Moreover, the method of moments, a rigorous numerical method based on integral equation, is implemented to simulate the micro-Doppler signature generated by the rotating blade in VHF/UHF band. The results show that it is feasible to obtain the rotating speed from the micro-Doppler measurement, which is also verified by the experiment.