Radar Cross Section Knott.pdf

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Radar cross section (RCS) is a comparison of two radar signal strengths. One is the strength of the radar beam sweeping over a target, the other is the strength of the reflected echo sensed by the receiver. This book shows how the RCS gauge can be predicted for theoretical objects and how it can be measured for real targets. Predicting RCS is not easy, even for simple objects like spheres or cylinders, but this book explains the two exact forms of theory so well that even a novice will understand enough to make close predictions. Weapons systems developers are keenly interested in reducing the RCS of their platforms. The two most practical ways to reduce RCS are shaping and absorption. This book explains both in great detail, especially in the design, evaluation, and selection of radar absorbers. There is also great detail on the design and employment of indoor and outdoor test ranges for scale models or for full-scale targets (such as aircraft). In essence, this book covers everything you need to know about RCS, from what it is, how to predict and measure, and how to test targets (indoors and out), and how to beat it.

Michael T. Tulley earned his MS in Electrical Engineering from the Georgia Institute of Technology in 1972. He spend 26 years at Georgia Tech conducting research in RCS, RCSR and radar system performance. In 1998, he joined the Institute for Defense Agencies and the Joint Commands. In 1997, he was elected a Fellow of the IEEE for his contributions to cross section technology.

Abstract:Radar cross section near-field to far-field transformation (NFFFT) is a well-established methodology. Due to the testing range constraints, the measured data are mostly near-field. Existing methods employ electromagnetic theory to transform near-field data into the far-field radar cross section, which is time-consuming in data processing. This paper proposes a flexible framework, named Neural Networks Near-Field to Far-Filed Transformation (NN-NFFFT). Unlike the conventional fixed-parameter model, the near-field RCS to far-field RCS transformation process is viewed as a nonlinear regression problem that can be solved by our fast and flexible neural network. The framework includes three stages: Near-Field and Far-field dataset generation, regression estimator training, and far-field data prediction. In our framework, the Radar cross section prior information is incorporated in the Near-Field and Far-field dataset generated by a group of point-scattering targets. A lightweight neural network is then used as a regression estimator to predict the far-field RCS from the near-field RCS observation. For the target with a small RCS, the proposed method also has less data acquisition time. Numerical examples and extensive experiments demonstrate that the proposed method can take less processing time to achieve comparable accuracy. Besides, the proposed framework can employ prior information about the real scenario to improve performance further.Keywords: near-field to far-field transformation (NFFFT); neural network; nonlinear regression; radar cross section (RCS) measurement; regression analysis

Conventional metasurface absorbers rely on high dissipation losses by incorporating lossy materials. In this paper, we propose a novel mechanism of absorption based on phase cancellation of polarization states of scattered fields emerging from adjacent L-shaped chiral meta-atoms (unit cells). A linearly polarized wave forms helicoidal currents in each meta-atom leading to diagonally polarized radiated waves. When phase cancellation is employed by reorienting four such meta-atoms in a supercell configuration, contra-directed chiral currents flow in adjacent cells to cancel all the radiated fields in far-field region leading to a minimal broadside radar cross-section. From the reciprocity, the currents that are induced in the meta-atoms produce a null towards the incident direction which can be utilized for infrared energy harvesting. Full wave electromagnetic simulation indicates near perfect resonant absorption around 52.2 THz frequency. Enhanced bandwidth is shown by adding smaller resonators inside the supercell in nested form leading to dual band absorption at 45.2 THz and 53.15 THz.

Conceptual demonstration of the RCS reduction (a) L-shaped Unit cell (meta-atom) of the chiral metasurface that resonates at 52.1 THz showing the dimensions and the x-polarized incident electromagnetic field (b) The phase-compensated supercell obtained by four folded L-shaped meta-atoms. (c) Normalized surface charge distribution on single-L unit cell at 52.1 THz. (d) Surface charge distribution on the phase-compensated unit cell showing the oppositely charged dipoles for L elements diametrically opposite to each other causing destructive interference in far-field. (e) The radiation pattern of the single-L unit cell at 52.1 THz showing a large backscatter of incident fields. (f) The radiation pattern for the phase-compensated supercell consisting of four L-shaped elements showing a dramatic reduction of the RCS in direction of incident fields at resonance frequency of 52.1 THz. (g) Spectrum of the monostatic backscattering radar cross section (RCS) of the two structures showing a null for phase-compensated geometry. Illustrations were created in Microsoft Powerpoint 365 [ -ww/microsoft-365/powerpoint].

In this paper, we proposed an infrared absorbing metasurface which is designed by exploiting two phenomenon as discussed above, i.e. the partial suppression of orthogonal electric field vector associated with the chiral metasurfaces43,44,45 and the phase manipulation based on the plane wave expansion46,48,51 (see Fig. 1). In particular, we tailor the phase response of a metasurface by arranging four chiral L-shaped elements in specific orientations to form a super unit-cell (more commonly termed as a supercell). This phase compensating arrangement creates a null in the broadside direction of the metasurface leading to suppression of the radar cross-section (RCS). Since the set-up is used in reflection mode, the reciprocity theorem forces the incident electromagnetic wave to be completely absorbed in the metasurface. The proposed metasurface exploits chirality in conjunction with phase manipulation and reciprocity. Therefore, the phase cancellation takes place on the unit cell basis and without the requirement of an extra absorbing layer as in54 which employs magnetic absorbing materials inside a ultrathin metasurface. With our approach, contra-directional chiral currents flow in adjacent unit cells that completely suppress the RCS with minimum dissipative losses. In another contemporary work55, the RCS reduction was accomplished by phase mixing from several two-layer super lattices with randomized arrangement. Hence the phase cancellation is a result of superposition of radiations from supercells arrays of different unit cell configurations sharing the same plane. These metasurfaces require more complex fabrication process and have larger apertures.

where R is the distance of the far-field point from the target at which RCS is calculated, \(|E_{s}|\) is the normalized back scattered far field and \(|E_{i}|\) is the normalized incident field. The monostatic RCS in Fig. 1g is found using back scattered far field \(|E_{s}|\) fields for the four-fold and single L-shaped designs. As depicted in Fig. 1g, a minima at 52.1 in the RCS curve of the phase-compensated supercell corroborates the null observed in the radiation pattern. Although, the supercell has almost four times physical cross section compared to the single-L configuration, the corresponding monostatic RCS is suppressed to 0.13 \(\upmu \text{m}^2\) which is about 50 times reduction from its value of 6.3 \(\upmu \text{m}^2\) for the single-L unit cell at the same frequency.

By assuming x-polarized electromagnetic wave illuminating the metasurface, the resulting co- and cross polarized reflection coefficients are calculated and are displayed in Fig. 3, along with the absorption coefficient A. It is interesting to note that the x-polarized incident field can be partially transformed into the y-directed reflected fields by the localized surface plasmon resonance current. This is possible due to the Fabry Perot resonances that arise due to interference originating from multiple reflections between metasurface and ground plane. As a result anti-clockwise rotational currents are excited in the silver L-shape nanoparticle (see the inset of Fig. 3) at the designed resonance of 54.2 THz. The resonant orthogonal reflected field components have equal magnitude and are perfectly in-phase i.e., \(\angle R_{yx}\) = \(\angle R_{xx}\) = \(-\pi \). Consequently, the in phase localized surface plasmon currents lead to linearly polarized reflected fields along diagonal axis along 135Â°, also shown in the inset of Figure 3. Looking at the absorption curve, it can be observed that the orthogonal polarization conversion leads to almost 25% absorption of the incident power which may be attributed to the non-radiative losses inside silver nanostructure. It should be noted that since the metasurface is diagonally symmetric, identical reflection response for the y-polarized incident waves should be expected i.e., \(R_xx =R_yy \) and \(R_yx =R_xy \). The L-shaped periodic elements can be reoriented to suppress overall scattered fields components from the metasurface as shown in the following section.

We propose a phase cancellation approach to reduce the radar cross section (RCS) of infrared chiral metasurfaces so that they behave as highly absorbing energy harvesters. The absorber design is based on the L-shape silver resonating nanostructure which supports localized surface plasmon current in lower infrared frequencies of 40 to 60 THz range. With a single L-shape unit cell, the metasurface is optically active and converts a x- or y-polarized incident wave to a diagonally polarized wave. The phase-compensated metasurface is designed by arranging four L-shape elements in a four-fold rotational symmetry to form a supercell. The incident x-polarized wave induces surface plasmon currents which form contra-directional rotational patterns cancelling the effect of each other. Consequently, two reflected waves having diagonally opposite polarizations are radiated which destructively interfere in the far field causing a significant RCS reduction leading to perfect absorption of incident electromagnetic radiation around 52.2 THz frequency. The rotational currents formed on the plasmonic unit cells can be rectified and utilize in energy harvesting. The absorption efficiency can be considerably improved adding more resonators in the supercell. It is shown that by nesting eight resonators in a supercell, a dual band metasurface absorber can be obtained which supports absorption bands at 45.2 THz and 53.15 THz frequencies. The proposed metasurface is based on destructive far field interference and hence does not require lossy materials to completely absorb the electromagnetic wave. Hence the phase compensation method offers efficient infrared energy harvesting at low cost while supporting minimum energy loss. Since the propose metasurface is optically active, it can demodulate several combinations of polarizations leading to higher efficiency compared to singly polarized structures. 2b1af7f3a8