computation Article Computation and Experiment on Linearly and Circularly Polarized Electromagnetic Wave Backscattering by Corner Reflectors in an Anechoic Chamber Mohammad Nasucha 1,2,*, Josaphat T. Sri Sumantyo 1, Cahya E. Santosa 1, Peberlin Sitompul 1, Agus H. Wahyudi 1, Yang Yu 1 and Joko Widodo 1 1 Center for Environmental Remote Sensing, Graduate School of Science and Engineering, Chiba University, Chiba 263-8522, Japan; [email protected] (J.T.S.S.); [email protected] (C.E.S.); [email protected] (P.S.); [email protected] (A.H.W.); aff[email protected] (Y.Y.); [email protected] (J.W.) 2 Department of Informatics, Universitas Pembangunan Jaya, Tangerang Selatan 15413, Indonesia * Correspondence: [email protected]; Tel.: +81-070-4404-4451 Received: 22 August 2019; Accepted: 21 September 2019; Published: 24 September 2019 Abstract: Electromagnetic wave backscattering by corner reflectors in an anechoic chamber is studied using our developed computational tool. The tool applies the Finite-Difference Time-Domain (FDTD) method to simulate the propagation of the wave’s electric and magnetic fields. Experimental measurement in an anechoic chamber is also carried out as a comparison. The two results show agreement, including the finding that the backscatter intensity variation amongst the four circularly polarized modes is significantly smaller than the variation amongst the four linearly polarization modes. Keywords: computation in radar; computation in synthetic aperture radar; backscattering by corner reflectors; simulation on circularly polarized electromagnetic waves 1. Introduction Understanding electromagnetic (EM) wave backscattering by corner reflectors is of interest to radar researchers. In particular, the backscattering of circularly polarized (CP) EM waves is of great interest. To study these CP EM waves, computations can be useful. The articles [1–9] report previously done computations on EM wave distribution in an anechoic chamber, but not in the context of backscattering by corner reflectors or in the context of CP EM waves. These works contributed to the development and performance improvement of the anechoic chamber. The articles [10–16] report computations and analyses on EM wave backscattering by corner reflectors. However, most of the reported computations for EM waves were done for non-polarized and linearly polarized waves. Until today, only [10] reports a computation on circularly polarized EM waves. It used the Method of Moments, and the report did not present any technical details about how the circular polarization was realized. To respond to this situation, our research aims to study circularly polarized EM wave backscattering by corner reflectors in an anechoic chamber through a computation using the Finite-Difference Time-Domain (FDTD) method. There has been no such work reported thus far. As an addition, we did the same computation for linearly polarized (LP) EM waves. An equivalent experiment has also been carried out to obtain a comparison. Computation 2019, 7, 55; doi:10.3390/computation7040055 www.mdpi.com/journal/computation Computation 2019, 7, 55 2 of 17 Computation 2019, 7, x FOR PEER REVIEW 2 of 16 The simulation was done using our developed computational tool, called Anechoic Chamber SimulationThe simulation Tool Version was 1 (AST_V1).done using Both our thedeveloped simulation computational and the experiment tool, called were doneAnechoic for four Chamber target types,Simulation i.e., triangular Tool Version trihedral, 1 (AST_V1). horizontal Both plate the bar,simulation vertical and plate the bar, experiment and sphere, were and done for all for linear four polarizationtarget types, and i.e., alltriangular circular polarizationtrihedral, horizontal modes. plate bar, vertical plate bar, and sphere, and for all linearThe polarization remainder and ofthis all circular articleis polarization arranged as modes. follows: Section2 addresses the method, Section3 reportsThe the remainder results, Section of this4 discussesarticle is arranged the results, as and follows: Section Section5 summarizes 2 addresses the contentthe method, of all sections.Section 3 reports the results, Section 4 discusses the results, and Section 5 summarizes the content of all 2.sections. Method 2.1.2. Method The Measurement Plan The measurement plan is depicted in Figure1. This was the scenario to be followed by both the computation2.1. The Measurement and the experiment.Plan It can be described as follows: The measurement plan is depicted in Figure 1. This was the scenario to be followed by both the i. The cuboid anechoic chamber size shall be 6.5 (length) 4.0 (width) 2.4 m (height). computation and the experiment. It can be described as follows:× × ii. The distance between the transmitter (TX) and the receiver (RX) antennas shall be 0.8 m. i. The cuboid anechoic chamber size shall be 6.5 (length) × 4.0 (width) × 2.4 m (height). iii.ii. TheThe distance distance between between the the transmitter target (corner (TX) and reflector) the receiver and antennas (RX) antennas in the orthogonalshall be 0.8 m. direction iii. Theshall distance be 3.0 between m. the target (corner reflector) and antennas in the orthogonal direction shall iv. beThe 3.0 vectorm. network analyzer (VNA)’s port 1 shall feed signals to the TX antenna. v.iv. TheThe vector RX antenna network shall analyzer feed signals (VNA)’s to port the VNA’s 1 shall portfeed 2.signals to the TX antenna. vi.v. TheThe RX turntable antenna shallshall befeed controlled signals to by the the VNA’s turntable port controller. 2. vii.vi. TheBoth turntable the VNA shall and be the controlled turntable by controller the turntable can be controller. driven by the Personal Computer (PC). viii.vii. BothPhase the shiftersVNA and shall the be turntable provided controller for the TX can and be RX driven antennas by the in Personal the case of Computer circular polarization (PC). viii. Phaseand shallshifters be installedshall be provided correctly for eachthe TX mode, and RX i.e., antennas RX right-handed in the case TX of right-handed circular polarization (RR), RX andleft-handed shall be installed TX right-handed correctly for (LR), each RX mode, left-handed i.e., RX TX right-handed right-handed TX (LR), right-handed or RX left-handed (RR), RX left-handedTX left-handed TX right-handed (LL). Phase (LR), shifters RX will left-handed not be needed TX right-handed for linear polarization (LR), or RX modes,left-handed i.e., RXTX left-handedhorizontal TX(LL). horizontal Phase shifters (HH), RXwill vertical not be TX needed horizontal for (VH),linear RX polarization horizontal TXmodes, vertical i.e., (HV), RX horizontaland RX vertical TX horizontal TX vertical (HH), (VV). RX vertical TX horizontal (VH), RX horizontal TX vertical (HV), ix. andThe RX transmit vertical power TX vertical shall be(VV). 5 dBm for all measurement cases. x.ix. TheThe transmit target shallpower be shall placed be on5 dBm a turntable for all measurement in such a way cases. so that elevation angle θ = 0◦, and be x. Therotated target in shall such be a way placed so that on thea turntable aspect angle in suchφ changes a way fromso that90 elevationto 90 (negative angle θ and= 0°, positive and be − ◦ ◦ rotatedsigns respectivelyin such a way represent so that the the aspect left and angle the rightϕ changes sides from of the –90° target) to 90° with (negative 5◦ increments. and positive xi. signsThe respectively EM wave frequency represent shall the beleft chosen and the in ri suchght sides a way of so the that target) 5λ < withtarget 5° size increments.< 10λ. xi. The EM wave frequency shall be chosen in such a way so that 5λ < target size < 10λ. FigureFigure 1.1. TheThe measurementmeasurement plan. plan. The The pyramidal pyramidal absorbers absorbers shall shall be be placed placed evenly evenly at at the the inner inner sides sides of allof all six six chamber’s chamber’s walls. walls. Definitions of the elevation angle θ and the aspect angle ϕ are explained in Figure 2. The targets were corner reflectors of four different shapes: Trihedral, horizontal bar, vertical bar, and sphere. To make it realistic for practices, every target was chosen to be large enough compared to the wavelength Computation 2019, 7, 55 3 of 17 Definitions of the elevation angle θ and the aspect angle φ are explained in Figure2. The targets Computationwere corner 2019 reflectors, 7, x FOR PEER of four REVIEW diff erent shapes: Trihedral, horizontal bar, vertical bar, and sphere.3 of 16 To make it realistic for practices, every target was chosen to be large enough compared to the wavelength but not too large to be carried in a radar mission. Here Here we we use use the the term term “med “mediumium size” to specify that 5λ < targettarget size size < 1010λλ.. The The targets targets and and their their descriptions descriptions are are listed listed in in Table Table 11.. TheThe horizontalhorizontal andand thethe vertical plate bars are simply the same object, only having didifferentfferent standingstanding orientations.orientations. Figure 2. Definitions of angles used in this work. The elevation angle θ is the angle between the target’s Figure 2. Definitions of angles used in this work. The elevation angle θ is the angle between the facing direction and the horizontal plane. The aspect angle φ is the electromagnetic (EM) wave incident target’s facing direction and the horizontal plane. The aspect angle ϕ is the electromagnetic (EM) wave angle in the azimuth point of view (in other words, the angle between the target’s facing direction and incident angle in the azimuth point of view (in other words, the angle between the target’s facing the vertical plane). Every measurement case in this study shall be done with θ = 0 and –90 φ 90 . direction and the vertical plane). Every measurement case in this study shall be done◦ with◦ θ≤ = 0°≤ and◦ –90° ≤ ϕ ≤ 90°.