An Ultra Wide Band Radar Absorber Based on Square Patch Resistive FSS

Chandrika Sudhendra1, Madhu AR1, ACR Pillai1, KARK Rao2 and T.S. Rukmini3

1AR Division, Suranjan Das Road, C.V. Raman Nagar, ADE, Bangalore

2Professor, PES College of Engineering, Mandya, Karnataka

3Professor, NMIT, Bangalore.

chansudhtumkur@gmail.com

Abstract: An ultra wide band (UWB) panel radar absorber (RA) based on square patch resistive frequency selective surfaces (FSS) is presented in this paper. The UWB panel RA is designed to realize an experimental radar cross section reduction (RCSR) of 10 dB (minimum) from 3 GHz. to 20GHz. The dielectric profile of UWB panel RA is designed as a four layer sandwich construction with thickness constraints and modeled as a transmission line equivalent circuit and simulated using HFSS. The crucial four resistive FSS layers each are developed as electrically thin PCBs (thickness = 0.2 mm) using conventional photolithographic technology and the panel RA is tested for its RCSR performance in the microwave anechoic chamber. The size of panel RA is 280 mm x 280 mm and the thickness is 14 mm. and the weight is 300 gm. The experimental and simulation results agree and are encouraging.

Key Words: Frequency selective surfaces, circuit analog radar absorbers, radar cross section reduction.

I. INTRODUCTIONRadar Absorber (RA) design and implementation assumes

strategic importance in the realm of stealth driven design of aircrafts and unmanned air vehicles (UAVs). The reduced skin reflection from conducting surfaces of air-vehicles translates to radar cross section reduction (RCSR) which results in reduced detectability by the enemy radar. RA designs have to address the ever increasing need for wider and wider absorption bandwidths coupled with constraints on weight and thickness. A paradigm shift in the design philosophy has emerged from realizing RAs as parasitic, add-on elements of an airframe to radar absorbing structures – (RAS), where the RAs are designed as multifunctional structures with primary RCSR characteristics along with mechanical rigidity and robustness of a multifunctional structure.

In a rigorous pursuit for the most optimized design of RAs with wide bandwidths, dielectric RAs have been vigorously explored for realizing the desired performance. The conventional Salisbury screen [1,2] comprises of ideally a 377 /square, infinitesimally thin resistive sheet, the spacecloth[3],

spaced from the conducting backplane whose RCS needs to be reduced, by a quarter wavelength thick dielectric spacer. Even in the most conventional Salisbury screen RA design, an accurate electromagnetic (EM) design of spacecloth is not available in open literature. This limitation in accurate design of spacecloth has been successfully addressed in our earlier paper [4], where novel chip resistor grid network on electrically thin RF substrates were conceptualized, developed and assembled as Salisbury screen RA. Multilayer Salisbury screens with multiple spacecloth layers known as Jaumann RA[5] have been reported for wide band RCSR performance. In a recent paper [6], a wide band three layer panel Jaumann RA with spacecloths based on innovative embedded passives resistor grid networks with experimental RCSR of 15 dB(minimum) from 2 GHz. to 14 GHz., for circular polarization is reported by the authors. However, a limitation in JA design is the multiple quarter wavelength thickness. To overcome the thickness limitation and realize wider bandwidths, circuit analog RA designs are reported in open literature, where the resistive spacecloth layers of JA are replaced by frequency selective surfaces (FSS). But the challenge in these RA designs is the realization of resistive FSS as pure FSS can only be used to derive various spatial filtering functions and does not absorb electromagnetic energy. In a recent paper [7]a three layer circuit analog RA based on fractal FSS and embedded passives resistors is reported by the authors with reduced thickness, for circular polarization.

In this paper, a three layer UWB panel RA with experimental RCSR of 15 dB (typical) is presented. In the following sections, the EM design and modeling of RA and full-wave analysis using HFSS™ is described. Next, the fabrication of FSS layers and assembly of RA is presented and RCS measurements are carried out on RA to verify the design and simulation.

II. UWB RADAR ABSORBER DESIGN AND SIMULATION

For a dielectric RAM such as described in this paper, which is broadband and non-magnetic, minimum thickness constraint is given by Rozanov [8] as

λmax Γ 0≤172 d (1)

Where,max is the wavelength at the lowest frequency, 0is the reflection coefficient in dB and d is the total thickness of RA. Hence, for desired RCSR and thickness, the lowest absorption frequency is constrained to a theoretical limit. Accordingly, the least thickness of a -10 dB wide-band dielectric RA such as proposed in this paper cannot be less than (1/17.2) of the largest operating wavelength (at 3 GHz. ) which is calculated to be 5.81 mm. The total thickness of RAM proposed in this paper is 14 mm. and hence does not violate the fundamental design rules given in [8].

The schematic of UWB panel RA is shown in figure 1(a) and the dielectric profile is given in figure 1 (b). The RA comprises of four FSS layers each comprising resistive square FSS patches. The FSS layers are each backed by dielectric spacers of four different thicknesses which are optimized for realizing the desired performance with thickness constraints. The transmission line equivalent circuit of RA is given in figure 2(a). Each resistive FSS layer is modeled as a series RC circuit in shunt with the short circuited transmission line. The resistive FSS layer has surface impedance given by:

ZFSS=R S + 1jωC S

(2)

Where, Rs is the surface resistivity and Cs is the capacitance of the FSS layer. The reflection coefficient in dB is given by

Γ=20 log10( ρ) (3)

Where, is the free-space reflection coefficient of RA.

Figure 1(a). Schematic of four layer UWB panel RA. Figure 1(b) Dielectric profile of UWB panel RA.

Figure 2(a). Transmission line equivalent circuit of UWB panel RA. Figure 2(b). Unit cell geometry model of UWB panel RA in HFSS.De-embedding of port also shown.

The dielectric spacers are modeled as transmission lines of various lengths represented by d1 to d4. The design is simulated using HFSS simulation software. Using Floquet’s theorem for

periodic surfaces such as resistive FSS described in this paper, the entire panel RA is analyzed by using a single cell geometry model in HFSS. The unit cell geometry model of UWB panel RA is shown in figure 2(b). The full design details of RA are given in table 1.

Table 1. Design details of 4 layer UWB RAM.patch length (mm)

Spacing between patches(mm)

Foam thickness

( (mm)Layer1 6.6 0.2 D1= 3.5Layer2 6.28 0.52 D2= 3.0Layer3 2.8 0.6 D3= 3.2Layer4 1.0 0.7 D4= 3.5

The optimized simulation plots of UWB panel RA is shown in figure 3. The resistive FSS layers are numbered in increasing order from the conducting backplane. A commercially available resistive sheet with surface resistivity of 100 Ohms/square is used for etching the resistive FSS layers. RF transparent, Rohacel™ foam with r= 1.03 and tanö= 0.0003 is used as the dielectric spacer. The size of the unit cell used for simulation is 6.8 mm. The total thickness of RA is 14 mm.

Figure 3. Optimized normal incidence simulation performance of UWB panel RA for TE and TM incidences.

FR4 substrates of thickness = 0.2 mm with r=4.4 and tanö= 0.02 is used in simulation and subsequent fabrication of FSS layers. From figure 3, it is observed that the design can be used to realize circular polarization from RA, as the RCSR performance of RA for both TE and TM incidences meet the desired specifications, for normal incidence. Extensive parametric studies are carried out in HFSS to assess the effect of various design parameters on the performance such as Angle Of Incidence (AOI), thickness variations of foam due to fabrication tolerances, effect of superstrate, variation in dielectric constant and loss tangent of the substrate etc. Some representative plots only are given in the paper but results are discussed in a later section. To study the effects of variation in AOI, the AOI is varied from 0 to 40 degrees and the simulation plot is given in figure 4, for TE incidence. The RCSR performance of RA for AOI variation, for TM incidence is shown in figure 5.

III. RA DEVELOPMENT AND RCS MEASUREMENTS ON UWB PANEL RA

The resistive FSS layers are designed as electrically thin PCBs using FR4 dielectric substrate of thickness 0.2 mms, using the PCB design software Visula v. 2.3. Conventional PCB fabrication technology is used for fabrication of resistive FSS PCBs. The four FSS PCB layers are bonded to the corresponding Rohacel foam dielectric spacers using a double sided Fixontape. Thus bonded four FSS backed foam layers in turn are bonded to the conducting back plane comprising of an EM conducting, tin plated copper foil available in (1ft. x 1ft.) size rolls. A photograph of the assembled panel RA is shown in figure 6.

Monostatic RCS experiments are carried out on the UWB panel RA in the microwave anechoic chamber to verify the design and simulation. The panel RAM is securely placed on an RF transparent thermocol stand on a single axis positioner and continuously rotated in azimuth from 0 to 360.

Figure 4. RA performance for variation in AOI from 0 to 40 degrees; TE incidence.

Figure 5. RA performance for variation in AOI from 0 to 30 degrees; TM incidence.

The conducting backplane serves as reference with which the RCS readings from RA side are compared. High directivity standard gain horn antennas are used for transmission and reception. Continuously variable phase shifter and attenuator are used in the two coupled ports of directional couplers connected to the transmitting and receiving horn antennas. Vectorial cancellation of the background at each measurement frequency is carried out to ensure accurate measurements. RCSR readings are taken with a frequency step size of 500 MHz. from 4 to 12 GHz. Figure 8 shows a representative RCSR plot of panel RA at 8.5 GHz. Experimental RCSR plots for panel RA are carried out for all other mentioned frequencies in C and X bands and are available.

Figure 6. Photograph of UWB panel RA. Size of panel RA = (280 mm x 280 mm). Thickness = 14 mm; Weight = 300 gm.Inset:zoomed cells for clarity.

Figure 7. Representative RCS measurement plot of UWB panel RA. Frequency: 8.5 GHz. Polarization: Vertical; Measured RCSR = 24 dB.

IV. DISCUSSION OF RESULTS i. A Radar absorber design and development for UWB

RCSR performance from 3 to 20 GHz., for circular polarization is presented in this paper. RCS measurements on panel RA have been completed in C and X bands. RCSR of 15 dB (minimum) from 6 to 12 GHz. has been measured and the results are available. The thickness of RA is 14 mm. and the weight is 300 gm. The relative RCSR bandwidth given by (fH/fL= 6.66), where fH = 20 GHz. and fL = 3 GHz. Reference is made to figures 4 and 5, which show the simulated RCSR plots for TE and TM incidence, with AOI variation from 0 to 30 degrees. From the figures it is noted that the panel RA design can be used to realize RCSR performance for AOI varying from 0 to 30 degrees, for both TE and TM incidence.

ii. It is noted that the square patch FSS based design is based on the capacitive method of design of wide band RAs as described in [9]. But the papers are bereft of any implementation details of wide band RA. It is observed that accurate and reliable translation of design to RA hardware is crucial in not only reducing the development cycle but also helps in establishing the processes required for realizing a flight worthy hardware.

iii. Reference is made to figure 7, which gives a representative RCS plot of RA. From RCS measurements on panel RA, it is observed that measured RCSR and simulated RCSR readings generally agree to the tune of +/- 1 dB, in both C and X bands. This is attributed to fabrication and assembly tolerances of the laboratory model of panel RA. It is observed that the realized experimental RCSR at band edge frequencies is lesser than the simulated RCSR by 2 dB.

iv. The thickness of wide band panel RA is 14 mm, with UWB RCSR from 3 to 20 GHz. This thickness qualifies the RA to be used for air vehicle stealth applications, especially for RCSR of wing leading edges. Also, for airborne applications, a carbon fiber reinforced plastic (CFRP) can be used as conducting backplane of RA as it satisfies both EM and structural requirements.

V. CONCLUSION The radar absorber described in this paper with its RCSR of

15 dBsm (typical) from 3 to 20 GHz. finds applications in airborne stealth with low weight due to the sandwich design and construction. The design can be easily up-scaled to design a flight-worthy sub-system with primary RCSR properties along with good structural characteristics.

ACKNOWLEDGMENTThe authors are indebted to Shri. P Srikumar, Outstanding

Scientist and Director, ADE for his guidance, support and according permission for presentation of paper in the conference. We record our grateful thanks to Shri KG Ramamanohar, Group Director, ADE for his support and guidance. Thanks are due to Dr. V.Ramachandra, Scientist G

and Head, FTTT division and Mr. Diptiman Biswas, Scientist E, for RCS measurements and Ms. Nagarathna R, TO A, for PCB layout design and Mr. Mahalingam, Scientist F OIC, PCB and EMI/EMC facility and his team for speedy and accurate fabrication of FSS PCBs.

REFERENCES

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3 Abouzhara, “Automated Wide-Band Surface Resistivity Measurements of Resistive Sheets”, IEEE Trans. on Instmn. and Mea., vol. IM 36, pp. 1031-1035, 1987.

4. Chandrika Sudhendra, Vibhor Mahule, ACR Pillai and Atanu Mohanty, “A Novel Space Cloth Using Resistor Grid Network for Radar Absorbers in Stealth Applications”, pp. 83-86,Proc. ICCSP 2011.

5 Munk B, Munk P, Prior J (2007) On designing Jaumann and circuit analog absorbers (CA Absorbers) for oblique angle of incidence. IEEE Trans Antennas Propag 55(1):186–193.

6 Chandrika Sudhendra,Atanu K. Mohanty,VibhorMahule,ACRPillai and KARK Rao, “Novel Embedded Passives Resistor Grid Network Based Wideband Radar Absorber”, in IEEE Intnl. Conf. on Electronics,Computing and Communication Technology – CONECCT 2014, Jan.06-07,2014, IISc., Bangalore.

7 Chandrika Sudhendra, P.Jose, ACR Pillai and KARK Rao, ”A Resistive Fractal FSS Based Broadband Radar Absorber”, Chapter 3, V. Sridhar et al. (eds.), Emerging Research in Electronics, Computer Science and Technology, Lecture Notes in Electrical Engineering 248, DOI: 10.1007/978-81-322-1157-0_3, ©Springer India 2014.

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BIO DATA OF AUTHOR(S)

Ms. Chandrika Sudhendra passed B.E. (EE) from UVCE, Bangalore and obtained M.Sc. (Engg.) from IISc, Bangalore. Presently Scientist ‘F’in Applied Research division, ADE, DRDO. Published more than 30 technical papers and seven of the papers have won various awards in international conferences. She is a fellow of IETE and member of IEEE.

Madhu AR passed B.E (ECE) from AIT, Chickmagalur. He obtained M. Tech (Communication Systems) from R.V. College of Engineering, Bangalore, during August 2013 and is presently working as SRF in ADE.

Dr. A. C. Radhakrishna Pillai obtained M.Sc. (Maths) from IIT, Kanpur and PhD (Maths) from IIT, Delhi. Area of specialization is numerical solutions of Differential equations. Presently Scientist ‘G’ in ADE, and is the Head of Applied Research division and Group Director (AWS). Published several papers in national and international journals. Current interests are in the areas of computational fluid dynamics, numerical design optimization and computational electromagnetics (RCS) for Low Observable applications.

Dr. K.A. Radha Krishna Rao is currently Professor in   E & C Engg at PES College of Engineering, Mandya. He obtained B.E. from Mysore University in the year 1984, Master of Technology from IIT Madras in 1989 and PhD from IISc, Bangalore in 2004. His research interests are in the area of signal processing. He has several journal and conference papers to his credit. He is also head of training & placement and corporate relations department of his institute.

Dr. Ms. T.S. Rukmini obtained PhD (Engineering) from IISc.,Bangalore. She is currently professor in E&C department at NMIT, Bangalore. Published several papers in national and international journals. Current interests are in the areas of CEM and conformal antenna design.