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International Journal of Advances in Engineering & Technology, Sept. 2013. ©IJAET ISSN: 22311963

FSS RADOMES FOR ANTENNA RCS REDUCTION Chandrika Sudhendra1, Madhu AR2, Mahesh A3 and AC Radhakrishna Pillai1 1

Scientist, Aeronautical Development Establishment, Ministry of Defence, DRDO, India 2 M.Tech Project Trainee, RVCE, Bengaluru, Karnataka, India 3 Assistant Professor, RVCE, Bengaluru, Karnataka, India

ABSTRACT In this paper, two different types of Frequency Selective Surfaces (FSS) panel radomes are presented. Both types of FSS radomes are designed as band pass spatial filters. The first designed example comprises of a single slot element (SSE) FSS radome for linear polarization with an RF transparency bandwidth of 1 GHz, with insertion loss <1 dB from 9.5 GHz. to 10.5 GHz. The SSE panel prototype FSS radome is developed as PCB on an FR4 substrate of thickness 0.4 mm. The second design comprises of Jerusalem Cross Slot (JCS) FSS radome with RF transparency bandwidth1 GHz with insertion loss <1 dB from 8.5 GHz. to 9.5 GHz. for circular polarization. A reflection loss of better than 10 dB is realized in both designs. The dielectric profile of the JCS FSS radome comprises of the FSS pattern sandwiched between two FR4 substrates of thickness 0.2mm. The electromagnetic designs are simulated using the 3D EM simulation software, HFSS v15. The FSS layers are designed as electrically thin PCBs using the Visula PCB layout design software and developed using conventional and highly accurate photolithographic technology. Both FSS radomes are developed as PCBs of size (280mm  280 mm). The FSS radome panels are tested for their performance in the microwave anechoic chamber. Experimental results agree closely with simulation results and are encouraging.

KEYWORDS: FSS, FSS radome, Jerusalem cross slot, Radar cross section (RCS) and RCS reduction (RCSR).

I.

INTRODUCTION

Frequency Selective Surfaces (FSS) are periodic surfaces in two dimensions and have inherent filtering characteristics. A two dimensional array of slots or patches etched on a microwave laminate can be designed to act as microwave band pass spatial filters or as band stop filters respectively. FSS are employed in the design of FSS radomes [1] where they need to de designed as band pass spatial filters. FSS radome design and development assumes importance as the FSS radomes are applied to reduce the enclosed antenna radar cross section (RCS) [1-2]. In this application, the FSS radome is designed as a band pass spatial filter with high transmission in the radiating frequency band(s) of the enclosed antenna and to provide reflection loss in the out of-band radiating regions of the antenna i.e. the antenna contributes the in-band RCS whereas the out-of-band RCS is dictated mainly by (i) the low-RCS shape of the radome and (ii) the reflection characteristics of the FSS radome. In a recent paper [3], an absorptive FSS radome is described with wide band absorptive property which performs the dual role of low insertion loss in the radiating frequency band of the enclosed antenna and also absorption of Electromagnetic (EM) signals in the out-of-band radiating frequency band of the antenna. Unlike the microwave filters with a designated input for desired output, FSS radome design is highly challenging as it needs to be designed for various Angles Of Incidence (AOI) and polarization in addition to stringent specifications of a normal radome for aircraft applications. FSS find applications as FSS radomes [4-10] and radar absorbers [11] wherein the FSS layers need to be realized as resistive FSS. Two basic complementary FSS geometries are the patch and slot in a free standing screen, and obey Babinet’s principle. A judicious selection of FSS geometry for meeting the desired EM specifications is the first step in FSS radome design. The analysis of FSS radome with complex geometries is carried out using full wave, 3D EM simulation software such as High Frequency Structure Simulator - HFSS.

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International Journal of Advances in Engineering & Technology, Sept. 2013. ŠIJAET ISSN: 22311963 HFSS is the industry standard for accurate simulation of passive and active RF/microwave devices. Further, the Floquetâ&#x20AC;&#x2122;s theorem enables analysis of the entire planar FSS structure by simulating single/unit cell FSS geometry. The HFSS simulation software aids not only in accurate simulation and sensitivity analysis but also helps in optimising all design parameters with virtual prototyping. In this paper, two types of prototype panel FSS radomes are designed and developed, viz., a Single Slot Element (SSE) FSS radome and Jerusalem Cross Slot (JCS) FSS radome to meet linear and circular polarization requirements respectively. In the first section the EM design, simulation in HFSS and fabrication and measurements of the dipole slot FSS radome are described. In the second section, the design, full wave simulation, fabrication and microwave measurements of the JCS FSS radome is described.

II.

EM DESIGN, SIMULATION AND MEASUREMENTS

A. Single Slot Element (SSE) FSS panel radome A Single Slot Element (SSE) FSS geometry is chosen to meet the linear polarization requirements and specifications such as insertion loss ď&#x201A;Ł 1dB and reflection loss better than 10 dB from 9.5 to 10.5 GHz. The resonant frequency, fc of the SSE FSS is given by đ?&#x2018;&#x201C;đ?&#x2018;? =

1 2đ?&#x2018;&#x2122;â&#x2C6;&#x161;đ?&#x153;&#x2021;đ?&#x153;&#x2013;

â&#x20AC;Ś â&#x20AC;Ś â&#x20AC;Ś â&#x20AC;Ś . (1)

where, l = length of the slot ď ­ = ď ­0ď ­r ď Ľ = ď Ľ0ď Ľr ď ­r = 1(non-magnetic material) ď Ľr = 4.4 (FR4 dielectric substrate) and ď ­0 and ď Ľ0 have the free space values. The SSE has polarization discrimination characteristics and hence was the preferred choice to meet the linear polarization requirements. The SSE FSS radome is modeled as unit cell geometry in HFSS v15 simulation software. The core EM solver in HFSS is based on Finite Element Method (FEM). FEM is used for rigorous and accurate simulation of 3D structures such as FSS radomes and radar absorbers in a broad frequency range for extracting the required S-parameters. Advanced computational EM solve techniques like Hybrid Finite Element and Integral Equation (IE) solver options available in HFSS v15 enable accurate simulation of electrically large and complicated geometries, radiation and scattering problems including nose cone FSS radomes. The model geometry is discretised into tetrahedral mesh with manual meshing to increase the accuracy. The advanced material library with comprehensive material data base containing permittivity, permeability, electric and magnetic loss tangent for common materials enables accurate simulation of material properties. Simulation of periodic surfaces such as FSS radomes is enabled with the unique Floquetâ&#x20AC;&#x2122;s port option. Various incident wave options such as plane wave, Hertzian dipole, cylindrical and Gaussian wave are available. The boundary conditions comprise of radiating and perfectly matched layers, sheet resistance etc., and the linked or periodic boundary conditions enables easy and accurate simulation of planar FSS radome. Also, the optimetrics option in HFSS enables sensitivity analysis while exploring the vicinity of the design point to determine the sensitivity of the design to small changes in variables. This option is particularly useful for determining the effect of process and fabrication tolerances by simply inserting these variables as parametric variables. The optimized S-parameters performance of the SSE FSS radome is shown in figure 1. The unit cell geometry model of the SSE FSS radome is shown in the inset of the same figure. It is noted that for a panel FSS radome design which is essentially a periodic surface, Floquetâ&#x20AC;&#x2122;s theorem for periodic surfaces enables analysis of entire surface by simulating a single cell. The FSS is idealized to be infinitely large and the analysis is then accomplished by analyzing a unit cell. The Floquetâ&#x20AC;&#x2122;s ports excitation are used exclusively with periodic structures defined by master slave boundaries. They contain plane waves whose frequency, phasing and the geometry of the periodic structure determine

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International Journal of Advances in Engineering & Technology, Sept. 2013. ©IJAET ISSN: 22311963 the propagation direction [12]. Floquet’s port excitation at the top and bottom of unit cell FSS geometry is shown in the figure along with linked master and slave boundaries for the sides of the unit cell. De-embedding of the port is also shown in the figure. One pair of linked boundaries is shown in the same figure, for clarity. The length and width of the SSE FSS are optimized to realize the desired performance. It is observed that the required insertion loss of < 1 dB is realized from the design. FR4 substrate with r = 4.4 and thickness 0.4 mm with tan = 0.02 is used in simulation. It is observed from the figure 1 that best reflection loss performance of 27 dB is obtained at 10 GHz. and the desired 10 dB reflection loss bandwidth is realized from 9.5 to 10.5 GHz, for normal incidence.

Figure1. Optimized HFSS simulation performance of S-parameters SSE radome Inset: Unit cell geometry model in HFSS.

In an operational scenario, the FSS radome may experience various angles of incidence (AOI) other than normal. Hence, to study the effect of the effect of variation in AOI on the design, the AOI is varied from -30 to +30. The simulation graphs are shown in figure 2.

Figure 2. HFSS simulation study of variation in AOI of SSE FSS radome.

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International Journal of Advances in Engineering & Technology, Sept. 2013. ©IJAET ISSN: 22311963 It is observed from figure 2 that for an AOI of +/- 10 degrees, the resonant frequency shifts to 9.98 GHz. but meets the reflection loss and insertion loss bandwidths from 9.5 GHz to 10.5 GHz. But for an AOI of 30, the resonant frequency shifts to 9.8 GHz. and hence the 10 dB reflection loss bandwidth correspondingly shifts from 9.3 GHz to 10.3 GHz. Figure 3 shows the parametric simulation performance of the SSE FSS radome. The slot length is varied and the slot width is kept constant at 2.25 mm. It is observed that as the slot length is increased, the resonance shifts to lower frequency as expected.

Figure 3. Parametric simulation studies of SSE FSS radome. Slot length varied. Slot width kept constant.

The SSE FSS radome is designed as a PCB using the PCB layout design software, Visula v 2.3 and developed using photolithographic technology. A photograph of the SSE FSS panel radome is shown in figure 4. FR4 laminate with copper thickness of 17.5 microns and dielectric thickness of 0.4 mm is used for fabrication of the FSS PCB. The dielectric profile of the panel FSS radome is shown in figure 5.

Figure 4. Photograph of the SSE panel FSS radome. Size of the FSS panel radome = 280mm  280 mm

Figure 5. Dielectric profile of the SSE panel FSS radome.

Comparisons between measurement and simulation results of the SSE panel FSS radome are tabulated in table 1. From table 1, it is observed that the simulated and measured insertion loss readings of the FSS panel radome agree very closely for vertical linear polarization.

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International Journal of Advances in Engineering & Technology, Sept. 2013. ŠIJAET ISSN: 22311963 TABLE I: Comparison of simulated and measured results of SSE panel FSS radome Frequency (in GHz) 9.5 9.6 9.7 9.8 9.9 10 10.1 10.2 10.3 10.4 10.5

HFSS Simulated insertion loss( dB) 0.7 0.5 0.4 0.3 0.3 0.2 0.3 0.4 0.5 0.7 0.9

Measured insertion loss( dB) 0.6 0.4 0.3 0.2 0.3 0.2 0.3 0.45 0.4 0.74 0.95

Good agreement in measured and simulated results is attributed to the very strict tolerances realized both in FSS PCB layout design and highly accurate photolithographic PCB fabrication technology. Also, the SSE FSS radome offers good polarization discrimination as the radome shows desired performance for TE incidence only. Hence, the SSE FSS radome is suited for linear polarization applications with RF transparency bandwidth of 1 GHz. in X-band. The reduced thickness of SSE panel FSS radome enables it to be easily conformed to curved surfaces.

B. Jerusalem Cross Slot (JCS) FSS Panel Radome A panel FSS radome was required for meeting similar RF transparency bandwidth requirements with circular polarization from 8.5 GHz. to 9.5 GHz. A Jerusalem Cross Slot (JCS) FSS geometry was chosen to meet the requirements of both RF transparency bandwidth and polarization specifications. Various modifications in basic JCS FSS geometry are reported in [13] with respect to the type of the element and its geometry, substrate and superstrate parameters and inter-element spacing, with the same unit cell size and periodicity. This has resulted in modified designs which obviously lead to a higher resonant frequency, bandwidth, increased null separation, dual band operation etc. The JCS geometry based FSS is referred to as parallel resonant screen (PRS) in [14] whereas the complementary FSS with Jerusalem cross patches is called series resonant grid. The grid impedance and the effective inductance is also derived in [14] and the TE incidence to the array of metal Jerusalem crosses corresponds to the TM incidence to the array of slots and vice versa. Further, the JC element FSS has been used to enhance the bandwidth for WLAN antennas as reported in [15]. To meet the specifications mentioned above, a simple JCS slot geometry was the right choice. The design centre frequency for the JCS FSS radome is taken to be is 9.5 GHz. The design details of a unit cell of JCS FSS geometry is given in figure 6.

Figure 6. Design of Jerusalem Cross slot FSS radome

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International Journal of Advances in Engineering & Technology, Sept. 2013. ©IJAET ISSN: 22311963 In the figure, 2L is the length of the slot, w is the width of the slot and p is the pitch or the size of the unit square cell. The optimized simulation performance of the JCS FSS radome is given in figure 7. In the inset of the same figure, a unit cell geometry model in HFSS is shown, with the master and slave boundaries for the four sides of the unit cell, Floquet’s port excitation for the top and bottom faces with de-embedding of port.

Figure 7. Optimised HFSS simulation performance of S-parameters of JCS FSS radome. Inset: Unit cell geometry model in HFSS.

The simulation performance for AOI variations of the JCS FSS is shown in figure 8. It is noted from the figure that the JC FSS geometry can be used without degradation in performance for an AOI variation from 0 to 30. A minor shift in resonant frequency is observed but the desired 1 GHz. bandwidth is maintained with insertion loss ( 1 dB) and reflection loss ( 10 dB) well within the specified bandwidth limits.

Figure 8. HFSS simulation study of variation in AOI of Jerusalem cross FSS

The performance of Jerusalem cross FSS for circular polarization is studied by simulating the FSS geometry for both TE and TM incidence in HFSS. The simulation performance for both TE and TM incidence is shown in figure 9. It is observed from the figure that similar S-parameters performance is observed for both TE and TM and hence, the JC FSS geometry is a good candidate for realizing circular polarization performance from the FSS radome.

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International Journal of Advances in Engineering & Technology, Sept. 2013. ŠIJAET ISSN: 22311963

Figure 9. TE and TM performance of the JCS FSS radome in HFSS v15.

A photograph of the prototype JCS FSS panel radome is shown in figure 10. The dielectric profile of the JCS FSS radome comprises of the JCS FSS pattern sandwiched between two FR4 substrates of thickness 0.2 mm each and is shown in figure 11.

Figure 10. Photograph of the fabricated JCS FSS radome for circular polarization

Figure 11. Dielectric profile of JCS FSS radome

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International Journal of Advances in Engineering & Technology, Sept. 2013. ŠIJAET ISSN: 22311963 The insertion loss measurements of the JCS FSS radome are carried out in microwave chamber and are tabulated in table 2. The simulated and measured results are tabulated observed that the results agree closely. It is noted from table 2, that the JCS FSS radome good candidate for circular polarization as it gives same performance for both TE polarizations.

anechoic and it is is a very and TM

TABLE 2: Comparison of simulated and measured results of JCS FSS panel radome. Frequency (in GHz)

9 9.1 9.2 9.3 9.4 9.5 9.6 9.7 9.8 9.9 10

III.

Simulated insertion loss (dB) (same results for TE and TM ) 0.71 0.58 0.46 0.38 0.32 0.30 0.30 0.34 0.42 0.5 0.6

Measured insertion loss (dB) TE 0.9 0.7 0.4 0.3 0.4 0.2 0.24 0.3 0.4 0.4 0.5

Measured insertion loss (dB) TM 0.9 0.7 0.4 0.3 0.4 0.2 0.3 0.3 0.4 0.4 0.5

DISCUSSION OF RESULTS

Two different types of FSS panel radomes are presented in the paper based on SSE and JCS element FSS geometries, for linear and circular polarization respectively. The following observations are in order: i. The SSE panel FSS radome based on dipole slot gives very good performance for vertical linear polarization, for normal incidence. The construction is fairly simple and offers the much desired polarization discrimination. ii. From table 1, it is observed that the simulation and measurement results of the SSE panel FSS radome agree very closely. This is attributed to the highly accurate and reliable translation of the design to hardware using photolithographic technology. iii. From figure 2, which gives the simulation performance of the SSE radome with respect to variation in AOI, it is observed that for an AOI of 30 degrees, the resonance frequency shifts to 9.8 GHz. Since the prototype SSE FSS radome was intended for a laboratory application, there was no requirement of AOI beyond 30degrees. iii. From table 2, which gives the simulation and measurement results of the JCS FSS radome, it is observed that the results agree closely and meet the specifications. With two dimensional symmetry in the pattern both TE and TM performance specifications have been met and the thickness of FSS radome is 0.4 mm. Hence this FSS radome is a good candidate for circular polarization.

IV.

CONCLUSION

Two different types of prototype panel FSS radomes are presented in this paper. The first designed example namely the SSE FSS panel radome with an RF transparency band width of 1 GHz. meets all specifications such as reduced thickness (thickness = 0.4 mm) for operation from 9.5 to 10.5 GHz., with insertion loss of < 1 dB in the band. Simulation results using the HFSS simulation software and the experimental results agree very closely. This FSS radome finds application where polarization discrimination is required, with vertical polarization. The second FSS panel radome based on JCS FSS geometry with similar RF transparency bandwidth requirement but in the frequency band from 8.5 GHz. to 9.5 GHz. meets all requirements. The two dimensional symmetry inherent in the JCS

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International Journal of Advances in Engineering & Technology, Sept. 2013. ©IJAET ISSN: 22311963 FSS geometry has been effectively used for realization of circular polarization performance. Insertion loss measurements carried out on the FSS panel radomes using high directivity standard gain horn antennas show that the measurement results agree closely with simulation performance. Commercially available FR4 substrates are used for fabrication of prototype FSS radome panels and the accurate PCB design and fabrication process adopted for realizing the FSS layer as electrically thin PCB ensure reliable and accurate translation of design to hardware. The FSS radomes find application in aircraft stealth technology for reduction of out-of-band RCS of stealth antennas.

V.

SCOPE FOR FUTURE WORK

1. For improved AOI performance, the SSE or the dipole slots can be arranged in a skewed lattice which helps in stabilizing the shift of resonance with variation in AOI [16]. 2. Since the FSS panel radomes were intended for laboratory application, FR4 substrate was used in fabrication. The FR4 substrate can be replaced by other commercially available microwave substrates with better dielectric loss tangent for best performance. 3. The JCS FSS radome design can be translated to be conformal to various curved surfaces such as cones, cylinders and hemispherical surfaces. These curved surfaces would then result in FSS radome structures. However, the periodicity in FSS would be lost and Floquet’s theorem would no longer be applicable.

ACKNOWLEDGMENT The authors are indebted to Shri. PS Krishnan, Distinguished Scientist and Director, ADE for his continued guidance, support and according permission for publishing the paper. We record our grateful thanks to Shri S. Gurudev, Group Director, ADE for his unstinted support and guidance. Thanks are also due to Dr. V. Ramachandra, Scientist G and Head, FTTT division for allowing us to use the anechoic chamber measurement facility, Ms. Nagarathna R, TO ‘A’ for PCB layout design and Mr. Mahalingam, Scientist ‘F’, OIC, PCB and EMI/EMC group and his entire team for speedy and accurate fabrication of PCBs.

REFERENCES [1]. Benedict A. Munk, Frequency Selective Surfaces: Theory and Design. New York: Wiley-Interscience, 2000. [2]. E. Martini, F. Caminita, M.Nannetti, and S. Maci “Fast Analysis of FSS radome for antenna RCS reduction” , pp. 1801-1804, IEEE Antennas and Propagation Soc. Int. Symp., 2006, DOI:10.1109/APS 2006.1710917. [3]. Filippo Costa and Agostino Monorchio, “A Frequency Selective Radome with Wideband Absorptive Properties”, pp. 2740-2747, IEEE Trans. Antennas Propag., Vol. 60, No. 6, June 2012. [4]. E A Parker and S M A Hamdy “Rings as elements for FSS”, Electronics Letters vol 17 ,pp 612-614, Aug-1982. [5]. A Roberts and R Mcphodran “Band pass Grids and Annular Apertures”, IEEE Transactions Antenna and Propagat., Vol. AP36, PP607 May 1988. [6]. T K WU “Frequency selective surfaces and grid array”, RW Electronics systems and technology divisions (John Wiley). [7]. Chandrika Sudhendra, Vasanth K.M, Vibhor M, ACR Pillai and TS Rukmini “Novel Metamaterials /FSS based single screen radome for stealth”, pp.319-322, Proc. IEEE Intnl. Symp. on Microwaves2010 - ISM-10. [8]. Chandrika Sudhendra, Shilpa NB, V.Mahule, D.Biswas, Nagarathna R and ACR Pillai, “FSS Band Pass Radome Based on Aperture Coupled Microstrip Patches for Stealth Applications”, pp.90-93,Proc. 5th Annual Conf., ATMS India 2012. [9]. Chandrika Sudhendra, Usha SP Nayak, V. Mahule, ACR Pillai and TS Rukmini , “A Tripole Slot FSS Based Band Pass Radome for Out-Of-Band RCS Reduction of Low RCS Antennas in Stealth Applications”, pp.74-77,Proc. 5th Annual Conf., ATMS India -2012. [10]. Vibhor M, Chandrika Sudhendra, Usha SP Nayak, ACR Pillai and TS Rukmini, “ A Novel Band Pass FSS Hybrid Radome for Reduction of Out-Of-Band RCS of Stealth Antennas", Presented in Int. Conf. on Microwaves, Antennas and Radio Science, ICMARS 2011, Jodhpur. The paper was awarded II prize in the session on Antenna Analysis, Synthesis and Measurements.

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International Journal of Advances in Engineering & Technology, Sept. 2013. ©IJAET ISSN: 22311963 [11]. 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: 10.1007/978-81-322-11570_3, ©Springer India 2014. The paper was awarded one of the best six papers in the ICERECT 2012 conference. [12]. ANSYS HFSS v. 15 EM Simulation software. [13]. Mirshahram Hosseinipanah and Qun Wu, “Novel Frequency Selective Surfaces Designing by Modification of Jerusalem Cross Slot”, Proc. ICMMT 2008, [14]. Constantin R. Simoski, Peter de Maagt and Irina V. Melchakova, “High Impedance Surfaces Having Stable Resonance With Respect to Polarization and Incidence Angle”, IEEE Trans. On Antennas and Propagation, Vol. 53, No.3,March 2005. [15]. Hsing-Yi Chen,Yu Tao,Kuo-Lun Hung and His-Tseng Chou, “Bandwidth Enhancement Using DualBand Frequency Selective Surface with Jerusalem Cross Elements for 2.4/5.8 GHz. WLAN Antennas”, pp. 1-4, IEEE Intnl. Conf. on Wireless Information Technology and Systems(ICWITS), 2010. DOI:10.1109/ICWITS.2010.5611978. [16]. P.Callaghan, EA Parker and RJ Langley, “Reduction of mode coupling effects in double layer dipole slot dichroics”, pp.701-704, IEE 7th International Conf. on Antennas and Propagation- ICAP 1991.

BIOGRAPHY OF AUTHORS Chandrika Sudhendra is Scientist F in Applied Research Division, ADE, DRDO. She has 30 technical papers to her credit and 7 of her papers have won various awards in international and national conferences. Her main research interests are in the design and development of radar absorbers and FSS radomes.

Madhu AR received B.E. from VTU, Belgaum in 2011. He is presently pursuing M.Tech in R V College of Engineering, Bangalore in Communication Systems. His fields of interests are Electromagnetics and digital communication.

Mahesh A has completed M.Tech from R V College of Engineering, Bangalore in Communication Systems. He is currently working as Asst. Professor in the department of Electronics and Communication engineering, RVCE Bangalore. His research interests are Electromagnetics and antennas.

A.C Radhakrishna Pillai is presently Scientist ‘G’ in ADE, and is the Divisional Head of Applied Research division and Group Director (AWS). He obtained M.Sc. (Maths) from IIT, Kanpur and PhD (Maths) from IIT, Delhi. Area of specialization is numerical solutions of Differential equations. Published several papers in national and international journals. Current interests are in MDO and computational Electromagnetics (RCS).

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