An Ultra Wide Band Radar Absorber Based on
Square Patch Resistive FSS
Chandrika Sudhendra1, Madhu AR1, ACR Pillai1, KARK Rao2 and T.S. Rukmini3
1
AR Division, Suranjan Das Road, C.V. Raman Nagar, ADE, Bangalore
2
Professor, PES College of Engineering, Mandya, Karnataka
3
Professor, NMIT, Bangalore.
[email protected]
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.
INTRODUCTION
Radar 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
 max 0 ≤172d
(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:
Z FSS
1
=R S +
j C 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
 = 20 log 10 ()
(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
Spacing
Foam
length
between
thickness
(mm)
patches(mm) ( (mm)
Layer1 6.6
0.2
D1= 3.5
Layer2 6.28
0.52
D2= 3.0
Layer3 2.8
0.6
D3= 3.2
Layer4 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 Fixontape. 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.
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.
ACKNOWLEDGMENT
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
The 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.
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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.