RCS reduction of integrated antenna arrays with resistive...
Embed Size (px)
Transcript of RCS reduction of integrated antenna arrays with resistive...

LUND UNIVERSITY
PO Box 117221 00 Lund+46 46222 00 00
RCS reduction of integrated antenna arrays with resistive sheets
Gustafsson, Mats
Published: 20050101
Link to publication
Citation for published version (APA):Gustafsson, M. (2005). RCS reduction of integrated antenna arrays with resistive sheets. (Technical ReportLUTEDX/(TEAT7135)/112/(2005); Vol. TEAT7135). [Publisher information missing].
General rightsCopyright and moral rights for the publications made accessible in the public portal are retained by the authorsand/or other copyright owners and it is a condition of accessing publications that users recognise and abide by thelegal requirements associated with these rights.
Users may download and print one copy of any publication from the public portal for the purpose of privatestudy or research. You may not further distribute the material or use it for any profitmaking activity or commercial gain You may freely distribute the URL identifying the publication in the public portalTake down policyIf you believe that this document breaches copyright please contact us providing details, and we will removeaccess to the work immediately and investigate your claim.
http://portal.research.lu.se/portal/en/publications/rcsreductionofintegratedantennaarrayswithresistivesheets(7191194594cf41998f1255e74de17e51).html

Department of ElectroscienceElectromagnetic TheoryLund Institute of TechnologySweden
CODEN:LUTEDX/(TEAT7135)/112/(2005)
RCS reduction of integrated antennaarrays with resistive sheets
Mats Gustafsson

Mats Gustafsson
Department of ElectroscienceElectromagnetic TheoryLund Institute of TechnologyP.O. Box 118SE221 00 LundSweden
Editor: Gerhard Kristenssonc Mats Gustafsson, Lund, December 5, 2005

1
Abstract
Edge diraction can be a major source of high radar visibility for integrated
array antennas. In this paper, it is shown that tapered resistive sheets at the
edges of the array can be used to transform the scattering properties of the
antenna to that of a metallic plate and hence reduce the radar cross section.
Numerical examples of an array of selfcomplementary patches and a bandpass
frequency selective radome are presented to illustrate the results. The RCS
reduction is very good for TE polarization but not as ecient for the TM
polarization.
1 Introduction
Antennas are a major potential source of high radar visibility on stealth objects.There are several phenomena, e.g., grating lobes, edge diraction, and surface waves,that contribute to the radar cross section (RCS) of an array antenna. The gratinglobes can occur if the interelement spacing is larger then half a wave length [2,9, 11]. Edge diraction can be interpreted as a diraction by the rapid changein the scattering properties between the antenna and its surrounding [7]. As anantenna is designed to absorb energy in its operational band, the inband diractionis signicant if the antenna is integrated in a nonabsorbing environment. The out ofband diraction can also contribute to the RCS if there is a phase dierence betweenthe reection from the antenna and the reection from the surrounding.
The specular reection is in general not a major problem for an integrated antenna as it is directed in the same direction as the specular reection of the body ofthe object, i.e., in a safe direction on a stealth object. Although the alignment canreduce the degrading eects of the diracted waves it is important to reduce theiramplitude as it is dicult to avoid backscattered waves as well as multiple scatteredwaves in the monostatic direction.
Tapered resistive edge treatment can be used to reduce edge diraction anddiraction from impedance discontinuities [6, 7]. The resistive sheet is highly conductive and very thin d 0, and is such that d = R1, see e.g., [7, 10, 12].They are commonly used in radar absorbing materials (RAM) such as Salisburyscreens and Jaumann absorbers [7]. They have also been used to taper the edges ofantennas to free space [2, 14]. Their scattering properties are analyzed in [4, 13].
In this paper, it is shown that a resistive sheet can transform the scatteringproperties of the antenna to the scattering properties of a perfectly electric conductor(PEC) in a controlled way. The resistive sheet transforms the reection coecientof the innite antenna along the inverted reactive circles in the Smith chart towardsthe 1 point as the resistivity decreases to 0. It is shown that the monostaticRCS reduces over a large frequency band and wide scattering angles as long as thephysical optics (PO) approximation is accurate. FDTD simulations of the RCS froma selfcomplementary patch array and an FSS radome are also presented to illustratethe results. The numerical examples indicate a 20 dB reduction of the monostaticRCS in the TE polarization but not as good for the TM case.

2
2 RCS and physical optics approximation
The RCS, , of an object is its equivalent area which if scattered isotropically wouldresult in the same scattered power density [8]. It can be determined as the quotientbetween the scattered wave and the incident wave, i.e.,
(r, k) = limr
4r2Es(r, r)2
Ei(k)2=
4
k2F s(r)2
Ei(k)2, (2.1)
where the incident wave Ei is a plane wave Ei(x; k) = E0eikkx. The RCS depends
on the polarization and frequency of the incident wave. For twodimensional objectsthe RCS is the equivalent length of the object.
Here, we analyze the contribution to the RCS from an antenna integrated in asurface of a PEC object. We assume that the antenna array is planar and that itis integrated in an innite planar PEC surface in the xyplane, see Figure 1. Thescattered eld can be determined from the currents on the surface of the object. Itis natural to consider the dierence between the scattered eld from the object withthe antenna and the scattered eld from the object when the antenna is replacedwith PEC.
The totally scattered eld can be represented with a Fourier type integral ofthe equivalent electric surface current, J , and magnetic surface current, M , overthe xyplane [11]. We get an innite planar PEC surface when the antenna isreplaced with PEC. The surface current on the innite planar PEC surface is givenby J0 = 2z H i or equivalently by a reection coecient 0 = 1. Subtractionwith J0 over the hole surface gives the antenna contribution to the scattered eld.The antenna contribution to the scattered fareld is, hence, given as a Fourierintegral over the xyplane, i.e.,
F s(r) =ik2
4r
R2
(M(x) 0r
(J(x) J0(x)
))eikrx dS. (2.2)
We analyze the RCS in an approximation where the local reection coecient isdetermined in the physical optics approximation (PO). For periodic antenna arraysand FSS, we use the reection coecients of the innite periodic cases. Observethat PO is not very accurate for the diracted eld. The physical theory of diffraction (PTD) could be used to improve the accuracy. However, PO illustrates thebasic phenomena and it is sucient for our analysis in this paper. Unfortunately,the PO approximation of the aperture currents is not unique and not necessarilyconsistent with the boundary conditions on the innite ground plane surroundingthe aperture [11, Ch. 16]. To obtain a scattered eld that is consistent with theboundary condition on the PEC screen, we need to use an approximation using onlymagnetic currents [11, Ch. 16]. The magnetic currents are given by
M(x) = 2(x)z Ei(x) = 2(x)z E0eikkx, (2.3)
where (x) is the reection coecient.

3
Antenna
PEC=1
=Al
d
d
a) b)
lx
y
Antenna
l dxd
=1( , ) A R =A
h
( , ) A RResistive sheet,
PEC
( , ) A R
x
y
z
x y
z
Figure 1: Illustration of the integrated antenna. The resistive sheet at the edgesof the antenna transforms the reection coecient of the antenna, A, to 1 overthe transition zone with width d. a) top view. b) side view.
The reection coecient can be dyadic. However, it is sucient to consider scalarreection coecients for the present analysis. The reection coecient depends onthe spacial coordinate x, the frequency f , direction k, and the polarization Ei. Wealso need to replace the surface current, J0, with an equivalent magnetic surfacecurrent, M 0. Here, we use that a magnetic surface current M 0 = 2z Ei givesan identical reected eld as the electrical current J0 = 2zH i. Substitution of themagnetic surface PO currents into (2.2) gives the PO approximation of the fareld,i.e.,
F s(r) =ik2
2r (z E0)
A
((x) + 1
)eik(kr)x dS (2.4)
where A denotes the aperture. This gives the physical optics approximation of theRCS
(r, k) =k2p2
A
((x) + 1
)eik(kr)x dS
2 (2.5)where p = r (z E0)/E0. For TE and TM polarization, we have p2 = cos2 rand p2 = cos2 i, where r and i denote the scattering and incident angles, respectively. Here, it is seen that the result is consistent with the boundary conditions, i.e.,the reection coecient approaches zero for grazing angles in the TE case. However,it is also well known that the accuracy of the PO approximation decreases for largescattering angles. This is also seen in the comparison with FDTD simulations inSection 4.2. In this paper, we focus on the monostatic RCS, i.e., r = k, of planarobjects in the xyplane with a scan angle where in both cases TE and TM we havep = cos .

4
RZLAntenna A
resistive sheet
resistance, R
refl.,
trans., =1+ R
R
R
v+u+
vu
Figure 2: Illustration of the scattering model with a resistive sheet in front of theantenna. The resistive sheet with resistivity R and the antenna with load ZL hasthe reection coecients R and A, respectively.
3 Resistive sheet
To reduce the diracted eld we need to construct a smooth transition over theinterface. Here, we consider a resistive sheet with high conductivity, i.e., thicknessd 0 and conductivity , with d and such that d = R1 is nite. Thereection coecient of the sheet is [10]
R =T
2R + T, (3.1)
where T is the transverse wave impedance [11], i.e., TE = 0/ cos and TM =0 cos where is the incident angle. It is easily seen that R is real valued and1 R 0. The corresponding transmission coecient is given by R = 1 + R.
We use a resistive sheet placed at the edges in front of the antenna array asseen in Figure 1. Let the resistance be zero (PEC) at the edge of the antenna andincrease to innity (air) at distance d from the edge. The reection coecient of thecombined resistive sheet and antenna structure is analyzed in the PO approximationwith the circuit model shown in Figure 2. The scattering properties of the antennaarray is modeled with the reection coecient A. The reection coecient is smallin the frequency band where the antenna is matched to the load ZL. However, asA depends on the frequency, the incident angle, and the polarization, it is dicultto utilize the specic structure of A in the analysis. Here, we consider A as anarbitrary complex valued number in the unit circle, i.e., A 1. The reectioncoecient of the combined sheet and antenna is obtained with a standard scatteringanalysis [11, Ch. 12]. The scattered waves depicted in Figure 2 are related asu = u+, v = Av+, v+ = Rv + Ru+, and u = Ru+ + Rv. Elimination ofthe v waves gives the total reection coecient
(A, R) = R +A
2A
1 RA=
R + A + 2RA1 RA
. (3.2)
We consider the map from A to for xed parameters R. This is a conformal map,mapping circles in the complex valued Aplane into circles in the complex valuedplane [1]. It is easily seen that (1, R) = 1, i.e., PEC is transformed to PEC.

5
a
0.20.40.60.8
(j j=1,0.2)A
( ,0.2) a
j j=1A
( ,1)=1 a
( ,0)= a a
0.0
Figure 3: Transformation of the reection coecient A 7 (A, R). An arbitraryreection coecient a is transformed along the 'inverted' reactive circles to the point1 as R : 0 1. The unit circle A = 1 is mapped into circles (A = 1, R)as illustrated for R = 0.2,0.4,0.6,0.8.
The transformation properties of the unit circle, A = 1, are constructed with theadditional relation (1, R) = (1 + 3R)/(1 R). This gives that the unit circle,A = 1, is mapped into the circle centered at
2R1 R
=T
R + Twith radius
1 + R1 R
=R
R + T. (3.3)
The circles corresponding to R = 0.2,0.4,0.6,0.8 are shown in Figure 3. Anarbitrary antenna reection coecient a is hence transformed towards the point 1,i.e., PEC, as R : 0 1 or equivalently R : 0. As the reection coecient ofthe resistive sheet, R follows a line, i.e., a generalized circle [1], the total reectioncoecient, , follows a circle towards 1. These circles are given by the 'inverted'reactive circles, see Figure 3. The eect of the resistive sheet can also be interpretedas a parallel coupling between the sheet resistivity R and the antenna impedanceseen from the outside, ZA = T(1 + A)/(1 A).
The RCS (2.5) is easily evaluated for an arbitrary transition zone. To illustratethe eectiveness of the conducting sheet, we consider an example with a piecewiseconstant, a linear, and cubic spline interpolation of the reection coecient, seeFigure 4a. The corresponding onedimensional Fourier transforms are shown inFigure 4b. Here, it is seen that the resistive tapering reduces the RCS. There is nomajor dierence between the three cases for low frequencies, i.e., > 0 lengthof transition zone. For higher frequencies, the improvement is larger for the smoothtransition, see also [7, Ch. 8].

6
1 0.5 0 0.50
0.5
1
1.5
2 4
80
60
40
20
0
+1 dB
y/ /0 0
Refg+1
Imfg
a) b)
Figure 4: a) transition of the reection coecient from A +1 to 0 with a piecewiseconstant, a linear, and a cubic spline interpolation. b) the corresponding Fouriertransforms in dB.
4 Numerical examples
Numerical simulations are used to illustrate the reduction of the RCS for an antennaarray and an FSS bandpass radome. To emphasize the edge diracted part, weconsider innite times nite arrays, i.e., periodic boundary conditions are used inone dimension. Here, the code periodic boundary FDTD (PBFDTD) developed byH. Holter [5] is used for the numerical simulations.
4.1 Selfcomplementary patch array
As a rst example, we consider an innite antenna array consisting of PEC patchesand dielectric sheets as depicted in Figure 5ac. The patches are fed at the corners ofeach patch. Here, we focus on a receiving array where the feeds are terminated in acharacteristic impedance Z0 = 120 . The patch array is almost self complementary,i.e., the PEC structure is almost identical to its complement. Hence, the impedanceof the patch array is frequency independent if situated in free space [3, 8]. As thepatch array radiates bidirectional, the ground plane under the patches denes theresonance frequency, f0, from h = 0/4 = c0/(4f0). The dielectric sheets above thepatch array act as a lter matching the antenna for a large range of frequencies.In analogy with quarterwave transformers in broadband matching, the sheets arechosen to be of equal optical thickness, i.e., a sheet thickness of di = h
1/2i is
used [3, 9, 11]. A typical reection coecient normalized to Z0 = 120 is shown inFigure 5b.
Here, we consider a patch array with dimensions a = 9.6 mm, b = 0.8 mm,and h = 13.6 mm giving the unitcell length l0 = 20.8 mm. This gives a resonancefrequency around 5.5 GHz and the onset of grating lobes at 7.5 GHz. Dielectric sheetswith permittivities 1 = 7 and 2 = 3 are used. We consider an array consisting of20 unit cells in the ydirection, giving l = 20l0 = 416 mm, and an innite amount ofunit cells in the xdirection, i.e., periodic boundary conditions in the xdirection. As

7
Resistive sheet (x)=1 A
h
dl
PEC
c)
a)
1
2
hpatches
2 4 6 8 1020
10
0dB
f/GHz
0
TM:60
TE:60
a
b
a
b)unit cell
PEC
PEC
E
k
r
ir
i
Er
Z0
Z0
Z0
Z0
l0
l0
y
x
z
yx
z
h2h1
Figure 5: Illustration of the integrated antenna array. a) top view of the elementgeometry. The innite array consists of a periodic repetition of square perfectlyelectric conductor (PEC) patches fed at the corners. An impedance Z0 = 120 isused to model the feed for the receiving antenna. b) calculated reection coecientfor the innite patch antenna as a function of the frequency for = 0, 60. c)scattering model of the patch array with resistive sheets at the edges. We use 20unit cells in the aperture, i.e., l = 20l0.
a plane wave in the yzplane impinges on the array, it is convenient to use the polarangles in the range /2 /2. The bistatic RCS of the patch array forthe incident angle 60 and TE polarization is depicted in Figure 6a. As expected,the specular reection at 60 dominates the bistatic RCS. The oscillations of theRCS away from the specular direction are due to the constructive and destructiveinterference of the edge diracted waves. The oscillations are more rapid for largearrays. The envelope of the RCS is highlighted to emphasize the amplitude of thediracted waves and at the same time deemphasize the dependence of the size ofthe array. The monostatic RCS is approximately 20 dBm at 3 GHz and 25 dBmat 5 GHz for the integrated array without tapering. With a linear tapering over twounit cells, i.e., d = 2l0 42 mm, the monostatic RCS reduces with approximately20 dBm.
The resistive tapering reduces the RCS by smoothing out the discontinuity between the antenna and its surrounding material. However, the RCS of an array can

8
90 60 30 0 30 60 9050
40
30
20
10
0
102GHz4GHz6GHz8GHz
10GHz
dBm
Grating lobes
2GHz
4GHz
6GHz
8GHz
10GHz
90 60 30 0 30 60 9050
40
30
20
10
0
3GHz
3GHz
5GHz
5GHz
dBm
2 taper0 taper
monostatic RCS
a) b)
Figure 6: Calculated (FDTD) bistatic RCS of a selfcomplementary patch arrayilluminated from = 60 in TE polarization. The envelope of the RCS is included toemphasized contribution from edge diraction. a) with and without resistive sheetsat 3 GHz and 5 GHz. b) linearly tapered with a resistive sheet over two unit cellsfor 2, 4, 6, 8, 10 GHz.
be signicant if the array supports gating lobes. These grating lobes can occur ifthe interelement spacing in the array is larger than half a wavelength. The patcharray supports grating lobes for frequencies above 7.5 GHz. The RCS of the selfcomplementary 24 patch array with a linear resistive tapering over the 2 edgeelements for an illumination from = 60 at 2, 4, 6, 8, 10 GHz is shown in Figure 6b.As seen in the gure, the monostatic RCS is very small for frequencies up to theonset of grating lobes at 7.5 GHz. The beam width of the grating lobes as well asthe specular lobe depends on the size of the array. The beam width decreases forlarger arrays.
4.2 FSS bandpass radome
We also consider the RCS of an FSS integrated radome. We consider a symmetrichybrid radome with fourlegged loop elements [8], see Figure 7a. The elements arearranged in a square grid with side length of l0 = 6.6 mm and they have a slot widthof 0.17 mm. The loop elements are placed in a 3 mm thick dielectric sheet withpermittivity r = 1.6. This gives a bandpass structure with passband from 8.5 GHzto 9 GHz as seen in Figure 8a.
The radome is integrated into a PEC structure and an antenna is placed underthe radome, see Figure 7b. Here, we use the patch array described in Section 4.1tuned to 8.5 GHz. The upper dielectric sheet is placed 5 mm from the inner side of theradome. We consider the three cases: without tapering, with a 26 mm linear taper,and with a 53 mm linear taper. The radome size excluding the taper is 332 mm . The nite length corresponds to 50 unit cells. The bistatic RCS is shown inFigure 8bcd for a TE wave at 45 and the frequencies 6, 8.5, 11 GHz. The envelopeof the RCS is highlighted to emphasize the amplitude of the edge diracted part. As

9
PEC
a) b)
Antenna
Radome FSS
Resistive sheet (x)=1A
PEC
E
k
r
ir
i
Erunit cell
l0
l0
y
x
z
0.17mm
2.4mm
dyx
z
3mm
5mm
9mm
l=50l0=332mm
Figure 7: Geometry of the FSS bandpass radome structure. a) unit cell. b) sideview with the radome and the antenna.
expected the specular reection is largest in the passband, i.e., at 8.5 GHz, wherethe radome discontinuity between the radome and PEC is large. For frequenciesoutside the passband, the radome is highly reecting and the discontinuity smaller.The eect of the tapering is negligible in the specular reection.
The monostatic RCS is also largest in the passband. Here, the eect of the tapering is considerable. As seen in Figure 8c, the tapering reduces the monostatic RCSwith 15 dBm to 20 dBm. The monostatic RCS is also reduced outside the passbandwith the tapering, however the improvement is not as large as the original RCS ismuch smaller. In Figure 9a, a comparison between the bistatic RCS calculated withFDTD and with the PO approximation is shown. The envelope of the FDTD andPO results are given by the solid and dashed curves, respectively. It is seen that thePO approximation gives a rough estimate of the RCS for the TE case.
The examples above illustrate the eectiveness of the resistive tapering and indicate that the PO analysis is reliable for TE polarization. The resistive sheets arenot as eective for the TM case. Figure 9bcd, shows a comparison between FDTDsimulations and PO approximation of the bistatic RCS of the FSS radome in TMpolarization. Here, it is seen that the PO and the FDTD results are only consistentin the specular direction. The PO approximation underestimates the monostaticRCS and the bistatic RCS for large scattering angles. It is also seen that the resistive tapering is not as ecient as in the TE case. This can be attributed to atleast two more or less related eects. First the TM polarization supports surfacewaves above the PEC ground plane. Secondly, the reection coecient of the resistive sheet approaches zero for grazing angles. Hence, a surface wave generatedby the edge diraction can propagate along the structure and is not absorbed bythe resistive sheet. To improve the RCS even further it is necessary to reduce thedegrading eect of the surface waves. One possible design is to continue the regionunder the PEC surface and ll it with RAM.

10
90 60 30 0 30 60 9050
40
30
20
10
0
10
90 60 30 0 30 60 9050
40
30
20
10
0
10
90 60 30 0 30 60 9050
40
30
20
10
0
10
0
48
dBm
6GHz048
11GHz048
8.5GHz048
0
04
48 8
dBmdBm
6 7 8 9 10 11 1230
20
10
0
PEC
dBa) b)
c) d)
TETM
f/GHz
Figure 8: Calculated (FDTD) bistatic RCS of an FSS bandpass radome. a) reection coecient of the innite times innite radome for 0, TE 45, and TM 45. bcd)a 50 array illuminated from = 45 in TE polarization for a linear resistivetapering of 0, 26, 53 mm. The envelope of the RCS is highlighted to emphasize thecontributions from the edge diraction and the grating lobes. b) 6 GHz. c) 8.5 GHz.d) 11 GHz.
5 Conclusions
In this paper it is shown that tapered resistive sheets can reduce the monostatic RCSof antenna arrays and bandpass radomes for TE polarization. The resistive sheettransforms the reection coecient of the antenna or radome to that of a PEC. TheRCS is reduced by elimination of the edge diracted eld. To obtain a low RCSfor higher frequencies it is also necessary to use a structure without grating lobes.The antenna performance is of course aected by the resistive sheet. However asthe transition zone is of the order of a wavelength, the eect should not be a majorconcern for large arrays. The eectiveness for TM polarization is not as clear andmore analysis is needed for this case.
Acknowledgment
This work was supported by the NFFP3+ project 521 SIGSUAV.

11
90 60 30 0 30 60 9050
40
30
20
10
0 TM: 11GHzFDTDPO
8
dBm
0
0
8
90 60 30 0 30 60 9050
40
30
20
10
0
10dBm
TM: 6GHz
FDTDPO
0
0
8
8
90 60 30 0 30 60 9050
40
30
20
10
0
10TM: 8.5GHz
FDTDPO
0
8
8
0
dBm
a) b)
c) d)
90 60 30 0 30 60 9050
40
30
20
10
0
10
dBm
0
4
8
TE: 8.5 GHzFDTDPO
Figure 9: Comparison between the FDTD and PO bistatic RCS of the FSS bandpass radome. a) TE polarization at 45 with edge tapers of 0, 26, 53 mm. bcd) TEpolarization at 45 a 53 mm with edge tapers of 0, 53 mm.
References
[1] G. B. Arfken and H. J. Weber. Mathematical Methods for Physicists. AcademicPress, New York, fth edition, 2001.
[2] J. David Lynch. Introduction to RF Stealth. SciTech Publishing Inc., 5601 N.Hawthorne Way, Raleigh, NC 27613, 2004.
[3] M. Gustafsson. Broadband array antennas using a selfcomplementary antenna array and dielectric slabs. Technical Report LUTEDX/(TEAT7129)/18/(2004), Lund Institute of Technology, Department of Electroscience, P.O.Box 118, S221 00 Lund, Sweden, 2004. http://www.es.lth.se.
[4] R. L. Haupt and V. V. Liepa. Synthesis of tapered resistive strips. IEEE Trans.Antennas Propagat., 35(11), 12171225, 1987.
[5] H. Holter and H. Steyskal. Innite phasedarray analysis using FDTD periodic boundary conditionspulse scanning in oblique directions. IEEE Trans.Antennas Propagat., 47(10), 15081514, 1999.

12
[6] E. F. Knott. Suppression of edge scattering with impedance strings. IEEETrans. Antennas Propagat., 45(12), 17681773, 1997.
[7] E. F. Knott, J. F. Shaeer, and M. T. Tuley. Radar Cross Section. SciTechPublishing Inc., 5601 N. Hawthorne Way, Raleigh, NC 27613, 2004.
[8] J. D. Kraus and R. J. Marhefka. Antennas. McGrawHill, New York, thirdedition, 2002.
[9] B. Munk. Finite Antenna Arrays and FSS. John Wiley & Sons, New York,2003.
[10] J. R. Natzke and J. L. Volakis. Characterization of a resistive half plane overa resistive sheet. IEEE Trans. Antennas Propagat., 41(8), 10631068, 1993.
[11] S. J. Orfanidis. Electromagnetic waves and antennas, 2002.www.ece.rutgers.edu/~orfanidi/ewa, revision date June 21, 2004.
[12] T. B. A. Senior. Backscattering from resistive strips. IEEE Trans. AntennasPropagat., 27(6), 808813, 1979.
[13] T. B. A. Senior and V. V. Liepa. Backscattering from tapered resistive strips.IEEE Trans. Antennas Propagat., 32(7), 747751, 1984.
[14] J. L. Volakis, A. Alexanian, and J. M. Lin. Broadband RCS reduction ofrectangular patch by using distributed loading. Electronics Letters, 28(25),23222323, 1992.