DTIC ADA422729: Bandwidth Limits and Other Considerations for Monostatic RCS Reduction by Virtual Shaping

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NSWCCD-64-TR-2004/04 Bandwidth Limits and Other Considerations for Monostatle RCE Reducatlon by Virtual Shaping

Naval Surface Warfare Center

Garderock Division
‘Weat Betheeds, MD 20817-5700

NSWCCD-64-TR-2004/04 January 2004
‘Survivabilly, Structures, and Materials Department
Technical Report

Bandwidth Limits and Other Considerations for
Monostatic RCS Reduction by Virtual Shaping

by
ALR. Swandic

20040503 056

Approved for puble rea; dlstibutlon fs untied
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7 ARE AND SUBTCE Ta CONTRACT HUMBER H
Borduidth ints and Other Considerations for Monostale ROS Reduction By

Viral Sharing

GT ONSET

PROGR ELEMENT MEER

= ROR PROCTER

Dr, domes R. Swan

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Contents

Abstract
Tntrodnetion .
Anpication of Refeterays Vrtal shaping
Bandwidd
Sidelobes
LacunarityiReflection from Exposed Dielectric Surface Area
Cost seni
Conclusion
References .

Figures

Figure 1. Refletaray concept. Frum RD. Javor, XD. Wu, and K, Chang
From THIE Tracs. AP43(8) pp. 952-939, Sepiember 1995

Figure 2. Geometry ofthe niceestipreflertaray. From DM. Porar, .D. Targonski,
and FLD. Sytigor, IKEE Trans, APAS2), pp. 287-296, February 1997.

Figuce 3. Phase af reflection cocficient at noma incidence fora petiodic amay
of square patches on grounded subate versus the patch side a at three
Feequenies (a= 14a, b= I mm.e,~ 1.05). From JA, Tncinay, EEE
‘Trans. AP49(10) pp. 1403-1410, October 2001

Figure 4. Phase of rfleotion coeficienl af nonmal incidence fora multilayer
periodic ruciure, dened in Figure 9(c) fortwo layers. vrwus the
Palch side ofthe eray closer to the ground plane. (25 =bi, 2

14 mun, by =hy = 3 me, = 1.05). (a) Two amay layers (ny

(6) Thro array layers (a = 0.73, O94, Fromm LAT
“Trans. AP4B(IO) pp. 1403-1410, October 2001

Figure 5. Refleced fickd phase versus patch length for an ifinite array of
anisrostrip patches wit plane wave incidence at various anges,
FromS.D, Targonsh and D.M. Pozar, ISEE Int. Symp. AP, Seatle, WA,
pp. TR20-1823, June 1998, ssn seo .

Tigure 6. Dual-potoried amay. From R.D, favor, X-D. Wa, and K Chang,
ABBE Trans. AP43(9) pp. 932-930, September 1995.

Figure 7. Mirosuip rflenarray with ectanguac patches of variable size to
control the phase. Fran 1.8. Eneinor, (EEE Trans, APAQ(LO)
pp. 1403-1410, October 2001...

igure &. Differential spatial phase delay ofrefleaarray. Peo S. Huang, Lie
Ant Symp. AP. Newport Beach CA, pp. 582-545, June 1995

igure 9, 1wo-layerrefcclaray using patches of variable size. (a) Refletaray
iTluminsled by a feed. (b) Multilayer structure, (¢) Periodic cel
From JA, Eusinar, IEEE Trans. AP49(10), pp. 1403-1410, October 2001

B
Contents

Figure 10 Thrce-layer sflecturay using patches of variable size.
(@) Reflectorray iluminated by a feed, (b) Multilayer strucwore,
From J.A. Encingr sad J.A. Zomoza, [EER Tram. APSI(7).
pp 1662-1654, Tuly 2003...

Page

19
Abstract

This work adtresses some major issues in the adaptation and application of microstrip
palch antenna aud refleetarmy technology ta radar cross section reduction by virtusl shaping
‘We define viral shaping la be any technique used fo vause the shape and orientation of an
object as it affects a scattered radar signal lo differ ftom the actual physical shape and orientation
‘of the object. This is accomplishod by introducing a linear phase gradient on the incident wave
‘sit i scotlered from various paris of the target surface, By thie method a vertical surface can be
‘made to appear to radar tobe skewed fom die vertical 16 is dome in conventional shaping for
‘adar cross section reduetion. Virwal shaping has the potential to reduce the megstive impacts
imposed by conventioal shaping on other ship design considerations such ws hydrodynamics,
stability, payloal, and arrangements while maintaining or cnbsacing the signature control
aspects. Micmasiip patch antenna and reflectartay technologies offer a theoretical and practical
basis for development of virtual shaping techniques; however, the goals of antenna design and
the adiacion characteristics of relectarrays when applied to conventional uscs of these
reclologies differ from those cousiderstions when applied o signature comiml and radar cross
section reduction, The major issues ofthese technologies thal become more eriticsl when
applied to virtual shaping incluile (2) Tumited bandwith, (2) high sidelobes, and (3) refleutions
ftom exposed dictecirie surfaces. This papct suramarizes efforts that have the poten to
extend these technologies to vietal shaping,

Introduction

“The two main methods to reduce the radar eross section (RCS) ofa target are shaping the
target geometry ta deflect the scutlered radar beam avsay from the incident direction (for
‘monostatic radars) and absorbing the incident radar beam in the target by using radar absorbing
‘materials or structures (RAM/RAS). Limits to shaping are impaved by the physical constraints
fof acro- and byelra-dyamics (for aircraft and for ships) and of intemal valume and stability
requirements forthe target platform. One possible way to circumvent these problems is to
‘decouple che radar scattering pattern fom the physical shape of the target by imposing a linear
phase gradient on the nge! ccatering elomicats; this gradieat serves to deflect the main lobe of
the soattered radar signal away ftom the radar receiver. Since absorption is per wavelength of
the incident radiation, sach a scheme my also he w aseful altemative for the absorption of low
frequency radar waves, which can require rather chick and heavy layers of materials. The
‘technique of causing the radar-apperent shape and orientation of an object to differ from its
mal physical shape and orientation by imposing a phase gradient om the reflestion from the
target surfaee will be referred to as vital shaping.

A proctical way'to produce a desired phase gradient on a surface (llat or eonformnal to &
‘curved surface) isto use a refleciarray composed of an array vf uicrostrip patches [1-3].
[Reflectarrays combine the properties and advantages ofcellectors and of phased array antennas,
‘Microstrip antennas are very thin conducting patches lying on a eonductor-backed dielectric
subinale thal ae fed by a coaxial probe through the substrate, Such a patch can also be excited
Iya epertare coupling a waveguide iio the substrate ar hy a microstrip transmission Tine on the
substrate surface that enmects to the mniorastrip palch. A reflectanray isa flat or conformal array
‘of microstrip palshes, with no feed lines or aperture coupling, on a thin conductex-backed
ielectvio substrate that is Ukuminated by a microwave horn antenna of other souree some
distance above the array. Esch patch of the array stalls the feed radiation with a different
‘phase so thatthe resulting array ecatcringis at difTereot angle than that of specular reflection
tnd the fh field appears to have been reflocted from a spherical or parabolic curved reflextor.
“The concept of a relectaray is shown in Figure | (taken ftom Reference 3), Radistion frum the
feed hom is incident om Uhe array, where each of the elements zeflets this radiation with a
different phase Gul is determined by the phase shifters. The geometry of a micros reflectarray
js shown in Fipire 2 (uiken fiom Reference 8). ‘Phe phases are chosen in this case so that the
reflected signal isa plane wave traveling in direction ro, The phase of the reflection coefficient
far radistion normally incident on a periodie array of square patchex vs. patch size for the three
frequencies 11.5, 12.0, and 12.5 GH2 is shovm in Figure 3; (has 4 muximom range tess than
‘360°, The place shifis at che same frequencies fora two layer and fora theee layer periodic amsay
are shown in Figure 4; Figures 3 and 4 arc taken from Reference 13. The additional layers
increase the muximum range of the phase shift and emooth the dependence of the phase variation
on palch size. This helps t increase the bandwidth ofthe reflectaray. As the soutee antenna
‘eed recedes fiom the array, ths system eventually comes to resemble that for monostatic RCS
seduction for an incident plane wave by relected beam deflstion threugh virtual shaping of The
reflectarray surface.

Application of Roflectarrays to Virtual Shaping

Roth microstrip antennas and reflectarrays have several desruble features, such as low
‘weight, small size, and ow prefile (60 they can be made conformal to an arbitrary surface), and
are thus quite migged. They also have Tow ost of design and of manftcture. Roth also have Ihe
major disadvantage ofa very narrow bandwidth, typically 2 few percent of the operating
frequency, Their broadside radiation pattern can also pessess high sidelobes. For many
applications, a wider bandwidth is required; im particular, this is true for RCS reduction by
virtual shaping of a surface by a relectaray of microstrip patches such as inthe S-band of 2— 4
GHz. For radar applicstions there are several related difficulties that must be overcome. These
inelude not only increasing the baadwidth for useful beam deflection, but slko (1) edacing the
sidelotes belins sire specified fevel, (2) minimizing Use reflection from the exposes dielecteic
area of the surCice, and (3) performing these functions tan affordable cost. These wilt be
addtessed in tum, starting with the major problem of increased bandwidth.

Bandwiath

‘There has been a great des af eff to wien the operational bandwidth of misrossip
antennas. All the methods used have tried te vervome the fundamental bandwith limitation st
doy the sual electial volume occupied by Ihe microstrip elements [4], Another way to express
{his ito realize that a mierostrp patch unten is #vesonan stuctare.ustelly with high Q [5],
thac is, do rntio of elecromaynetic energy stored in the vicinity of the etch to fhe
clootroiagnetic energy radiated of scattered (or otherwise dissipated) by the patch per eycle is
‘ery large, The three main approaches to producing microstrip antennas with improved
immpetence bandwidth ae [l-
1) Use of an increased antenna volume, ether by increasing the arca or by stacking,
2
such as by parasitic clements, overlay techniques, stepped substrates, ot an annular ring.

2) Use of matching techniques, such as employing multiple resonance effects or wing
the feed probe as a matching device

3) Use of elements with high interna! losses, such ax patch spirals or curved sections

‘Ae there is a strong correspondence betsicen the velusce occupied by a microstrip
element and its impedanes bandwith, utilization of the maximum volume available is «fist step
{in conformal (or plaran) antermmn design (6)

Sines a reflectarray is merely a reflecting devine, not raditing antenna, theres no need
for amy eure injection, This elimnntes one ofthe major causes of naerovs bandwith, but also
Drechudes widening the bandwidth by the use of paral elemens ot by using the food probe as
2 matching deviee. A eomron problem wih incfeasiug antenna volume by wing wicker
Substrate is the increased sifice wave loss for high permavty substrates an greater Teed.
probe loses for low permivity substrates [4]. As the subset thickness increaxes te resonant
Fespency decrees, so tat te frequency bandwidth around the rexonunt frequency inetcases
7A

ach microstrip patch slement ofthe rellectarray pasa different phave othe field it
scatters, For monoslaie RCS redtetion, a planc wave incident within some range of angles 10
the sort ofthe smay surface ust be deflected through an angle different than that af specular
fefleetion and large enough o avoid being bockscotered to the radar wensmitertaciver. A
quantitative satement of is requirement is that an S-band plane wave incident a any angle ®:in
the range -0y <0, ~the i, that ee Oy, he reflected outside this sang, s0 thatthe reflected
angle (ao longer equi o he specular reflection angle 6,) 8 ¢ Sy. The refleved field phase ve.
Zatch Jeng for an infinite array of micros pulches with plane waves incident at various
‘ules shown in Figure & (akon from Reference 38). To be specifi, leta wave incident at
“#6 bereflectod tthe angle 8, = y+ AB, where 40 is Jang chong te deflect ze main reflected
lobe otiside of Os. Since the specular reflection angle for this extreme ease is 8-0 — the
hace yradient mux deflect the reflsction through an angle 20y ~ 88. This sets the condition on
the linear phase gradient ofthe may that wl exist forall incident angles, € 4, vir, 2X
(sind, + vin8.) +ap'=0 forthe main lobe, where A is the ttl relative phase shift fference
‘over the enti refletasay surfice of extent L for wavelenath 2 of the radiation, For 3GH2
radiation, 2.= 10ony for L= 10m, Gy = 15°, ond AO= 5°, 8,= 040 20° and ab=20%60.),
‘which is many tens of times a 2a phase change, Hence. cir a very thiek multilayer of patches
ora set of suburays, each with a 2x or small multiple there) max itoum relive phase
<lifexence, 8 required to produce the necessary phase Uilerence, Such a malilayer may be
physically too thick tobe praciea, while aa assembly of such subarays produces @ much wider
Inn lobe seatcring pattern. The wid ofthe subareys in terms of relative phase shift may not
be, a least approximately, coustant forall froquoncies of invest ehh can drastically alter the
scattering pattern for he etre arsy. The seme pane dierence will exist fo all other incident
sxigles 1h particu, for a nonmally incident wove (@,= 0), the reflected wave wil be
approximately at angle 8, 28y,+A@. 354 similarly, a wave incident sf =O will be
reflected at approximately 8, = 38yy+ A@ = 50°.

‘One method to control this reflection phase i to altach a stub, of various lengths, to each
‘miczostip patch, as is shown in Figure 6 (taken from Refercuce 3). A better method ie to use
ppatehes of varions sizes to construct ths array. as shown in Figure 7(iaken from Reference 13).
‘Hoth mthods introgiuec « small phase shift in the resonant frequency of the element, which
produces a change in the phase af the scattered file [8]. The use of patches of variable size has
several advantages over the use of stub-tuned patches, auch as a wider bandwideh, much smaller
‘eross polarization, and aa need for surface space devcred to the stabs [9,101

‘The bandwidth ofa microstip reflectaray is limited primarily by two factors, the narrow
‘bandwidth of the microstrip clement itself andthe differential delay ofthe array {11}. This baler
‘concep is shown in Figure § (taken from Reference 11), For any puriculue incidence angle, the
UA (~ 0) dependence means thatthe relative phase difference of the incident wave is frequency
dependent [12]. For RCS reduction problems, this latter limit on the bandwidth is related to the
aperture fll dmc, the time (L’c}sin@, required fora plane wave incident al angle 8; to luminets
or to fill ho extize aperture of extent 1, wider bandwidh can be obtained by using a thick
substrate forthe palch, by stacking multiple patches, ond by using sequentially rotated subarray
clements [11]. The use of multiple layers of patches allows the phase of the reflected wave 10
‘vary over a range greater (or even mach greater) than 2n snd provides @ smoather dependence on
patch size [14,14]. Examples of two. and Umee-layer reflectarvays are shown in Figure 9 (taken
Trura Reference 13) and Hew 10 {taken fiom Refereacel4). Corupntation for analyzing
seaneting by maldlayered periodic structures bas also becn treated [15], as has analysis of
stacked microstip patches [16].

‘Another posable solution to the bandwidth problem isthe ure ofa Fact reflecurray,
‘hati, an array whose elements are distrbuted in 9 Bactal pattem. A fractal is an object with
<ilatiowtvansltion invariance (self similarity) of some sort: it ean be considered fo bave
structure at all (r fora wide range of) length scales [17,48]. Since the Factal patern is self
simile, it prodjoes a sel? similar scattering rater: for evecy change in wraysHength that outs
the soale of elf simitarity, dhe scattering pattern wilt be the same except for a widening af the
scattering lobe strnerurc dc tothe finite sizc of the factal objoc, and forthe cffects of
dispersion of the dielectric permitivity

Since a narrow bandwidth is such a severe constraint, especially for RCS applications of
icrostip relostarray devices, the fret order of business is lo detersine, or a lest to estienute,
he muximum pinaible bundsvidth range for which such a device can reduce by a specified
umount the RCS af a ship sinuctue, There are several related topics in various arcas of physics
that are velevani to the determination of this Kitt, ‘hese indude causality conditions [19-23},
time delay [24-26}, dwell time (27), and partial wave phase sbifis [28-30]. By causally
‘coudition is micant that there cun be no wcallared wave hefore the incident wave reaches the
soatforer [22]. A diffeenl form of Lhis condition is given by van Kampen {21], who requites that
the tolal probability outside the scatterer never exceed its iniual value; he also $20] considers the
scattering of photons by a spherically symmetric scatterer outside of whieh the interaction is
cexacily zero. Gell-Mann et af, [23] are more striagenc in that they require not only that waves
‘camo be scatiered before the incident wave atzives, but alsa that even after the arrival one unust
‘wait the appropriate rine wo recive a signal, Their causality requiterent is the quantum
‘mechanical formulation ofthe condition that waves do not propagate faster than the velocity of
Wight

‘The concept of time delay in sullering as inode by Eisenbud and Wigner [28]
Nonsstativistic quanlurm.r-wave soattering produces a phase shift (Fal energy E:the time
delay is given by At= 2(h2R) nya, where h is Phanch’s eomscant. Fora plane-wave packet
incident on a spherically symmetric seater that produces seating amplitude £8) in the
direction ©, the ime delay in Uns direction is piven by [1 | At ~ 2¢v2n) feng (FO) /0P. The
differenl clefivitims or inerpreations ofthe delay time arc discussed by Nuskenzveig [24]. Tle
goes on lo extend his previous work [25] to find the average time delay in electromagnetic
Seatering by spherically symmetric, eflotion-inverint,nov-absorbing seater. For the th
‘rtial wave in each polarization the tine delay isthe sper average of dy fs over the
Fncdeat wave packet, where ry isthe conceponding phase sifLund athe angular fequency. A
review of trae delay and elated concep, inching their many ypplications in physics, is given
by Carvatho and Nessenzveig [26]

Formany af the prublems ieated in these refereuces, spherical symmetry ofthe neuterer
or ofthe sentering potenti allows a straightforward partial wave expansion of the scattered
field, Tor the reflectarray problems of intcrest here, it may be betler ro use a plane wave angular
spectrum expansion forthe seatcred waves [32,33], There ix sill a seatterer of finite extent, but
itis no louger spherical, The reflectarray van be confined roa finite region that is bounded by z
{0 2=h in the z-direction (normal to the reflectatray surfiec) and by |x| <a.|y| <bior
Ixt+yl 9° +b’) ") in the transverse directions + and y. .Au increase in the thieluess
2 of the refloctaray can inuretse the bundwidth (reeall Fignres 3 and 4 for normal incidence),
‘while an increase inthe transverse dimension can deerease the bandwidth forall but nonmatly
incident radiation (due to the differential spatial phase delay (11,12).

Sidelobes

JLow sidelobes for antennas have been of imerest for elo rejection. They une alsu a
goed counter a ECM (electenic countermcasore), such us jamming 134]. ‘The two parts to
Aesigning a low sidelobe antomne pre (1) to choose the correct itmmination fuction to obtsin the
desired dosign (ermar-fre} sidelobes, and (2) ra control the phase and amplitude ervos Unat
produce the random sidelobes, which fundamcotally limits sidelobe performance [34]. The
ithmination function is defited and limited only by the allowed range of the incidence angles,
€ {2,30 it eannot he completely known, Purely random errors produce random sidelobes,
‘while corelated tandom: errors produec sidelobe energy thut is concentrated at discrete locations
in the fr field; this gives vise to higher sidelobes in a miled numberof locations [35]. The
‘actus influencing the realizable sidclobe perfomance of a microstip antenna array are
discussed by Pozar and Kanfinan [36]. It-should also be noted that they conehe their paper
with de comment [36]: “Finally, it might be noted thac this work represents a good example of
hove theoretical antlyses, on which most of the results in this paper are based, eat be appHed 10a
‘Practical hut difficult problem in antenna engineering that would probably remain unsolved if
sautacked with purely empirical techniques.”
“The use ofa fractal uray cun uo aid in the suppression of sidelober. Kim und Jugyard
[37] deseribe the ulvantages ofa factat array: “While random arrays are robust with respect to
‘loment location errors and failures they arc characterized by relatively high sidefobes. Uniform
‘or tapers periodic arrays posscss aclaively low sidclabos but ate sensitive to errors in location
tend tothe Yalues of the cxcitation currents, Here the virtues of eandom and periodic arays are
‘eombined by interjecting slf similarity jnlo randorn array theory to control the sidclobe radiation
pactern.” Kor example. an infinite Cantor array hos the some array factor at an infinite nvmnber of
‘bands (it is a multiband system, nol a frequency-independent syster); its behavior will he The
same at several bands, but will not be frequency-independent within each band [38]. A band
ted reatization of such a fractal structure will have similaviey propestcs through as many
bands as ilertioms used in tue constzactn oF the array [38], As the frequency of the radiation is
reuced (sw is wavelength is increased), the width ofthe main scattering lobe will increase. ‘The
‘secondary lobes af the seattoring pattem will uo be high; this Fearue is related to the array
characteristic known as Inconarity, A fructal stretute possesces high lacunarity von it has large
‘gaps between the differnt Fractal substructures (38)

Lacunartty/Reflection Fram Exposed Dielectric Surface Area

“Ve spatial scale of u fractal ceffectaay is set by the froquonsy range over which i is
‘operate, The highest operating frequency datcrmines the size of the smullest elements through
Their resonant frequency’ Tho size of the array and ity operational bandwidth detennine the
‘murabar of ireations of the gencrating pattem. For a fact array of sicrostrip patches on a
‘conduetar-backed dicleetrie sheet or substrate, high lacunarity can also produce atthe higher
‘operating frequencies significant specular veflecion from the large area obure divlectie surthce
between the conducting patches. Radiation incident on this bare dielectric will be reflected from
the bare surface or wil enter the substrate and wil then bc rcflected from the conducting ground
plane, In both casos there will be specular reflection that mnst he reduced in intensity. Ifthe
speculurlyceflecied field is included in the analysis [9] of un infinite artay of microstrip patches,
then atthe design frequency of the reflectarmay the scallered field from the patches is of nearly
‘equal amplimde and bas on approcimate phase difference of 180 degrees compared tothe
specularly reflected field; this effectively eliminates the spocular reflection [39] und may be
‘useful forthe problems considered here, The RCS of'¢yelch on an isotrapic substeate has been
calculated and compared to messinemen|s [40]. One possible way to address the problem of
scattering from a large surfice area of bure dielectric is to use a series of multilayers saranged in
such a way that they form a thick slab of a “tree” rather than just a“two"-dimenvicnal fructal
object. For example, u Cantor bar fetal vay could be placed at aright angle to anather such
array sbove (or belaw) it to reduce the amount of baro dielectric, Similarly, a series of layers
‘paved om the Sierpinski carpet can be arrmnged to form aslah of a “three” dimensional fractal,
somewhat lke the Menger sponge [17]. One way to achieve this isto displace each layer of
smicrosuips with respect to ike neighboring layers.

Cost

‘The lectmotogy for microstrip antounas and reflectarrays has existed for several decades;
construction methods are well known, and one of the major advantages of this technology is