571975.pdf

Hindawi Publishing CorporationInternational Journal of Antennas and PropagationVolume 2013, Article ID 571975, 8 pageshttp://dx.doi.org/10.1155/2013/571975

Research ArticleBroadband EM Performance Characteristics of Single SquareLoop FSS Embedded Monolithic Radome

Raveendranath U. Nair, J. Madhumitha, and Rakesh M. Jha

Computational Electromagnetics Laboratory, Centre for Electromagnetics, CSIR-National Aerospace Laboratories (CSIR-NAL),Bangalore 560017, India

Correspondence should be addressed to Raveendranath U. Nair; [email protected]

Received 15 June 2012; Revised 11 February 2013; Accepted 18 February 2013

Academic Editor: Stefano Selleri

Copyright © 2013 Raveendranath U. Nair et al. This is an open access article distributed under the Creative Commons AttributionLicense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properlycited.

Amonolithic half-wave radome panel, centrally loadedwith aperture-type single square loop frequency selective surface (SSL-FSS),is proposed here for broadband airborne radome applications. Equivalent transmission linemethod in conjunction with equivalentcircuit model (ECM) is used for modeling the SSL-FSS embedded monolithic half-wave radome panel and evaluating radomeperformance parameters. The design parameters of the SSL-FSS are optimized at different angles of incidence such that the newradome wall configuration offers superior EM performance from L-band to X-band as compared to the conventional monolithichalf-wave slab of identical material and thickness. The superior EM performance of SSL-FSS embedded monolithic radome wallmakes it suitable for the design of normal incidence and streamlined airborne radomes.

1. Introduction

Enhancement of radome performance parameters overbroadband is a major challenge in the design of airborneradomes formodern radar antenna systems. Over broadbandof frequencies, major goals to achieve are high transmissionefficiency, low cross-polar levels, and low boresight errorcharacteristics with possible modifications of radome wallconfigurations. Many techniques were reported for modi-fying the radome wall configurations to enhance the EMperformance of radome walls [1, 2]. Generally, arrays of thinparallel metallic wires and wire meshes were used for thebroadbanding of radomes.The application of single-wire gridand double-wire grid was proposed for the broadbanding ofthin dielectric slabs [3]. The EM performance of modified A-sandwich panels bymatching core to that of the skinwas stud-ied by Chase [4]. Monolithic radome panels, centrally loadedby a periodic array of conducting inclusions, offered a widerfrequency bandwidth compared to the conventional radomedesigns of the same material and thickness [5, 6]. In someother applications, resonant walls made of perforated metalsheet structures were embedded within the dielectric wall orfixed behind the dielectric radome panel. Such structures had

narrow bandwidth but provided better performance at thehigh-incidence angles.

Most of the reported works focused on the enhancementof transmission efficiency of the radome panels. Even thoughthe transmission efficiency of the radome wall configurationscan be improved with the inclusion of metallic structures,other performance parameters like boresight error, cross-polar level, and so forth, may degrade drastically. Thisnecessitates the development of new techniques, which cansimultaneously improve the EM performance parameters ofradome walls. In this regard, FSSs find potential applicationsin the design of high-performance radomes due to theirinherent frequency selective characteristics [7]. In our previ-ouswork reported elsewhere [8], circular FSSwas used for theenhancement of EM performance parameters of monolithichalf-wave radome over X-band. For the equivalent trans-mission line model of circular FSS embedded radome wallstructure, the circular FSS was considered as an inductiveelement and hence the inductive susceptance of circular FSSwas incorporated in the EM analysis.This representationmaynot be valid for all the frequencies considered in the X-band.

In the present work, novel application of aperture typesingle square loop (SSL) frequency selective surface for

2 International Journal of Antennas and Propagation

𝑑FSS

𝑑𝑝

𝑔

𝑤

(a) (b)

Figure 1: (a) Schematic of the single square loop (SSL)-FSS and (b) Monolithic slab of half-wave thickness. SSL-FSS is centrally loaded w.r.t.the midplane. Here 𝑝: periodicity; 𝑑FSS: thickness of SSL-FSS; 𝑔: gap length; 𝑑: length of SSL element; and 𝑤: width of SSL element.

FSS

EM wave

(a)

𝑍𝑠 𝑍𝑠

𝑍𝑠 𝑍𝑠

𝑋𝑟Source

Φ1 Φ1

𝑍0

𝑍0

(b)

Figure 2: (a) Schematic of the monolithic half-wave slab centrally loaded with SSL-FSS and (b) its equivalent circuit.

broadbanding of monolithic half-wave radome wall config-uration is presented (Figure 1). Here the reactance (whichconsists of both inductive and capacitive components) ofthe SSL-FSS element is included in the EM performanceanalysis of SSL-FSS embedded radome wall. Hence, it offersa better representation of the SSL-FSS element in the equiv-alent transmission line model of the cascaded radome wallconfiguration (Figure 2).

Monolithic half-wave radome wall is generally used forthe design of streamlined nose-cone radomes as it providessuperior EM performance at the design frequency and suf-ficient structural rigidity for catering extreme aerodynamicstresses at high Mach speeds. But the major disadvantage ofmonolithic half-wave wall is very narrow bandwidth, whichlimits its applications. Since modern airborne radar antennashave multiband functional requirements, such designs maynot be sufficient. Hence, in the present work, the designparameters of SSL-FSS embedded monolithic half-wave wallconfiguration are optimized such that they provide superiorEM performance as compared to monolithic radome panelalone over the entire frequency range from L-band to X-bandand over a range of incidence angles.

It is observed that for a given radome wall configurationwith identical materials and dimensions, the radome EMperformance characteristics are generally superior for parallelpolarization, as compared to perpendicular polarization at a

given incidence angle [1]. In other words, the performancedegradations for a given radome wall configuration may bemore for the perpendicular polarization case as compared tothe parallel polarization case. Hence, in the present work, theEM performance parameters for perpendicular polarizationare analyzed.

2. EM Modeling Aspects

Monolithic half-wave radome wall considered is made oftypical glass epoxy (𝜀

𝑟= 4.0; tan 𝛿e = 0.015). The radome

wall thickness (𝑑m), optimized for power transmission at themidfrequency ofX-band and at normal incidence, is 7.44mm.It is selected based on the fact that the half-wave radome wallthickness at the lower frequency range is very large, therebymaking the radome panel heavy and conflicting with theweight requirements. The single square loop FSS, consistingof periodic arrays of square loop apertures, is centrally loadedin the monolithic half-wave radome wall.

The SSL-FSS embedded monolithic radome wall is con-sidered as an equivalent transmission line with differentsections corresponding to slab and FSS structure (Figure 2).The change in the characteristic impedance of the free spaceand monolithic half-wave wall is a major source of reflectionof the wave incident on the structure. In the present radome

International Journal of Antennas and Propagation 3

wall structure, SSL is symmetrically loaded in the midplaneof the monolithic slab.

As compared to the free space, different layers of SSL-FSS embedded monolithic wall can be considered as lowimpedance lines connected end to end. Hence, the wholeconfiguration can be represented by a single matrix obtainedby the multiplication of matrices corresponding to the indi-vidual layers.

Since the SSL-FSS is located in the midplane of themonolithic slab, it is considered to be made up of twoidentical sections with SSL-FSS in between them. Let 𝑍

0be

the characteristic impedance of free space. The characteristicimpedance of each half-section of monolithic slab is repre-sented by 𝑍

𝑠. In the equivalent transmission line method,

the matrix consisting of 𝐴𝑖, 𝐵𝑖, 𝐶𝑖, and 𝐷

𝑖will represent the

𝑖th layer of the radome wall. Let 𝑡 and 𝑑FSS represent thethickness of each half-section ofmonolithic slab and SSL-FSS,respectively. Here, 𝑡 = (𝑑m − 𝑑FSS)/2.

One half-section of the wall is represented by

[𝐴1

𝐵1

𝐶1

𝐷1

] =[[[

[

cos Φ1

𝑗𝑍𝑠

𝑍0

sin Φ1

𝑗𝑍0

𝑍𝑠

sin Φ1

cos Φ1

]]]

]

, (1)

where Φ is the electrical length corresponding to each layer,which is a function of the complex permittivity, 𝜀

∗, ofdielectric layer, the angle of incidence, 𝜃, and the thicknessof the dielectric layer, 𝑑. The other half-section of the wall isrepresented by

[𝐴2

𝐵2

𝐶2

𝐷2

] =[[[

[

cosΦ2

𝑗𝑍𝑠

𝑍0

sinΦ2

𝑗𝑍0

𝑍𝑠

sinΦ2

cos Φ2

]]]

]

. (2)

The interaction of incident EM waves with aperture type FSSis represented as a wave propagating along a transmissionline, with shunt reactance representing the FSS. The EMcharacteristics of single unit cell of SSL-FSS are assumed tobe same as that of infinite periodic array.Themonolithic half-wave slab exhibits a capacitive reactance, while the aperturetype SSL-FSS exhibits inductive reactance.The design param-eters of the aperture type SSL-FSS are optimized in sucha way that the inductive reactance of SSL-FSS matches thecapacitive reactance of the entire monolithic slab to providebroadband EM performance. Let 𝐴FSS, 𝐵FSS, 𝐶FSS, and 𝐷FSSbe the elements of the matrix representing aperture type SSL-FSS. Then, the SSL-FSS is represented by the following:

[𝐴FSS 𝐵FSS𝐶FSS 𝐷FSS

] = [

[

1 0

1

𝑗𝑋𝑟

1]

]

. (3)

Here, 𝑋𝑟represents the reactance of the SSL-FSS, which

is a function of inductive reactance 𝑋 and capacitive sus-ceptance 𝐵. The equivalent circuit modeling of SSL-FSS was

Table 1: Design parameters of SSL-FSS.

Angle ofincidence

Length,𝑑 (mm)

Width,𝑤 (mm)

Gaplength,𝑔 (mm)

Period,𝑝 (mm)

Thickness,𝑑FSS (mm)

0∘ 13.20 3.0 2.0 15.20 3.030∘ 10.25 3.0 1.0 11.25 3.045∘ 12.00 3.0 1.0 13.00 3.060∘ 17.00 3.0 1.0 18.00 3.0

discussed in [9]. Using this modeling approach, 𝑋 and 𝐵 aregiven by

𝑋 =𝑝 cos 𝜃

𝜆[ln(

1

sin (𝜋𝑤/2𝑝))] ,

𝐵 =4𝑝 sec 𝜃

𝜆[ln(

1

sin (𝜋𝑔/2𝑝))] .

(4)

Here, 𝑤, 𝑔, and 𝑝 are the width, gap width, and period ofthe SSL-FSS, respectively. 𝜆 is the wavelength of the incidentEMwave. In order to eliminate spurious reflections and phasedistortions, the medium present in the aperture sectionsof the SSL-FSS is assumed to be the same as that of themonolithic radome wall. The entire monolithic half-wavewall with SSL-FSS is represented by [10] as follows:

[𝐴 𝐵

𝐶 𝐷] = [

𝐴1

𝐵1

𝐶1

𝐷1

][[

[

1 0

1

𝑗𝑋𝑟

1

]]

]

[𝐴2

𝐵2

𝐶2

𝐷2

] . (5)

Using (3), the 𝐴, 𝐵, 𝐶, and 𝐷 parameters of the final matrixare computed.The power transmission coefficient is given bythe following:

𝑝𝑡𝑟

= [4

(𝐴 + 𝐵 + 𝐶 + 𝐷)2] . (6)

The power reflection coefficient is given by the following:

𝑝𝑟𝑓

= [𝐴 + 𝐵 − 𝐶 − 𝐷

𝐴 + 𝐵 + 𝐶 + 𝐷]

2

. (7)

The phase distortions are determined by the insertionphase delay (IPD) of the radomewall. For the present radomewall configuration, two symmetric dielectric layers and oneFSS layer are cascaded. Hence, the insertion phase delay isgiven by the following:

IPD = −∠𝑇 −2𝜋

𝜆(2𝑡 + 𝑑FSS) cos 𝜃. (8)

Table 1 shows the optimized design parameters of theSSL-FSS over 1 GHz–12GHz at normal incidence, 30∘, 45∘,and 60∘. Here, width 𝑤 and 𝑑FSS are kept constant from thefabrication point of view. Other design parameters (𝑑, 𝑔, and𝑝) are optimized in such a way that the resulting FSS-basedradome wall configuration would provide maximum powertransmission and minimum power reflection and insertionphase delay over the specified frequency range.


1 2 3 4 5 6 7 8 9 10 11 120

20

40

60

80

100

Slab alone

SSL-FSS embedded slab

Frequency (GHz)

Pow

er tr

ansm

issio

n (%

)

(a)

1 2 3 4 5 6 7 8 9 10 11 120

20

40

60

80

100

Slab alone


Frequency (GHz)Po

wer

refle

ctio

n (%

)(b)

1 2 3 4 5 6 7 8 9 10 11 120

50

100

150

200

Slab alone


Frequency (GHz)

Inse

rtio

n ph

ase d

elay

(deg

)

(c)

Figure 3: (a) SSL-FSS embedded monolithic half-wave slab design results in higher power transmission characteristics at normal incidenceover a broadband (1 GHz–12GHz) as compared to the slab alone case. (b) SSL-FSS embedded monolithic half-wave slab design results inlower power reflection characteristics at normal incidence over a broadband (1 GHz–12GHz) as compared to the slab alone case. (c) SSL-FSSembedded monolithic half-wave slab design results in superior insertion phase delay characteristics at normal incidence over a broadband(1 GHz–12GHz) as compared to the slab alone case.

3. Numerical Results and Discussion

Here, ∠𝑇 is phase angle associated with the voltage transmis-sion coefficient of the entire radome wall. It is noted that thelength, 𝑑, and gap width, 𝑔, of SSL-FSS are critical designparameters and any slight variations in these dimensions alterthe EM performance parameters drastically.

The EM performance parameters (power transmission,power reflection, and insertion phase delay) of themonolithicconfiguration with SSL-FSS are computed for perpendicularpolarization over a range of incidence angles (0∘, 30∘, 45∘,and 60∘). The incidence angles in the range 0∘–30∘ are typicalfor normal incidence radomes (hemispherical and cylindri-cal shapes), 0∘–45∘ for paraboloidal radomes, and 0∘–60∘


1 2 3 4 5 6 7 8 9 10 11 120

20

40

60

80

100

Slab alone


Frequency (GHz)

Pow

er tr

ansm

issio

n (%

)

(a)

1 2 3 4 5 6 7 8 9 10 11 120

20

40

60

80

100

Slab alone


Frequency (GHz)Po

wer

refle

ctio

n (%

)(b)

1 2 3 4 5 6 7 8 9 10 11 120

50

100

150

200

Slab alone


Frequency (GHz)

Inse

rtio

n ph

ase d

elay

(deg

)

(c)

Figure 4: (a) SSL-FSS embedded monolithic half-wave slab design results in higher power transmission characteristics at 30∘ over a broadband(1 GHz–12GHz) as compared to the slab alone case. (b) SSL-FSS embedded monolithic half-wave slab design results in lower power reflectioncharacteristics at 30∘ over a broadband (1 GHz–12GHz) as compared to the slab alone case. (c) SSL-FSS embedded monolithic half-wave slabdesign results in superior insertion phase delay characteristics at 30∘ over a broadband (1 GHz–12GHz) as compared to the slab alone case.

for the highly streamlined radomes (conical, ogival radome,etc.). A comparative study of the EMperformance of SSL-FSSmonolithic half-wave radome with that of monolithic slabalone is presented in the following sections.

3.1. EM Performance at Normal Incidence. The power trans-mission characteristics of the monolithic slab alone and SSL-FSS embedded monolithic slab over the frequency range

1 GHz–12GHz at normal are shown in Figure 3(a). Thepower transmission characteristic of monolithic slab alone,at normal incidence is high, around 95% at 1 GHz, anddecreases gradually to the minimum around 5GHz. There-after, the power transmission efficiency gradually increasesto a high value at the design frequency (10GHz) of themonolithic slab. With the incorporation of SSL-FSS, thetransmission efficiency of the monolithic half-wave panel hasbeen improved considerably throughout the frequency range,


1 2 3 4 5 6 7 8 9 10 11 120

20

40

60

80

100

Slab alone


Frequency (GHz)

Pow

er tr

ansm

issio

n (%

)

(a)

1 2 3 4 5 6 7 8 9 10 11 120

20

40

60

80

100

Slab alone


Frequency (GHz)

Pow

er re

flect

ion

(%)

(b)

1 2 3 4 5 6 7 8 9 10 11 120

50

100

150

200

Slab alone


Frequency (GHz)

Inse

rtio

n ph

ase d

elay

(deg

)

(c)


and it is superior to those for the slab alone case. However,the transmission efficiency of SSL-FSS embedded structureshows gradual decrease from 1GHz to 12GHz.

The power reflection is almost negligible in the frequencyrange from 1GHz to 10GHz and it shows slight increasebeyond 10GHz (Figure 3(b)). For monolithic slab alone,the power reflection characteristic demonstrates oscillatorynature. It starts increasing from 1GHz, reaches maximum(around 35%) at 5GHz, followed by decrease to a very

low value at the design frequency of 10GHz, and increasesthereafter. It is interesting to note that the power reflectionof SSL-FSS embedded structure is very low (well within 5%)throughout the frequency range.

3.2. EM Performance at 30∘. The EM performance charac-teristics of the SSL-FSS embedded and slab alone structuresat the incidence angle 30∘ are shown in Figures 4(a)–4(c).Similar to the normal incidence, the power transmission


1 2 3 4 5 6 7 8 9 10 11 120

20

40

60

80

100

Slab alone


Frequency (GHz)

Pow

er tr

ansm

issio

n (%

)

(a)

1 2 3 4 5 6 7 8 9 10 11 120

20

40

60

80

100

Slab alone


Frequency (GHz)

Pow

er re

flect

ion

(%)

(b)

1 2 3 4 5 6 7 8 9 10 11 120

50

100

150

200

Slab alone


Frequency (GHz)

Inse

rtio

n ph

ase d

elay

(deg

)

(c)


efficiency of SSL-FSS embedded panel is much better thanthat of slab alone. As compared to the normal incidencecase, the power transmission efficiency of SSL-FSS has beenimproved.

The power reflection of SSL-FSS embedded panel isalmost zero throughout the entire frequency range, whichis highly desirable for radome applications. This is in con-trast to the slab alone case for which the power reflectioncharacteristic is oscillatory. Very low reflection is essential for

reducing the sidelobe level degradation and elimination offlash lobes. As in the case of normal incidence, the IPD ofSSL-FSS embedded panel is lower than that ofmonolithic slabalone structure. But the IPD of SSL-FSS embedded panel hasbeen reduced as compared to that of normal incidence case.

3.3. EM Performance at 45∘. The power transmission, powerreflection, and IPD characteristics of SSL-FSS embedded


monolithic slab and slab alone at 45∘ are shown in Figures5(a)–5(c), respectively. Similar to the previous cases, thepower transmission of SSL-FSS embeddedwall ismuch betterthan that of slab alone throughout the frequency range.The power reflection for the modified structure is also verylow (close to zero) in the entire frequency range. Regardinginsertion phase delay, the SSL-FSS embedded wall showsbetter performance as compared to the slab alone.

3.4. EM Performance at 60∘. TheEMperformance character-istics of both SSL-FSS embedded and slab alone structuresat high incidence angle 60∘ are shown in Figures 6(a)–6(c).As compared to the previous incidence angle cases, there isa decrease in the power transmission efficiency of SSL-FSSembedded radome panel. Even though the power reflectionof SSL-FSS embedded wall is very low as compared to that ofslab alone case, it has increased in the high-frequency region.

The IPD of SSL-FSS embedded wall is also lower thanthat of slab alone. It is observed that the IPD of SSL-FSSembedded structure tends to fall in the higher frequencyend. These results indicate that the SSL-FSS embeddedmonolithic half-wave wall will offer better boresight error(BSE) characteristics (which is an associated parameter ofIPD), as compared to monolithic radome case alone.

4. Conclusions

The EM analysis shows that this novel application of SSL-FSS radome design is capable of achieving broadband-ing of monolithic half-wave radome panels over a widerange of incidence angles such as those found in advancedaerospace/aircraft applications. Thus this radome structurehas overcome the major limitation of the conventionalmonolithic half-wave radome wall, that is, the very narrowbandwidth. Considering the EM performance parameters,the SSL-FSS is found to be a better choice as comparedto circular FSS [8] for radome broadbanding application.Moreover, the presence of FSS in the radome wall offersmore structural rigidity, which an advantage for the airborneapplications. The present work also shows that the SSL-FSSembedded monolithic half-wave radome wall configurationhas potential application in the design of both the normal-incidence radomes and the highly streamlined nose-coneradomes.

References

[1] R. H. J. Cary, “Radomes,” in The Handbook of Antenna Design,A. W. Rudge, K. Milne, A. D. Olver, and P. Knight, Eds., PeterPeregrinus, London, UK, 1982.

[2] D. J. Kozakoff, Analysis of Radome Enclosed Antennas, ArtechHouse, Norwood, Mass, USA, 2009.

[3] M.A. Teichman, “Designingwire grids for impedancematchingof dielectric sheets,”Microwave Journal, vol. 11, pp. 73–78, 1968.

[4] V. A. Chase, “Investigation of improved composites for radomeapplications,” in Proceedings of 13th Symposium on Electromag-neticWindows, pp. 23–28, Georgia Institute of Technology, 1976.

[5] A. Frenkel, “On the scaling of inclusions size in metal-dielectricwindows at normal incidence,” IEEE Antennas and WirelessPropagation Letters, vol. 1, pp. 26–27, 2002.

[6] A. Frenkel, “Thick metal-dielectric window,” Electronics Letters,vol. 37, no. 23, pp. 1374–1375, 2001.

[7] B. A. Munk, Frequency Selective Surfaces: Theory and Design,Wiley, New York, NY, USA, 2000.

[8] R. U. Nair, T. Madhulatha, and R. M. Jha, “Novel applicationof circular frequency selective surface for broadbanding ofmonolithic radome,” in Proceedings of the 7th InternationalSymposium on Management, Engineering and Informatics (MEI’11), vol. II, pp. 40–44, Orlando, Fla, USA, 2011.

[9] M. Ohira, H. Deguchi, M. Tsuji, and H. Shigesawa, “Analysisof frequency selective surface with arbitrarily shaped elementby equivalent circuit model,” Electronics and Communicationsin Japan, vol. 88, no. 6, pp. 9–17, 2005.

[10] F. Chen, Q. Shen, and L. Zhang, “Electromagnetic optimaldesign and preparation of broadband ceramic radome materialwith graded porous structure,” Progress in ElectromagneticsResearch, vol. 105, pp. 445–461, 2010.

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