Publicações em Antenas

Number 8 | February 2013

Analysis of Omnidirectional

Antennas with Radome Operating in LMDS Band for Signals of Digital TV

Úrsula C. Resende

Fernando J. S. Moreira

Jose R. Bergmann

Publicações em Antenas

Number 8 | February 2013

Analysis of Omnidirectional Antennas with Radome

Operating in LMDS Band for Signals of Digital TV

Úrsula C. Resende

Fernando J. S. Moreira

Jose R. Bergmann

CRÉDITOS

Publisher:

MAXWELL / LAMBDA-DEE Sistema Maxwell / Laboratório de Automação de Museus, Bibliotecas Digitais e Arquivos

http://www.maxwell.lambda.ele.puc-rio.br/

Editor: Jose Ricardo Bergmann

Capa:

Ana C ris tina Cos ta Ribeiro

© 2009 IEEE. Reprinted, with permission, from IEEE INTERNATIONAL MICROWAVE & OPTOELECTRONICS CONFERENCE (IMOC 2009), 2009. This material is posted here with permission of the IEEE. Such permission of the IEEE does not in any way imply IEEE endorsement of any of Pontifícia Universidade Catolica do Rio de Janeiro’s. Internal or personal use of this material is permitted. However, permission to reprint/republish this material for advertising or promotional purposes or for creating new collective works for resale or redistribution must be obtained from the IEEE by writing to pubs-permissions@ieee.org. By choosing to view this document, you agree to all provisions of the copyright laws protecting it.

Analysis of Omnidirectional Antennas with RadomeOperating in LMDS Band for Signals of Digital TV

Ursula C. ResendeDept. Electrical Engineering

CEFET-MGBelo Horizonte, MG, Brazil

resendeurs ula@des.cefetmg.br

Fernando J. S. MoreiraDept. Electronics Engineering

UFMGBelo Horizonte, MG, Brazilfernandomoreira@ufmg.br

Jose R. BergmannCETUCPUC-Rio

Rio de Janeiro, RJ, Brazilbergmann@cetuc.puc-rio.br

Abstract- This work considers the use of dual-reflector antennasfor omnidirectional coverage in a LMDS system base station tooperate with the distribution of digital TV signals. We investigatethe performance of omnidirectional antenna with and withoutdielectric radome. Numerical simulations based the method ofmoments are presented to illustrate the usefulness of theomnidirectional antennas to operate in LMDS systems. Theanalyses are conducted for a 20% bandwidth. It is used a radomewith thickness equal to 'A./2 and relative permittivity equal to 2.08.The analyses intend to demonstrate the radome influence in theantenna behavior across the operation band and to identify thebest radome thickness.

Keywords-Method of moments; omnidirectional reflectorantennas; radomes; LMDS, digital TV

I. INTRODUCTION

The recent growth of internet-related applications and theintroduction of digital TV by satellite with new interesting andinteractive possibilities have stimulated of development of newradio-based technologies such as local multipoint distributionsystems (LMDS). This technology has the advantage of a high­capacity of downlink and can be shared by many users in aflexible manner. LMDS is the broadband wireless technologywith cellular architecture that provides flexible high capacityconnections. The systems employ a point-to-multipointbroadcast downlink and can be used to deliver digital TVsignals, high-speed data, voice signals or video. The frequencyallocations are in the 28-29 GHz band in the United States [1].The European Standard for Antennas for Point-to-Multipointfixed radio systems proposes the use of several frequencybands (from 11 GHz up to 60 GHz), for which it makes sensethe use of reflector antennas as a feasible solution [2],[3].

Many antennas have been designed for these wireless point­to-multipoint applications based on the cellular radio concept.The cell base-station antenna is required to give uniformcoverage in the azimuth plane, which is achieved either with anomnidirectional antenna at the center of the cell or somesectorial coverage antennas. In [4] some antennasconfigurations are investigated for LMDS systems operation.For terminal multiflare horns with dielectric lens and shappedGregorian splash reflectors are presented. For base stationsomnidirectional single and dual shapped reflector solutions aredescribed. An omnidirectional antenna consisting of an axially­fed dual-rotationally symmetric reflector configuration was

978-1-4244-5357-3/09/$26. 00©20091 EEE

proposed [5] as an inexpensive solution for areas of limitedtraffic. In [6] a reflector antenna is presented as a feasiblesolution for LMDS systems base station with a reconfigurablenumber of sectorial beams. The antenna is an improved versionof the hourglass reflector antenna.

LMDS systems band frequency operation requires the useof reflector antennas as a feasible solution. Reflector antennascan yield compact designs capable of providing the widebandwidth required. Omnidirecional dual-reflector antennas aremore compact than single reflectors [7]-[10] and may reducethe feed return loss [8],[9]. There are four different types ofaxis-displaced dual-reflector antennas for omnidirectionalcoverage: OADC (omnidirectional axis-displaced Cassegrain),OADG (Gregorian), OADE (ellipse), and OADH (hyperbola)[7]-[10]. Among them, the OADC and OADE can lead tocompact geometries [8]-[10].

Radomes are used to protect antennas from a variety ofenvironmental and aerodynamical effects and may be designedto support the subreflector. However, they create undesirableblockage due to interactions of the antenna fields with theradome. Needless to mention, the radome affects the antennapattern and the feed return loss across the bandwidth. A carefulanalysis of the antenna-radome system is thus appropriate [11].Several techniques have been developed for the analysis ofantenna-radome structures to provide accurate design tools.Surface integral equations, numerically evaluated by themethod of moments (MoM), are one of the most accurateanalysis tool.

As OADC antenna configuration leads to compactgeometries, we investigate this antenna, with and withoutdielectric radome, to operate in LMDS systems. The antennageometry, illustrated in Fig. 1, is basically the same studied in[12]. However now we used a more efficient coaxial hom anddifferent radome thickness are analyzed in a different operationband. In this work we intend, beyond to demonstrate theradome influence in the antenna behavior across the operationband, also to identify the best radome thickness. The accurateanalysis is based on the electric (EFIE) and magnetic (MFIE)field integral equations, numerically evaluated by the methodof moments (MoM) technique. The OADC antennaconfiguration is excited by a TEM coaxial hom and is used aradome with relative permittivity equal to 2.08.

83

Vs = 80.26 mm

Subreflector /

z.

Ds = 175.48 mm

d= Ad/2

- r'''''''-''"' fo,,_~e:.;f:ix=at::.:io::.:n ~:.- =;:;::::=- .----__

Radome R;=4.5mm_

RE=11 .6mm_1

Coaxial Horn

"-J-1rr- Ds=5

.1 mm

IBC for TEM mode absorption

WA=69.94 mm

'I

Figure I Omnidirectional antenna geometry (OAOC)

II. SURFACE INT EGRA L EQUATIONS

MoM solutions involving the EFIE and MFIE lead to manydifferent formulations (13], [14]. The EFIE is the choice foropen conducting shells. For closed conducting surfaces thecombined field integral equation (CFIE) avoids spuriousresonances [14]. The CFIE is a linear combination ofEFIE andMFIE. For dielectric bodies, EFIE and MFIE can be linearlycombined in several forms [13]. One of the most adoptedcombinations are MUller and PMCHWT formulations [13].

Present numerical evaluation of the formulations isperformed by the MoM technique. Triangular basis functions(TBF) are employed for the equivalent current representationand Galerkin's method is adopted to numerically evaluate theMoM coefficients. All integrals appearing in the MoM full­matrix elements are evaluated by Gaussian quadratures withappropriate singularity treatment [15].

III. OMNIDIRECTIONAL ANTENNA WITHO UT RAOOME

The OADC antenna shown in the Fig. 1, without theradome, is analyzed for operation in a LMDS system across theoperation band of 26 to 34 GHz. The antenna dimensions aredescribed in Fig. 1 and were chosen for D, = 17.57Ae, DM =17.57Ae, Vs = 8.03Ae, WA = 7Ae, DB = 0.255Ae, R,= 0.45Ae andRE = 1.16Ae, where Ae is the wavelength in the vacuum forcentral frequency of de band, 30 GHz. It is used the hompresented in [16]. The radiation patterns obtained from theMoM analyses across the operation band are illustrated in Fig.2 and the Fig. 3 shows the maximum gain and the angle wheremaximum gain occurs (MGA). As it can be verified the overall

antenna performance (levels and spatial distribution of mainlobe and sidelobe) is not significantly affected across theoperation band. This fact enables this antenna configuration foroperation in LDMS systems, in principle.

IV. OMNIDIRECTIONAL A NTENNA WITH RAOOME

In this section the electrical performance of the OADCconfiguration with radome, illustrated in Fig. 1, is investigatedfor operation in the same frequency range. It is employed aradome with relative permittivity of 2.08. The radomethickness (d) is chosen for minimal reflection (i.e, the radomeis a half-wavelength, Ad, window), where Ad is the wavelengthin the dielectric [15]. In this work, three different radomethickness (d) are investigated, as described in Table 1. TheseThree values were calculated using wavelength in the dielectricradome at 26,30 and 34 GHz, respectively.

The radiation patterns of the antenna at 26, 30 and 34 GHzare shown in Figs. 4--6, respectively. These demonstrate thatantenna electric behavior (levels and spatial distribution ofmain lobe and sidelobe) is not significantly modified by theradome presence. Tables 2 and 3 and Fig. 7 show themaximum gain and angle where maximum gain occurs (MGA),respectively, at 26, 30 and 34 GHz for the radome thickness dpresented in Table 1. It can be observed that across theoperation band the antenna characteristics are kept withoutimportant modifications.

Fig. 8 illustrates the feed return loss across the bandwidth.The oscillatory behavior of the return loss across the band isexpected and caused by the electromagnetic coupling among

2009 SBMO/IEEE MTT-S International Microwave & Optoelectronics Conference (IMOC 2009) 84

--Without radome

-- d=3.99 mm

- - d=3.47 mm

. d=3.06mm

--Without radome

-- d=3.99mm

- - d=3.47mm

. d=3.06mm

II

I I I- - -1- - - T - - 1- - -

I I II I I

l I I I; 1 J.. __ --l _

I I II I I

I- _I _

III

80 90 100 110 120 130 1408(Degree)

708050

5

o

-5

10

o

10

5

-15

-10

-10

in:s-o:·iii -5o

Figure 6. Radiation pattern: DADC antenna with radome at 34 GHz

80 90 100 1108(Degree)

Figure 5. Radiation pattern: DADC antenna with radome at 30 GHz

34

120

3332

80 100

8(Degree)

Maximum Gain

60

29 30 31Frequency (GHz)

Angle where maximum gain occurs

28

40

2726

91

15

10

5

0

iii -5"0C·iii -10o

-15

-20

-25

Figure 2. Radiation patterns: DADC antenna whitout radome operatingin the LDMS system (26 to 34 GHz)

11.5

11

m"0

10.5C·iiio

10

9.526 27 28 29 30 31 32 33 34

Frequency (GHz)

Figure 3. DADC antenna whitout radome operating in the LDMS system (26to 34 GHz): a) Maximum gain b) AMG

T ABLE I. RADOME T HICKNESS

TA BLE Il l. DADC A NTENNA WITH R ADOME: MGA

TAB LE II. DADC A NTENNA WITH RADOME : MAXIM UM GAI N (oBI)

Rad ome Fr equeney (GH z)thi cknessd(mm) 26 30 34

3.99 87.8 90.6 90.6

3.47 88 90.2 9 1.8

3.06 88 89.8 91.4

Rad ome Frequeney (GHz)thi cknessd (mm) 26 30 33

3.99 10.28 10.11 9.97

3.47 9.28 10.49 10.05

3.06 9.90 10.15 9.98

Frequeney x, Radome thiekness(GHz) (mm) d(mm)

26 11.53 3.99

30 9.99 3.4734 8.82 3.06

--Without radome

-- d=3.99 mm- - d=3.47mm

d=3.00mmI

I

I I I I,~ __ L __ --J 1 _

I I II I II I II I

-- ------I I

5

10

80 90 100 110 120 130 140O(Degree)

Figure 4. Radiation pattern : DADC antenna with radome at 26 Gl lz

2009 SBMO/IEEE MTT-S International Microwave & Optoelectronics Conference (IMOC 2009) 85

34

II

I

I

I

I

I

I

I

33

--Sem radome- - 3.99 mm- - - -3.47mm

- - 3.06mm

,

/ '"\

29 30 31 32Frequency (GHz)

28

\ /

27

rr

r

/

rr,,

-25

-10

iii':g, -15:2.3E~ -20c::

-5 .-- ---,-- - r-- ---r- - .---- ----,-- - --.-- -----r- - ,Retum Loss

3433321

2827

-- Without radome

- - d=3.99mm9 L-----'-----L-----Y - - - - d=3.47 mm26 29

1--d=3.06mm

Angle where maximum gain occurs

91

9.5

ijj 10.5Ciii 1019

11.5 .-- ---,-- - r-- ---r- - .---- ----,-- - --.-- -----r- - ,

11

Maximum Gain

Figure8. Return Loss: OADC antenna whit radome.34333229 30 31Frequency (GHz)

282787 L-_---'---__L-_---'-__-'---_---'-__-'----"----'-__--'

26

Figure 7. OADC antennawhit radomeoperating in the LDMSsystem (26 to34 Gllz): a) Maximumgain b) AMG

feed structure, reflectors, and radome. As it can be verified atfrequencies of 26, 30 and 34 GHz the value of return loss forantenna without radome is approximated equal to antenna withradome with thickness equal to 3.99, 3.47 and 3.06 mm,respectively. This fact demonstrates that for frequency forwhich the radome was specified (frequency ofradome sintony ­FRS) it is almost invisible for antenna system (i.e., thereflections are minimum). When the frequency deviates fromthe FRS, the feed return loss increases, as expected. Across theoperation band, the smallest return-loss variation occurs for theradome with FRS in the center frequency (30 GHz). Antennaswith a FRS radome in the extremes of the operation bandpresent return loss above -1 OdS for some frequencies; whereasfor the radome with FRS at 30 GHz the return loss peak is-8.5dS, remaining below -1 OdS at most of the operation band.So, from the return loss point of view, the best radomethickness is equal to 3.47 mm.

V. CONCLUSION

This work studied dual-reflector antennas foromnidirectional coverage in a LMDS system to operation withsignals of digital TV. The performance of omnidirectionalantenna with and without dielectric radome was investigated.The rigorous analysis was based on electric (EFIE) andmagnetic (MFIE) field integral equations, numericallyevaluated by the MoM technique. The antenna systems wereexcited by an omnidirectional TEM coaxial hom. It wasverified that the electrical performance of the analyzedantennas was not significantly modified across a 20% operationbandwidth. From the retrun loss point of view, considering thefrequencies values analyzed across the operation band, the best

radome thickness is equal to 3.47 mm (llod/2 where A.d is thewavelength in the dielectric for central frequency of theoperation bandwidth).

REFERENCES

[I] A. Nordbotten, "LMDS Systems and Their Application," IEEECommun. Mag., vol. 38 no. 6, June 2000, pp. 150-54.

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[3] Antennas for Point-to-Multipoint Fixer Radio Systems in the II GHz to60 GHz Band. Part2: 24 GHz to 30 GHz, 2002.

[4] L. Haro, A. G Pino, 1. L Besada, A. M. Arias, 1. O. Rubinos, "AntennasFeasibilityStudy for a LMDS CommunicationSystem," IEEE Antennasand Propagation Society International Symposium, 1999.

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[6] J. A. Martinez-Lorenzo, M. Arias, O. Rubinos, J. Gutierrez, and A.Garcia-Pino, "A shaped and reeonfigurable reflector antenna withsectorial beams for LMDS base station," IEEE Trans. Ant. Prop., Vol.54, No.4, pp. 1346-1349, April 2006.

[7] F. 1. S. Moreira and 1. R. Bergmann, "Classical axis-displaced dual­reflector antennas for ominidirectional coverage," IEEE TransactionsonAntennasand Propagation, vol. 53, no. 9, pp. 2799-2808, Sept. 2005.

[8] J. R. Bergmann and F. 1. S. Moreira, "Simples design equations forominidireetional axis-displaced dual-reflectorantennas," MicrowaveandOptical TechnologyLetters, vol. 45, no. 2, pp. 159-163, April 2005.

[9] F. J. S. Moreira and J. R. Bergmann, "Axis-displaced dual-reflectorantennas for omnidirectional coverage with arbitrary main-beamdirection in the elevation plane," IEEE Transactions on Antennas andPropagation, vol. 54, no. 10, pp. 2854-2861 , October2006.

(10] F. J. S. Moreira, A. Prata, Jr. and 1. R. Bergmann, "GO Shaping ofomnidirectional dual-reflector antennas for a prescribed equi-phaseaperture field distribuition," IEEE Antennas and Propagation, vol. 55,no. I, pp. 99-106, January2007.

[IIJ V. B. Yurchenko, A. Altintas, and A. I. Nosich, "Numericaloptimizationof a cylindrical reflector-in-radome antenna system," IEEETransactions on Antennas and Propagation, vol. 47, no. 4, pp. 668-673,April 1999.

[12] U. C. Resende, F. J. S. Moreira, and O. M. C. Pereira Filho, "Analysis ofOmnidirectional Dual-Reflector Antennas with Radomes," 2007International Microwave and Optoelectronics Conference (IMOC 07),Salvador, BA, Brazil,pp. 70- 74, October 2007.

[13] J. Mautz and R. Harrington, "Electromagnetic scattering from ahomogeneous material body of revolution," AEO - Int. Journal ofElectronics and Communication, vol. 33, pp. 71-80, 1979.

(14] J. Mautz and R. Harrington, "Il-ficld, E-field and combined fieldsolutions for bodies of revolution," IEEE Trans. Antennas Propagat. ,vol. 27, no. 4, pp. 157-164, July 1979.

(151 U. C. Resende, Analise de antenas refletoras circularmente simetricascom a presenca de corpos dieletricos, Doctorate Thesis, Dept. ofElectronics Engineering, UFMG, Minas Geriais, Brazil, May 2007.

(16] J. R. Bergmann and F. 1. S. Moreira, "An Omnidirectional ADEReflector Antenna," Microwave and Optical Technology Letters, vol.40, no. 3, pp. 250-254, February. 2004

2009 SBMO/IEEE MTT-S International Microwave & Optoelectronics Conference (IMOC 2009) 86