Radial Line Slots Array Antenna (RLSA) Performance based on Different Dielectric Constants @ 12.5GHz

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World Applied Programming, Vol (3), Issue (6), June 2013. 205-211 ISSN: 2222-2510

©2013 WAP journal. www.tijournals.com

205

Radial Line Slots Array Antenna (RLSA) Performance

based on Different Dielectric Constants @ 12.5GHz

S.Z Iliya T.A. Rahman O. ElijahWireless Communication Centre,

Faculty of Electrical Engineering,Universiti Teknologi Malaysia (UTM),

81310 Skudai, Johor, Malaysia

Wireless Communication Centre,



Wireless Communication Centre,



[email protected] [email protected] [email protected]

Abstract: This paper suggests the importance of dielectrics materials in the optimization of RLSA antennas. Simulations of Ku band (11-14) GHz 20 degree beam squinted RLSA designs on CST 2010 using different

dielectric constants between εr = 1.4 and εr = 3.0 is done in this submission. Simulation results showed that dielectric constants have great effect in the design and overall radiation performance of the RLSA antennas. At

12.5GHz resonant frequency, higher dielectric constant records a drop in both efficiency and directivity of the

RLSA antennas.

Keywords: RLSA, Squint angle, Dielectric constant, Antenna performance, ku band, transient analysis

I. INTRODUCTION

Dielectric materials are widely used in the design of radial line slot array antennas (RLSA). Their primary effect is toreduce the risk of grating lobes formation [1] in the design which affects the overall performance of the antenna. They

provide a measure of their effect on a capacitor and relates to the permittivity of the material. Permittivity is a quantity

that describes the effect of a material on an electric field: the higher the permittivity, the more the material’s ability toreduce any field set up in it [2]. RLSA antennas are forms of planar antenna [3] their feed probe interfaces between the

coaxial feed line and the radial guide forming the body of the RLSA antenna. The feed converts power from TEM

transmission line mode into a TEM cavity mode, travelling outward within the slow wave structure (dielectric material)

whilst causing minimal reflection back into the coaxial transmission line. Fig 1 is a typical schematic of the RLSA radialcavity feed containing dielectric material.

Figure 1. Structure layout of RLSA cavity feed [4]

II. SLOTS ARRANGEMENT ON UPPER PLATE

Slots arrangements on the upper plate (radiating surface) are designed such that it couples much energy into the radiated

pencil beam as possible. Any energy not radiated by the slot surface is either absorbed by the guide’s wall or reflected back to the feed. Energy lost at the walls either escapes through the open edges of the radial cavity or is dissipated in

absorbing material placed at the cavity edges. The slots intercepts current on the upper waveguide surface to produce

radiation of desired polarization, thus coupling from the cavity to the slots occurs via the magnetic field given by theseexpressions [4]:

(2)

(2) (1)1 max

(1)1 1

1 max

( )( ) ( ) ( )

( )

k k k

k

H H H H

H

(1)



S. Z. Iliya, et al. World Applied Programming, Vol (3), No (6), June 2013.

206

Figure 2. Side and top view of a beam-tilted LP-RLSA [5].

Where:

2

g

k

Is the wave number in the radial cavity;(1)

1( ) H is the Hankel function of the first kind order 1,

(2)

1( ) H

is

Hankel function of the second kind order 1, and is the complex reflection coefficient for the interface formed at the

cavity boundary, max this interface is left open. For stability it is assumed that the argument k is sufficiently larger

than 1 [4] this allows for simplifying assumptions of the Hankel function as:

3( )(1)

41

3( )(2 )

41

2( )

2( )

j

j

e H

e H

(2)

Equation (1) can then be re written as:

max

3 3( ) ( [ 2 ]4 4

2 2( )

j k j k

k k e e H

(3)

The first term of equation (3) represents the primary outward travelling wave which is excited by the disc ended probe,

while the second term represents the secondary inner travelling wave which is considered as being produced by the

reflections from the cavity boundary at max

. It is ensured that the cavity is well terminated with absorbing materials

to reduce the reflection coefficient to a level considered negligible , for further simplification, it is assumed that the

reflection coefficient is significantly small ( 0 ) thus neglecting the second term we have the cavity wave

expression as:

3( )4

2( )

j k

k e H

(4)




207

Since the field has only components [4], there only exist surface currents associated with the field as radial currents

proportional to H .Software codes for the design implementation of RLSA antennas on CST 2010 is been written in

visual basic by [6]. High speed computers with recent specification: Intel i7, windows 7, 64 bits, 8 Gigabytes RAM

speed is utilized as platform for CST 2010. The code takes into account all the theoretical basis of the RLSA designtheories for the simulations of the antenna radiation performance at various frequencies of interest to the designer.

In these submission dielectric constants ( r ) within the range of values 1.4 3.0r are used as slow wave factors in

the design simulation of a 20 degree beam squinted RLSA antenna for direct broadcast service (DBS) applications at aresonant frequency of 12.5GHz, this gives a comparative platform for the different dielectrics explored in this

submission. Effects on the performance of a 20degree beam squinted RLSA antenna is simulated using CST 2010’s

transient solver and results itemized on table and graphs below. Interest is focused on directivity, efficiency and returnloss values recorded.

Table I. Varying dielectric constants of a 20 degree beam squinted LP-RLSA antenna design and related performance

Dielectric Constant (r ) Directivity (dBi) Radiation Efficiency (dB) Total Efficiency (%)

1.4 24.31 -0.2431 63.811

1.5 26.26 -0.2002 65.132

1.6 25.90 -0.2064 66.114

1.7 27.70 -0.1931 69.438

1.8 27.82 -0.2027 70.113

1.9 29.43 -0.2199 75.422

2.0 29.51 -0.2107 80.156

2.1 30.77 -0.2130 87.619

2.2 31.06 -0.2375 91.979

2.3 31.85 -0.2360 93.469

2.4 32.40 -0.2407 91.081

2.5 32.60 -0.2434 94.549

2.6 32.35 -0.2515 91.981

2.7 29.55 -9.5990 10.967

2.8 30.74 -0.2740 91.482

2.9 29.83 -0.2831 91.112

3.0 28.85 -0.2962 89.953

From table I; worst antenna efficiency performance is recorded at dielectric constant value ( r ) = 2.7. This dielectric

value recorded a contrasting constant return loss performance of -200dB across the entire band of frequencies covered inthis study. Due to scaling it is not possible for this return loss value to be seen amongst others in fig 4. This probably

would have been the cause in the poor efficiency value recorded for this dielectric constant in this study.

Fig 3 shows S11 simulations for dielectrics constants (ε) between 1.4 and 3.0 respectively@ 12.5GHz.




208

Frequency (GHz)

11.5 12.0 12.5 13.0 13.5

S 1 1 ( d B )

-50

-40

-30

-20

-10

0

f(GHz) vs er=1.4

f(GHz) vs er=1.6

f(GHz) vs er=1.8

f(GHz) vs er=2.5

f(GHz) vs er=2.9

f(GHz) vs er=3.0

Figure 3. S11 performance of some dielectric materials with constants values between ( r ) =1.4 and ( r ) =3.0.

As seen from fig 3.Best return loss performance for 12.5 GHz resonant frequency is recorded when a dielectric constantvalue of 2.5 was used in the design simulations of a 20 degree beam squinted LP-RLSA antenna with an efficiency value

of 94.549%.

Fig 4 is the simulated S11 results for dielectric constants between ( r ) = 2.3 and ( r ) =2.8 @12.5GHz

Frequency (GHz)

12.0 12.2 12.4 12.6 12.8 13.0

S 1 1

( d B )

-50

-40

-30

-20

-10

0

f(GHz) vs er=2.3

f(GHz) vs er=2.4

f(GHz) vs er=2.5

f(GHz) vs er=2.6

f(GHz) vs er=2.7

f(GHz) vs er=2.8

Figure 4. S11 dielectric performance @12.5GHz resonant frequency with dielectric constants between ( r ) = 2.3 and ( r ) = 2.8respectively.

It is also clearly seen from fig 4 that optimal S11 performance for 12.5GHz resonant frequency is achieved when a

dielectric with constant value ( r ) =2.33, this gave an efficiency of 93.469%.

Figs 3 and 4 clearly shows poor return loss performance recorded at dielectrics with low constants values less than ( r )

= 2.0. This poor performance may not be associated with losses in dielectrics since lower dielectric constants generally

exhibit low associated losses [2], [7], [8]. At resonant frequency dielectric losses tend to be more [2].

Fig 5 shows the radiation performance in terms of gain (dB) in relation to the beam squinted angle of 20deg considered

in this submission when ( r ) =1.4, ( r ) =2.5, and ( r ) =3.0.




209

ThetaT (degree)

-100 -50 0 50 100

D i r e c t i v i t y ( d B

i )

-30

-20

-10

0

10

20

30

ThetaT vs gain (dielectric =1.4)



Figure 5. Simulated Directivity (dB) versus 20deg beam squint angle.

Fig 6 is the radiation performance @20deg beam squinted angle, with ( r ) = 2.3, ( r ) = 2.5 and ( r ) = 2.8.

ThetaT (degree)

-100 -50 0 50 100

D i r e c t i v i t y ( d B i )

-40

-20

0

20




Figure 6. Simulated Directivity (dB) @ 20deg beam squinted angle.

III. SIMULATION, RESULT AND DISCUSSION

Figs 3 and 4 shows the return loss performance of some selected dielectric constants as seen from table I. with lower

dielectric constants at 20deg beam squint angle, deterioration in S11 is noticed. As the values of the dielectric constantsincreases, an improvement in S11 is seen from the CST 2010 simulations. A gradual drop in performance @ dielectric

values of ( r ) =3.0 and higher dielectric values is observed. This signifies poor RLSA antenna performance at higher

constants values. This may be attributed to the high losses incurred with higher dielectric constants values at resonant

frequencies [2]. It is worth noting that at higher values of dielectric constants, it is possible to reduce the size of the

antenna as recorded by [9], this would assist immensely in reducing aperture blockage associated with parabolic reflectorantenna.

Fig 7 shows variation in antenna directivity with increasing value of dielectric constant ( r )




210

1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 324

25

26

27

28

29

30

31

32

33

epsr

D i r e c t i v i t y ( d

B )

Figure 7. Directivity versus dielectric constants

Fig 7 displays antenna performance in terms of directivity as values of dielectric constant is increased from ( r ) =1.4 to

( r ) =3.0. It is easily seen that directivity tend to drop as values of dielectric constants is raised higher, beyond ( r )=3.0 for the Ku band frequency considered in this study.

Fig 8 shows the variation in efficiency as dielectric constants is increased

1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 310

20

30

40

50

60

70

80

90

100

epsr

e f f i c i e n c y ( % )

Figure 8. Efficiency versus Dielectric constants

From fig.8 a breakdown in efficiency is obvious at a dielectric constant value of ( r )=2.7; an improvement in efficiency

is seen as the dielectric constant is increased further. From ( r )= 2.8 and above a diminishing trend in efficiency is

observed as values of dielectric constants increases further from ( r ) =3 and above.

VI. CONCLUSIONS

CST 2010 transient solver’s simulations in this study have shown how important the choice of dielectric can be in RLSAdesigns. The choice as seen from the study has immense effects in the overall performance of the RLSA antenna; thus

informing why a careful identification and selection of dielectric constants values is necessary for an efficient design. It

is therefore recommended for RLSA designers to study the properties of the various dielectric constants based on theresonant frequency considered in studies of interest for effective and efficient antenna performance.




211

ACKNOWLEDGEMENTS

The authors wish to acknowledge all contributors of this study.

REFERENCE

[1] Prototypes Development for Reflection Canceling Slot Design of Radial Line Slot Array (RLSA) Antenna for DirectBroadcast Satellite Reception. 2003. Asia-Pacific Conference on Applied Electromagnetics (APACE 2003), Shah Alam,

Malaysia

[2] [Teaching and learning packages © 2004-2012 University of Cambridge

[3] ZhiNing Chen and Michael Y. W. Chia “Broadband Planar Antennas Design and Applications” Institute for Infocomm Research, SingaporeCopyright © 2006 John Wiley & Sons Ltd, The Atrium, Southern Gate, Chichester,West Sussex PO19

8SQ, England Telephone (+44) 1243 779777

[4] Davis, P., A linearly polarised radial line slot array antenna for direct broadcast satellite services. 2000. PhD ThesisSubmitted to the Department of Computer Science and Electrical Engineering, The University of Queensland, Australia

[5] Jose I. Herranz, Member, IEEE, Alejandro Valero-Nogueira, Member, IEEE, Felipe Vico, Member, IEEE, andVicent M.Rodrigo, Member, IEEE .Optimization of Beam-Tilted Linearly Polarized Radial-Line Slot-Array Antennas.2010. Antennasand Propagation Society International Symposium (APSURSI), 2010 IEEE . 2010. IEEE.

[6] Purnamirza, T., T. Rahman, and M. Jamaluddin 2012. The extreme beamsquint technique to minimize the reflection coefficient

of very small aperture radial line slot array antennas. Journal of Electromagnetic Waves and Applications, 26(17-18): p.

2267-2276.[7] Kaoru Sudo*, Jiro Hirokawa, and Makoto Ando. 2005. Analysis of a Slot Pair Coupling on a Radial Line Filled with Double-

Layer Dielectric Department of Electrical and Electronic Engineering, Tokyo Institute of Technology 2-12-1, O-okayama,

Meguro-ku, Tokyo, 152-8552 , Japan E-mail: ksudo©antenna.ee.titech.ac.jp

[8] J. M. Fernández González, P. Padilla, G. Expósito-Domínguez, and M. Sierra-Castañer, Member, IEEE. 2011. LightweightPortable Planar Slot Array Antenna for Satellite Communications in X-Band. IEEE Antennas and Wireless Propagation

Letters, vol. 10.

[9] M.D Rafi Ui Islam, Tharek Abd Rahman. 2008. A Novel and Simple Design of a Multilayer Radial Line Slot Array (RLSA)

Antenna Using FR-4 Substrate. 2008 Asia-Pacific Symposium on Electromagnetic Compatibility and 19th International Zurichsymposium on Electromagnetic compatibility.

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Radial Line Slots Array Antenna (RLSA) Performance based on Different Dielectric Constants @ 12.5GHz