Hepta-band DRA Loaded Minkowski Fractal Slot Antenna

S. Dhar, K. Patra, S. Sankaralingam, B. Gupta, D.R. Poddar

Dept. of Electronics and Telecommunication Engineering Jadavpur University, Kolkata – 700 032, India

sayan.dhar@gmail.com, kaushikpatra294@gmail.com, slingam.ju@gmail.com, gupta_bh@yahoo.com,

drp_ju@yahoo.com

R. Ghatak Dept. of Electronics and Communication Engineering

National Institute of Technology, Durgapur Durgapur – 713 209, India

rowdra.ghatak@ece.nitdgp.ac.in

Abstract—A hepta-band dielectric loaded Minkowski Fractal Slot Antenna is presented in this article. Fractal geometry is utilized to generate multiple frequency bands conforming to various wireless standards. The idea behind placement of the dielectric resonator is to increase the impedance bandwidth at the upper frequency band. The proposed antenna exhibits a hepta-band performance over the frequency range of (900 MHz -7.5 GHz) with a gain variation of 1.1 – 3.1 dBi.

Keywords—Dielectric Resonator Antennas, Minkowski fractal, Slot Antenna

I. INTRODUCTION With an increased demand of bandwidth for voice and data

applications wideband and multiband antennas are being preferred over single band antennas. Also with most systems supporting multiple wireless standards it is imperative to employ antennas which can cover these bands. However, some wideband antennas tend to cause interference with other antennas by radiating outside its intended band of frequencies. In this respect multiband antennas may be used to avoid this problem by using an antenna to radiate at specific frequencies only. Multiband antennas have been a topic of extensive research over the past decade, with techniques such as etching slits on the radiating patch or on the ground plane [1-2], using stacked patches [3-4], reactively loading the antenna with shorting pins [5-6] and employing fractal geometry on antennas [7-8].

Since, B.B. Mandelbrot [9] coined the term fractal to describe a class of complex geometries that are created through successive iterations of applying a geometric generator to a simple initiator geometry, antenna designers have utilized these structures to obtain multiband behavior making use of their inherent self-similar properties. This article, presents an extension of the previous work done by the authors on multiband behavior of fractal geometry on a CPW fed slot loop antenna [10, 11]. In this work the authors try to overcome the problem of low gain by placing a dielectric resonator on top to enhance the gain as well as the bandwidth of the antenna at the higher frequencies. Apart from exhibiting multiband behavior, fractal geometry also results in miniaturization of the antenna

structure. The dielectric loading is used to enhance the bandwidth in the upper frequency band. The proposed antenna is designed to cover GSM 900 (890-960 MHz), PCS 1900 (1850-1990 MHz), IEEE 802.11b/g/n (2.4 – 2.485 GHz), WiMAX 3.5 (3.4 – 3.6 GHz), IEEE 802.11a/h/j/n (5.15-5.85 GHz) among others.

II. ANTENNA DESIGN PROCEDURE The geometry of the Minkowski fractal slot loop antenna is

shown in Fig. 1 while its fabricated prototype is shown in Fig. 2. The fractal slot loop is etched on an FR4 substrate with relative permittivity εsub = 4.4 and thickness t = 1.6 mm having dimensions L x W = 100mm x 100mm. The 50Ω CPW feed line is designed to have a central conductor width of s = 4.2 mm and a gap width of g = 0.4 mm. The linear dimensions of the slot loop antenna are chosen such that its perimeter is approximately equal to one guided wavelength. Considering

(a)

Fig. 1 Top view and side view of the proposed CPW fed Minkowski fractal slot antenna loaded with a dielectric slab

(a) (b)

Fig. 2 Fabricated prototype of the proposed antenna (a) top view and (b) bottom view

978-1-4673-5820-0/13/$31.00 ©2013 IEEE

the frequency of operation to be 2.4 GHz, dimensions of the slot line maybe calculated using the formula λg = c/ (f√εeff) where λg is the guided wavelength, c is the speed of light in vacuum, , f is the frequency of operation and εeff is the effective permittivity of the slot line. Parametric studies reveal that the optimum dimension of the fractal slot is about 95 mm. The dimensions of the slot loop are thus given by Lo = Wo = 25mm, with a slot width of sw = 0.4mm. Minkowski fractal is formed by displacing the middle one-third of each straight segment (indentation length) by some fraction called the indentation width. Indentation factor (i) is defined here as the ratio of indentation width to the indentation length. The resulting structure has five segments for every one of the previous iteration, but not all of the same scale. Changing the indentation factor results in a change in the resonant frequency which allows for an added degree of freedom when designing an antenna. The dimensions of the dielectric resonator antenna are calculated using the dielectric waveguide model [12] such that it resonates at a frequency of 5 GHz.

222zyx

rrr kkkckcf ++==

εε (1)

Where, ( ) ( ) 2201tan;

2; yryyzx kkBkk

Hk

Ak −−=== εππ (2)

The dimensions thus calculated are A = 33 mm, B = 30 mm and H = 5 mm, respectively.

III. PARAMETRIC STUDY

A. Stub Length A rectangular slot loop antenna is designed as the initiator

to the consequent fractal designs. Since a CPW line is used to feed the slot loop there is a change in the characteristic impedances of the two lines which degrade the overall matching. Hence, a tuning stub is added to the CPW line of length lf to improve impedance matching. Fig. 3 shows the reflection co-efficient characteristics of the slot loop antenna for various lengths of the tuning stub. It is observed that with an increase in the stub length, the matching improves till a critical length after which the matching slowly deteriorates. The stub acts as a parallel impedance to the existing CPW fed slot loop, which tends to reduce the effective impedance seen

by the CPW and hence improves matching.

B. Minkowski Fractal Geometry It is seen that the rectangular slot loop antenna does not

yield sufficient bands to cover all the necessary wireless bands. To increase the number of bands fractal geometry is incorporated on the slot boundary. Fractal geometry increases the effective path length of the structure thus providing an increased path length for the current which results in frequency reduction. Successive fractal iterations increase the path length further which results in further reduction of frequency. This reduction of frequency results in a larger number of frequency bands to be packed within a specified frequency range. Variation of the reflection co-efficient with respect to fractal iterations is shown in Fig. 4. In our study the indentation factor is specified as fraction varying from (0.1 to 0.9). Variation up

Fig. 3 Simulated S11 (dB) for different tuning stub lengths

Fig. 4 Simulated S11 (dB) for fractal indentations

Fig. 5 Current distribution for various fractal iterations

Fig. 6 Measured S11 (dB) of the Minkowski fractal slot loop antenna with

and without DR loading

to the second iteration is considered in this study keeping in mind fabrication tolerances. Fig. 5 shows the surface current distribution along the slot geometry indicating an increase in the current path leading to an increase in the frequency bands.

IV. RESULTS AND DISCUSSIONS After completing the parametric studies, the optimized

parameters are used to design a prototype antenna. Fig. 6 shows a comparative study between a DR loaded fractal slot loop antenna and a fractal slot loop antenna without any loading. Fig. 7 gives the simulated and measured results of the dielectric loaded second order Minkowski fractal slot loop antenna which appear to be in good agreement. It may be

observed from the resonance characteristics that fractal geometry results not only in miniaturization of the slot antenna but also produces multiple bands which may be tuned by changing the indentation ratio to obtain desired frequency response. The dielectric resonator loading of the antenna results in an increase in the impedance bandwidth at the upper frequency owing to its relatively smaller Q value than a slot antenna. Fig. 8 shows the normalized radiation patterns of the fractal DRA at their respective resonant frequencies. Gain measurement is performed using the gain transfer method where a standard gain horn antenna is used as reference. As noted earlier, the antenna radiates broadside at all frequencies. The radiation pattern observed in 5-6 GHz band corresponds to the dielectric loading. Table I lists the simulated and measured resonance and gain characteristics of the prototype antenna. It

Fig. 7 Simulated and measured S11 (dB) of the designed antenna

(a)

(b)

(c)

(d)

(e)

(f)

(g)

Fig. 8 Normalized radiation patterns at (a) 0.95 GHz, (b) 1.9 GHz, (c) 2.45 GHz, (d) 3.4 GHz, (e) 4.21 GHz, (f) 5.54 GHz

and (g) 6.06 GHz

may be noticed that the antenna gain is almost constant over the entire frequency band.

V. CONCLUSION

An investigation on the use of dielectric loaded fractal slot loop antenna for multi-band performance is performed in this work. Minkowski boundary slot etched on an FR4 substrate fed by CPW is proposed and characterized. Parametric studies are carried out to investigate the antenna design parameters. The fabricated prototype yields a hepta band performance for a -10 dB reflection co-efficient. The antenna radiates with an acceptable gain of 1.1 dBi to 3.1 dBi over different frequency bands.

ACKNOWLEDGMENT This research work was supported by the INDO –

TUNISIAN JOINT PROJECT [Ref: INT / Tunisia / P – 02 / 2012 dated 01.03.2013].

REFERENCES [1] Muhammad R. Khan, Mohamed M. Morsy, Muhammad Z.Khan and

Frances J. Harackiewicz, “Miniaturized Multiband Planar Antenna for GSM, UMTS,WLAN, and WiMAX bands”, IEEE International Symposium on Antennas and Propagation (APSURSI 2011), Washington, USA

[2] M. F. Abedin and M. Ali, “Modifying the Ground Plane and Its Effect on Planar Inverted-F Antennas (PIFAs) for Mobile Phone Handsets”,

IEEE Antennas and Wireless Propagation Letters, vol. 2 2003, pp. 226-229

[3] Han-Cheol Ryu, Hee-Ran Ahn, Sang-Hwa Lee and Wee Sang Park, “Triple-stacked microstrip patch antenna for multiband system”, Electronics Letters, vol. 38, 2002, pp. 1496-1497

[4] J. P. Damiano, J. Bennegueouche, and A. Papiernik, ‘‘Study of Multilayer Antennas with Radiating Elements of Various Geometry,’’ Proc. IEE, Microwaves, Antennas Propagation, Pt. H, vol. 137, no. 3, 1990, pp. 163–170.

[5] Junho Yeo, R. Mittra, Yoonjae Lee, S. Ganguly, “A Novel Modified Sierpinski Patch Antenna using Shorting Pins and Switches for Multiband Applications”, IEEE International Symposium on Antennas and Propagation Society (APS), no. 4, Texas, USA, 2002, 90-93

[6] Peter Callaghan and John C. Batchelor, "Dual-Band Pin-Patch Antenna for Wi-Fi Applications”, IEEE Antennas and Wireless Propagation Letters 7, 2008, pp. 757-760.

[7] Steven R. Best, “On the Multiband Behavior of The Koch Fractal Monopole Antenna”, Microwave and Optical Technology Letters, Vol. 35, 2002, pp. 371-374.

[8] Deepti Das Krishna, M. Gopikrishna, C. K. Anandan, P. Mohanan, and K. Vasudevan, “CPW-Fed Koch Fractal Slot Antenna for WLAN/WiMAX Applications”, IEEE Antennas and Wireless Propagation Letters, vol. 7, 2008, 389-392.

[9] B.B. Mandelbrot, The Fractal Geometry of Nature, W. H. Freeman, New York, 1983.

[10] S. Dhar, S. Maity, R. Ghatak, B. Gupta and D.R. Poddar, “A CPW Fed Slot Loop Minkowski Fractal Antenna with Enhanced Channel Selectivity,” International Conference on Communications, Devices and Intelligent Systems, 2012, pp. 542 – 545.

[11] S. Dhar, R. Ghatak, B. Gupta and D.R. Poddar, “A Wideband Minkowski Fractal Dielectric Resonator Antenna,” IEEE Transactions on Antennas and Propagation, vol. 61, no. 6, 2013, pp. 2895 – 2903.

[12] K.M. Luk and K.W. Leung, Dielectric Resonator Antennas, Research

Studies Press Ltd. 2003

TABLE I COMPARISON BETWEEN SIMULATED AND MEASURED RESONANCE AND GAIN CHARACTERISTICS

Simulated Measured Resonant

Frequency (GHz)

0.85 2.2 2.99 3.72 4.28 5.15 6.03 0.95 1.9 2.45 3.4 4.21 5.54 6.06

-10 dB Impedance Bandwidth

(MHz)

15 50 38 42 80 770 285 27 35 57 150 160 720 575

Gain (dB) 0.4 1.7 2.2 2.4 1.5 3.0 2.4 1.1 1.4 2.5 3.1 1.9 2.8 2.1