A High Gain Compact Coplanar Stripline Fed Antenna for Wireless Applications

A High Gain Compact Coplanar Stripline fed Antenna for Wireless Applications

Roshna T.K.*1, Deepak U.2, Sujith R.3and Mohanan P.4

1. Centre for Research in Electromagnetics and Antennas (CREMA), Department of Electronics, CUSAT Cochin-22, Kerala, India. 1. [email protected] 2. [email protected] 4. [email protected]

3. Angstrom Laboratory, Department of Engineering Sciences, Uppsala University, Box 534, SE-751 21 Uppsala, Sweden. [email protected]

Abstract

This paper proposes a novel, open stub loaded high gain, asymmetric coplanar stripline antenna for IEEE 802.11a, HIPERLAN2, HiSWANa, WiMAX (IEEE 802.11e) and ISM 5.8 application bands. The proposed antenna has an overall 3 dimension of 24×23×1.6mm when printed on a substrate of permittivity (εr) = 4.4 and has a 2:1 VSWR bandwidth of 23% at 5.35 GHz. The antenna exhibits a peak gain of 5.20dBi at 5.35 GHz and a good omni-directional radiation with an efficiency of 84%, which are validated by measurements.

1. Introduction

Recently, owing to rapid developments in WLAN and ISM communications, high gain compact planar antennas operating in 5-6 GHz band are becoming demanding in practical applications. Combining resonant structures with microstrip line slot antennas for efficient radiation in some desired bands has been studied before [1-4]. In [5], a wideband rectangular slot antenna with U-shaped strip for WLAN application is presented. The overall size of this antenna is 75×75 mm2 and is not well suited for practical applications. This paper presents a novel, open stub loaded planar antenna which meets the bandwidth requirements of many application bands, while belonging to the class of simply-structured and compact coplanar strip fed antennas. In the proposed antenna the actual radiation element is the arc shaped structure and hence it excites the un-guided mode of propagation. An open stub of length λg/4 is used for impedance matching. To reduce the overall size of the antenna a slanted stub is used in this study. The antenna has a 2:1 VSWR bandwidth of 23.69% from 4.6-5.38 GHz covering IEEE 802.11a (5.15-5.35 GHz, 5.725-5.825 GHz), HIPERLAN2 (5.45-5.725 GHz), HiSWANa (5.15-5.25 GHz), WiMAX (5.25-5.83 GHz) and ISM 5.8 (5.725-5.825 GHz) communication bands.

2. Antenna Design & Geometry

Figure 1: Geometry of the proposed antenna (W=23mm, L=24mm, Sl=24mm, Sg=1mm, Sw=3mm, Rl=8.85mm, Rw=2.83mm, Rp=9.25mm, Ar1=6.5mm, Ar2=8.5mm, As=1.5mm, h=1.6mm)

Figure 1 depicts the geometry of the proposed antenna and its optimized parameters. The arc shaped strip of radius Ar1, at the left side of the coplanar strip is employed to produce the resonance between 5-6 GHz, while a quarter wave (λg/4) open stub connected to the right side of the coplanar strip improves the impedance matching for the above produced resonance. The

978-1-4673-5225-3/14/$31.00 ©2014 IEEE open stub of size Rl×Rw is placed at a distance λg/4 away from the feed point in x-direction to obtain adequate impedance matching. Stripline gap (Sg) and stripline width (Sw) is optimized for getting maximum impedance matching. The resonant frequency is inversely proportional to the stripline length as expected. The prototype is fabricated on a FR-4 substrate of 3 relative permittivity (εr) 4.4 and loss tangent (δ) 0.02. The antenna requires a small footprint of 24×23×1.6mm only and is fed by a 50Ω coaxial cable.

3. Results And Discussion

Figure 2: Simulated reflection characteristics of different antenna elements.

The stub loaded asymmetric coplanar stripline antenna is simulated using Ansoft HFSS. Various parameters of the proposed antenna are optimized to obtain the resonance in the above mentioned application bands. The variation in the reflection coefficient with the evolution of the present antenna from the guided coplanar stripline is shown in Figure 2.

It is clear from Figure 2 that the conventional coplanar stripline antenna hardly has a resonance in the 5-6 GHz band, while by introducing an arc on the left side, the resonance in the 5-6 GHz band has been introduced, with poor impedance matching. The bandwidth improvement with better impedance match is achieved by inserting a slanted quarter wave (λg/4) open stub on the right side of the coplanar stripline. The optimized design is fabricated and experimentally verified using Agilent PNA E8362B network analyzer.

Figure 3: Measured reflection and radiation characteristics of the proposed antenna

Figure 3 depicts the measured reflection and radiation characteristics of the proposed antenna. It is evident from the graph that the proposed antenna can effectively operate over 4.6-5.83 GHz frequency band, with 2:1 VSWR bandwidth of 23.69% about the center frequency 5.35 GHz.

(a) (b) Figure 4: Effect of open stub (a) Position (Rp) & (b) Length (Rl), on the reflection characteristics.

The influence of open stub position (Rp) and length (Rl) is exhibited in Figure 4 (a) & (b) respectively. It is observed that the location of the stub has large influence on the impedance characteristics of the antenna. The stub should be placed at proper location at which the impedance is matched with 50Ω. Impedance matching of the antenna with the length of the stub is shown in Figure 4 (b). It is found that the position of open circuited λg/4 stub from the feed should be around λg/4, where λg is the guided wavelength corresponding to the center frequency of the antenna.

Figure 5: Surface Current Distribution of the antenna at 5.35 GHz

The surface current distribution of the proposed antenna at 5.35 GHz is illustrated in Figure 5. It is evident from the current density plot that the resonance at 5.35 GHz is due to the arc shaped strip. Figure 6 (a) & (b) represents the measured radiation patterns at the center frequency. It is apparent that the radiation pattern is omni-directional in nature with reasonable cross polarization level in the proposed application band. The cross polarization level for E-plane (Y-Z plane) is 16dB and for H-plane (X-Z plane) is 19dB. A small tilt in the E-plane radiation pattern is due to the asymmetry of the structure.

(a) (b)

Figure 6: Measured radiation pattern of proposed antenna at 5.35 GHz (a) E-Plane (b) H-Plane

(a) (b)

Figure 7: Measured (a) gain and (b) radiation efficiency of the proposed antenna

Measured gain and radiation efficiency of the proposed antenna is depicted in Figure 7 (a) & (b) respectively. A high gain of 5.20dBi and a good radiation efficiency of 84.75% are obtained. The radiation efficiency is measured using Wheeler Cap Method [7]. The proposed antenna is small in size and provides high gain.

4. Conclusion

A novel open stub loaded asymmetric coplanar stripline antenna for wireless communication has been proposed and studied experimentally. With the use of an arc shaped strip and an open stub on either side of coplanar stripline, a 2:1 VSWR bandwidth of 23.69% is achieved. This is enough for covering IEEE 802.11a, HIPERLAN2, HiSWANa, WiMAX (IEEE 802.11e) and ISM 5.8 application bands. The proposed antenna also provides high gain and good radiation efficiency.

5. Acknowledgment

The authors acknowledge the University Grants Commission (UGC) and Department of Science and Technology (DST), Government of India for the financial assistance.

6. References

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