compact serrated notch band mimo antenna for uwb applications · 2016. 4. 10. · uwb applications....

VOL. 11, NO. 7, APRIL 2016 ISSN 1819-6608

ARPN Journal of Engineering and Applied Sciences ©2006-2016 Asian Research Publishing Network (ARPN). All rights reserved.

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COMPACT SERRATED NOTCH BAND MIMO ANTENNA FOR UWB APPLICATIONS

M. V. Reddiah Babu1, Sarat K. Kotamraju1, B. T. P. Madhav1, S. S. Mohan Reddy2,G. V. Krishna1,

M. V. Giridhar1 and V. Sai Krishna1 1Department of Electronics and Communication Engineering, K L University, AP, India

2Department of Electronics and Communication Engineering, SRKR Engineering College, Bhimavaram, India E-Mail:[email protected]

ABSTRACT

A compact UWB antenna is been designed to notch Wi-Max (3.3GHz-3.7GHz) and W-LAN (5.15GHz-5.85GHZ) operating bands. The antenna comprises of two square slottedmonopoles with serrated edges on the patch surface and T-shaped stub as defected ground structure. Coplanar waveguide feeding is used in the antenna structure at two ports with the impedance of 50 ohms. Both simulation and measurement aredone to study the antenna parameters like return loss, radiation-characteristics, impedance matching and isolation between the two ports. To enhance isolation a slot is cut on the T-shaped ground surface. Two inverted L strips are added on either sides of the groundplane and a slot cut on the ground plane finally form T-shape defected ground structure. The proposed antenna notches two application bands in the UWB range with low mutual coupling which makes the antenna a suitable model for desired applications. Keywords:serrated antenna, notch band, ultra wideband (UWB), T-shaped stub, defected ground structure, mutual coupling.

1. INTRODUCTION

UWB technology is a rapid growing field which makes use of wide frequency bands to transmit signals at low energy level. It has promising applications in short range, high data rate transmission, Radar imaging and cancer sensing etc. Like other wireless communication systems ultra wideband (UWB) systems suffer from multipath fading. It is well known that Multiple Input Multiple Output (MIMO) technology which involves multiple antennas to be installed both at the transmitter and the receiver can be used to provide multiplexing gain and diversity gain to improve capacity and link quality of wireless systems [1-3].UWB combined with MIMO is gaining importance in today’s field of wireless communication. To further investigate, antennas with wide impedance bandwidth and good isolation are required.

UWB systems using huge bandwidths already have high data rates, so MIMO technology can be used to fade counter-measure through diversity gain. The basic concept of MIMO diversity is to use multiple antenna elements to transmit/receive signals with different fading characteristics. However for portable devices where the space is very limited, installing MIMO antennas with low coupling is always a great challenge for antenna designers. Various MIMO antennas have been studied for uses in portable devices in different wireless systems such as LTE, UMTS and WLAN [4-6].These studies reveal that MIMO technology employed with UWB system would provide superior channel capacity over that used in narrow band systems. A number of studies were carried out in order to reduce coupling between the antenna elements in MIMO-UWB antennas [7-11].

The UWB has gained importance when the Federal Communication Commission (FCC) has assigned

the frequency band ranging from 3.1 GHz-10.6 GHz for commercial UWB communication applications [12-13].In this unlicensed frequency band designated by FCC, there exists other wireless communication systems such as IEEE 802.16 Wi-MAX system operating in the frequency band of 3.3 GHz-3.7 GHz and WLAN for IEEE 802.11a operating in the frequency band of 5.15 GHz-5.85 GHz; thus the UWB system is prone to be under interference with the Wi-Max and WLAN systems. A feasible solution to this problem is to design a UWB antenna with band-notched characteristics. In [14-15], MIMO antennas were studied to suppress interference from WLAN systems by employing a special feature of notches. In [16],by etching two SRR slots on the radiators we obtain the desired band-notched characteristic in the WLAN band centered around 5.5GHz.A UWBMIMO antenna with perpendicularly placed antenna elements is studied in [17-18] for portable UWB applications. The antenna has a compact size of about 26×40 mm2.The first smallest UWBMIMO antenna with and without notches are discussed in [19-20].The antenna was designed with a compact size of about 27×30 mm2.

In this work, a MIMO notched antenna on defected ground structure with compact size is proposed. The size of the proposed antenna is 13%-15% smaller in electrical size than the earlier proposed models. Defected ground with a strip is used to create the Notch band with efficiency of 8% much smaller compared with earlier designs. Initially a square patch employing strips in the ground plane is used in the MIMO antenna with T-shaped defected ground structure to notch W-LAN band. The proposed antenna covers the entire UWB ranging from 3.1 GHz - 10.6 GHz. The edges of the radiating patch is moulded into serrated shape and by employing slot in the

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patchelement dual band notching in WiMAX and WLAN is obtained. 2. ANTENNA DESIGN AND GEOMETRY

The basic model of the proposed MIMO antenna on defected ground structure (DGS) is as shown in Figure-1.The antenna occupying a compact size of about 22×36×1.6 mm3 which is smaller in size compared to most of the UWB antennas. The radiating element of the monopole antenna is initially square shaped but the proposed model took a modification of serrated edges along three sides of the patch. The proposed antenna is as

shown in Table-1.Designing the Ultra Wide bandwidth for the UWB antenna is not such a problem and can be achieved through matching using etching a ground slot under the feed line, adjusting the gap between the ground and the radiator and also tapering the feed line. The dimensions of the final design are listed in Table-1 and the basic antenna compared with the proposed model is shown in Figure-1.In our MIMO antenna shown in Figure-1 (b) the square shape monopole radiators make a transformation to serrated shape along the three sides of a square. The dimension of the slot which creates a notch in the Wi-MAX band is depicted in Figure-1(c).

(a) (b)

Figure-1. MIMO Antenna (a) Basic model (b) Proposed model.

Figure-1(c). Dimensions of the slot.

Table-1. Dimensions of the proposed antenna in mm.

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The fundamental resonant frequency of a planar monopole antenna can be approximated by [13]

Fr= GHz (1)

Where lgandlp respectively denote the length of

the ground plane and the radiation patch respectively and d represents distance between them.

rg= (2)

rp= (3)

Ag and Ap denote the Area of the ground plane

and the radiation patch respectively. lg, lp, d, rg, rp are in centimeters. Also εreis the effective dielectric constant given by εre=(εr+1)/2.

Generally speaking,to get a notch in the frequency band of 3.3 GHz-3.7 GHz the elective length of the slot is calculated from the design equation as

Leff = (4)

Where c is the speed of light, fois the center frequency of the notch band and length of the slot is about half the wavelength (λ/2) at 3.5 GHz. 3. RESULTS AND DISCUSSIONS

The basic antenna model is compared with the modified antenna and the obtained results are placed in Figure-2. The results indicate that the basic antenna has bandwidth (S11<-10dB) from 2.1 GHz to 11.2 GHz, which almost covers the FCC Ultra wideband, whereas the proposed model has bandwidth ranging from 2.1 GHz-12.8 GHz. This clearly indicates that the proposed antenna is having improvement of 1.6% in the bandwidth compared to basic model. Also the mutual coupling (in terms of S12) is below -15dB from 2.4GHz to more than 12.4 GHz. To eliminate interference from the WLAN band, a notched band is created from 5.15 GHz to 5.85 GHz by employing two inverted L strips on either side of the slot in the ground plane. The basic antenna undergoes some modifications in the patch to improve the impedance bandwidth and also employing slots to create a notched band from 3.3 GHz to 3.7GHz to eliminate interference from the Wi-MAX band which is clearly shown in Figure-2(a).

Figure-2(a). Reflection coefficient of the basic antenna and proposed antenna.

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Figure-2(b). S12 parameter of the proposed antenna.

Figure-3. VSWR of Basic antenna and proposed antenna.

The VSWR characteristics of the basic antenna and the proposed antenna are compared in Figure-3.The VSWR is less than 2 (VSWR<2) in the proposed frequency band of operation. The parametric analysis is carried out for the ground plane and the slot in the patch. Figure-4(a) shows the simulated S-parameters for different values of length of the ground plane lg1.With the increase in the value of lg1 the desired S-paremeters are not obtained. Figure-4(b) and Figure-4(e) shows the simulated S-parameters for different values of length of the slot Ls and width of the slot Ws respectively. As value of Ls and

Ws is slightly increased the notch band slightly shifts. The simulated S-parameters with different values of Lt1 are shown in Figure-4(c). The notched band from 5.15 GHz to 5.85 GHz shifts as the value of Lt1 is increased. In the study of the proposed antenna, the value of Lt1 is set to 1.75 mm to obtain a notch band in the 5.15 GHz-5.85GHz to suppress interference in the WLAN band. Figure-4(d) shows the simulated S-parameter by varying the width of the T-shaped ground stub (Wg2).The desired S parameters are obtained by choosing the value of Wg2=20 mm which is clearly evident from the figure.

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Figure-4(a). Parametric analysis of S11 with varying Lg1.

Figure-4(b). Parametric analysis of S11 varying Ls.

Figure-4(c). Parametric analysis of S11 varying Lt1.

The parametric analysis carried out with respect to the slot in the patch is as shown in Figure-5. Figure-5(a) shows the simulated S-parameters for different values of

sl1. As the value of sl1 is varied the S- parameters are varied greatly. The optimum value of sl1 which creates a notched band is set to 7 mm. Figure-5(b) shows the

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simulated S-parameters of the antenna with respect to sl2. Figure-5(c) indicates the variation of S-parameters in the first notched band as the value of sl3 is varied. Figure-5(d)

clearly the left shift of the bands as the value of the slot width i.e. Wl1 is varied. For the desired performance the value of Wll1 is set equal to 0.2 mm.

Figure-4(d). Parametric analysis of S11 varying Wg2.

Figure-4(e). Parametric analysis of S11 varying Ws.

Figure-5(a). Parametric analysis of S11 varying Sl1.

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Figure-5(b). Parametric analysis of S11 varying Sl2.

Figure-5(c). Parametric analysis of S11 varying Sl3.

Figure-5(d). Parametric analysis of S11 varying Wl1.

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Figure-6. 2-D Radiation patterns with port 1 excited and port 2 terminated with 50-ohm load: (a)2.5 GHz;(b)3.8 GHz;(c)4 GHz;(d)8.5 GHz.

The radiation patterns of the proposed MIMO

antenna are simulated. With port 1 excited and port 2 terminated with a 50-Ω load, the radiation patterns at frequencies of 2.5 GHz,3.8 GHz,4 GHz and 8.5GHz are shown in Figure-6(a),(b),(c),(d) respectively. The radiation characteristics indicate a quasi omnidirectionalpattern in

the H-plane and dumbbell shape pattern in the E-plane in the higher frequency bands. At the notched frequency the radiation patterns are very narrow. Figure-7 shows the 3-D (Three dimensional) radiation characteristics of the proposed antenna at operating frequency bands.

Figure-7. 3-D polar plots at (a)2.5 GHz, (b)3.8 GHz, (c)4 GHz,and (d)8.5 GHz with port-1 excited and port-2 terminated with 50-ohm load.

Figure-8 shows the surface current distributions

of the proposed antenna at the center of the two notched frequency bands i.e. 3.5GHz and 5.4GHz and also at 4.2GHz.It is clearly evident from the figures that current

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coupled from Patchelement1 to patchelement2 is less reducing coupling between the two ports.

Figure-9 shows the gain of the proposed antenna with respect to the frequency of operations. A peak realized gain of 4 dB is attained from the proposed antenna.

Figure-9.Simulated gain vs Frequency of basic antenna and proposed antenna. By setting the value of Sl2=7.4 mm we obtained a notched band from 3.3GHz-3.7GHz to suppress interference from Wi-Max band, which is evident from Figure-10.

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Figure-10. S parameters varying slot length Sll2.

Figure-11. Fabricated MIMO antenna front view, Figure-12. Fabricated MIMO antenna back view

Figure 11 shows the prototyped MIMO antenna

on FR4 substrate front view. The rectangular patches are designed with serrated edges and impedance matching is achieved with proper feed width and length. Figure 12 shows the MIMO antenna back view with defected ground structure. Figures 13 and 14 shows the measured S12 and

VSWR of the proposed antenna model. It is been observed that the proposed antenna is showing estimated results when compared with the simulation results obtained from the HFSS tool. Antenna is rejecting proposed bands and passing the other bands as per the specifications.

Figure-13. Measured S12 of the proposed antenna on ZNB 20 VNA.

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Figure-14. Measured VSWR of the proposed antenna on ZNB 20 VNA. 4. CONCLUSIONS

An Ultra wideband antenna is designed to notch Wi-Max (3.3GHz-3.7GHz) and W-LAN (5.15GHz-5.85GHZ) frequency bands. Proposed antenna is prototyped on FR4 substrate and tested on ZNB 20 VNA. Both simulation and measurement results are showing good agreement with each other. Average gain of 3.5 dB is attained in the operating band and at notch band antenna is showing poor gain. The proposed antenna can be used to notch the desired bands and applicable for wireless communication applications. ACKNOWLEDGEMENTS

Authors like to express their gratitude towards the department of ECE and management of K L University for their continuous support during this work. Further Madhav likes to express his gratitude to DST through FIST grant SR/FST/ETI-316/2012. REFERENCES [1] L. Zheng and N. C. Tse. 2003. Diversity and

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compact serrated notch band mimo antenna for uwb applications · 2016. 4. 10. · uwb applications....

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