A Very Compact UWB Antenna with L Shaped Resonators for Dual Band Rejection

A Very Compact UWB Antenna with L Shaped Resonators for Dual Band Rejection

Surya Rajan

M.Tech Scholar (Wireless Technology) Department of Electronics

Toc H Institute of Science and Technology Ernakulam, Kerala, India

Prof. A. K Prakash Professor (P.G studies) Department of Electronics

Toc H Institute of Science and Technology Ernakulam, Kerala, India

AbstractWe present here a very compact Ultra-Wideband (UWB) microstrip fed monopole antenna with dual band rejection characteristics. The antenna has been fabricated on a FR4 substrate with a thickness of 1.6 m.m.The dimension of the antenna is 22*14 m.m2.For rejecting Wimax(3.3-3.6 GHz) we used a quarter wavelength open slot stub on the patch and for WLAN(5.15-5.85 GHz) rejection we used a half wavelength resonating L shaped structure on bottom side of antenna. The antenna is simulated using HFSS VER 13 software and the VSWR band and radiation pattern at different frequencies are evaluated. The frequency measured showed a bandwidth of 7.97 GHz over 3.0911.06 GHz with a Voltage Standing Wave Ratio(VSWR) less than 2 with two eliminated bands centered at

1. GHz and 5.5 GHz to reject the aforementioned frequencies. The omnidirectional radiation patterns and compact size of the proposed antenna make it suitable for Wireless Personal Area Network(WPAN) UWB applications.

   KeywordsUltra Wide Band, Beveling technique, Dual Band Rejection,Quarter wave Resonator, Half Wave Resonator, Microstrip fed, VSWR.

   1. INTRODUCTION

      Since U.S. Federal Communications Commission (FCC) allocated an unlicensed spectrum of 3.1 -10.6 GHz for ultra- wideband (UWB) commercial purposes, both academical and industrial researches into UWB technology has risen drastically [1]. Among the proposed UWB antennas design for UWB, printed monopole antennas is a well suited candidate for future UWB applications when compact size, stable radiation pattern and easiness of fabrication is to be considered [2-3].Anyway the design of UWB antennas faces many problems of which most headache for designers is the interference with co-existing bands such as wireless local area networks (WLAN), WiMAX.Therefore it is essential need for UWB antennas to be band notched for interfering bands.

      Many antenna designs are proposed with single and dual band rejection characteristics. Some antennas are also proposed with multiple band rejection. Most of the designers have used half wavelength and quarter wavelength open stub slots of various shape like U, L, C, E [4-11] etc.Some of them have also used Split Ring Resonators (SRR) [12] also. A detailed study of different parametric element is done by [13].

      It is found that using most of these designs a major part of useful bandwidth is rejected along with rejection bandwidth. Also the radiation pattern is also distorted at the higher frequencies. So there is a need for a compact and simple co- existing band rejected UWB antenna with stable radiation pattern.

      In this paper we first design a square monopole antenna and then will expand its bandwidth using techniques like beveling technique and truncating ground technique to achieve UWB bandwidth. Then we propose a simple and very compact microstrip fed UWB antenna with dual band rejection at 3.5 GHz and 5.5 GHz. The dual band stop characteristics is obtained by using a open quarter wavelength resonator on patch for 3.5 GHz and a newly proposed L shaped half wave resonator on bottom side near ground plane for 5.5 GHz rejection.

   2. ANTENNA GEOMETRY AND DESIGN

      1. Basic UWB Antenna.

         In this section the basic UWB antenna without band notch is described. Figure 1 and 2 shows the geometry of basic square monopole antenna and figure 3 and 4 shows its transformation to fully UWB bandwidth using ground truncation and beveling technique. The substrate used is FR4 material of r of 4.4 and thickness 1.6 m.m.with size 22*14 m.m2. The patch design equations are taken from [14]. A 50 microstrip feed line is designed with a width of 2 m.m and length 9 m.m which is taken from [15].Then the ground truncation and beveling techniques are used to extend bandwidth.

         1. (b)

            Fig. 1.Basic UWB square monopole antenna

            1. Front view

            2. Bottom view

            Name

            m2 11

            Y

            X

            7m.010 3.0700 1.9611

            .9600 2.0110

            6.00

            5.00

            VSWR(1)

            4.00

            3.00

            2.00

            vswr

            m1 m2

            HFSSDesign1 ANSOFT

            Curve Info VSWR(1)

            Setup1 : Sw eep

            Fig. 2.Modified UWB square monopole antenna

            1. Front view Beveled Patch

               1.00

               2.00 4.00 6.00 8.00 10.00 12.00 14.00

               Freq [GHz]

               Fig. 4. VSWR Bandwidth of Modified UWB square monopole antenna

               In the simple square monopole antenna shown in Fig.1.

            2. Bottom view-Truncated Ground

            TABLE 1.Dimensions of UWB antenna without notches

            Dimensions	Values (mm)
            Width of the Substrate (Ws)	14
            Length of the Substrate (Ls)	22
            Patch length and width(L=W)	12
            Feed width(Wf)	2
            Feed length	9
            Ground length,Lg	5
            Width of Truncated Ground slot,Wd	4
            Length of Truncated Ground slot,Ld	3
            Top Length of beveled patch,Lb1	6
            Top Length of beveled patch,Lb2	7.8

            The optimized values of square monopole UWB antenna and modified UWB antenna without notches are given in the Table1.

            there is a discontinuity in the connecting point between the microstrip-feed-line and the patch, that is an important factor for lowering the bandwidth and degrading the radiation pattern at the higher frequencies. To eliminate this issue, the square monopole patch antenna in Fig. 1.(a) is turned to beveled structure in Fig. 2(a) which provide a smooth transition from microstrip feed to patch and improves impedance matching .Bottom side in Fig.1(b) is reconstructed to truncated ground structure behind the microstrip feed line as shown in Fig. 2(b) which act as an impedance matching element, since the truncated ground creates a capacitive load that neutralizes the inductive effect of microstrip feed line which results in a purely resistive input impedance. Together this two modifications result in an improved impedance matching and hence an improved VSWR bandwidth as shown in Fig. 4. ranging from 3.07-11.6 GHz compared to bandwidth of basic square monopole patch which ranges only from 3.55-8.79 GHz. The antenna is designed in x-y plane which has normal direction towards z axis.

      2. Dual Band Notched Antenna.

         In this section details about our dual band notched antenna is described in detail.

         Name X Y

         9m.010 3.5500 2.0105

         m2 8.7900 1.9886

         8.00

         7.00

         VSWR(1)

         6.00

         5.00

         4.00

         3.00

         2.00

         vswr

         m1 m2

         HFSSDesign1 ANSOFT

         Curve Info VSWR(1)

         Setup1 : Sw eep

         Fig. 5.Modified UWB square monopole antenna with dual band notches

         1. Front view

         2. Bottom view

            1.00

            2.00 4.00 6.00 8.00 10.00 12.00 14.00

            Freq [GHz]

            Fig. 3.VSWR Bandwidth of Basic UWB square monopole antenna

            Fig. 5. shows the final design of dual band notched antenna which is optimzed by taking several things into consideration like optimum rejected bandwidth, stable radiation pattern etc.

            The dual band notching is achieved by cutting horizontal

            L shaped slot in the patch for WiMax rejection (3.5 GHz) and providing an inverted L shaped resonator on bottom side of the antenna for rejecting WLAN(5.5 GHz).The L shaped slot in the patch act as a open stub quarter wavelength resonator at WiMax frequency. Similarly the L shaped resonator at bottom side will act as a half wave resonator at

            6.00

            5.00

            VSWR(1)

            4.00

            3.00

            2.00

            VSWR

            HFSSDesign1 ANSOFT

            Curve Info VSWR(1)

            Setup1 : Sw eep

            WLAN frequency. So the notching frequency at which maximum rejection happens can be found using the following equations:

            1.00

            2.00 4.00 6.00 8.00 10.00 12.00

            Freq [GHz]

            Fig. 6. VSWR Bandwidth of proposed dual band notched antenna.

            Where

            =

            1

            4 1

            2

            =

            2 2

      Ln1=Lq1+Wq1+Wq (3)

      Ln2=Lr1+Wr1+Wq (4)

      2

      = +1

      (5)

      where fr1 and fr2 are resonant frequencies at 3.5 and 5.5 GHz respectively,c is the velocity of light in free space,eff is the effective dielectric constant calculated by (5),r is the dielectric constant of substrate which is given as 4.4and other parameters are given in fig. 5.

      TABLE 2.Optimized dimensions of UWB antenna with dual band notch

      Dimensions	Values (mm)
      Slot lengtp on patch (Lq1)	11
      Slot lengtp on patch (Wq1)	2
      Slot Width (Wq)	.5
      Slot lengtp on ground plane(Lr1)	10.5
      Slot lengtp on ground plane(Wr1)	4.5
      Distance from top of substrate to Lresonator(W2)	7.3
      Distance from edge of substrate to Lresonator(Lx)	7.8

      Fig. 6. shows the VSWR graph of our proposed antenna with dual band rejections characteristics. The length of quarter wave open resonator for rejecting WiMax at 3.5 GHz and WLAN at 5.5 GHz are optimized in such a way that the interfering band is rejected and also the positions of resonators are selected like that there is least coupling between the notching structures and a stable radiation pattern is maintained.

      Fig. 7. Surface Current pattern of proposed dual band notched antenna at

      3.5 GHz.

      Fig. 8. Surface Current pattern of proposed dual band notched antenna at

      5.5 GHz.

      From Fig. 6. it is clear that our proposed antenna reject the interfering WiMax and WLAN frequencies with VSWR>2 while maintaining the elsewhere in UWB bandwidth from 3.1 GHz-10.6 GHz with a VSWR< 2.

      For better understanding of the band notching characteristics the current pattern of proposed antenna at 3.5 and 5.5 GHz shown in Fig. 7.It can be seen that current is concentrated around the open quarter wavelength resonator around 3.5 GHz and around the half wavelength resonator on bottom plane at frequencies around 5.5 GHz.

   3. RADIATION PATTERN OF PROPOSED ANTENNA

      1. (b)

         (c) (d)

         (e) (f)

         Fig. 9. Radiation patterns at different frequencies proposed dual band

         notched antenna.

         (a) 3.1 GHz (b) 3.5 GHz (c) 4.5 GHz (d) 5.5 GHz (e) 7.5 GHz (f) 10 GHz

         Fig. 9. shows the radiation pattern in E and H plane of proposed antenna at various frequencies. It should be noted that the radiation pattern is distorted at notching frequencies and stable for other frequencies. So antenna can be said to be omnidirectional in UWB band.

   4. CONCLUSION

Design of a modified UWB antenna and a dual band notched antenna for rejecting WiMax and WLAN frequencies are discussed in this paper. A quarter wave length open stub resonator on patch is used to reject 3.5 GHz WiMax frequency whereas a half wave resonator is used at the bottom plane to reject the WLAN frequency at 5.5 GHz. The interesting point to be noted is that the antenna proposed is a very compact antenna of dimension 14*22*1.6 m.m3 which makes it suitable for UWB WPAN applications. Also we have obtained a constant radiation pattern and VSWR bandwidth with notched bands at >2 and UWB band <2, when the proposed antenna is simulated using HFSS VER 13 [16].

REFERENCES

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11. Mohammad Ojaroudi, Changiz Ghobadi, and Javad Nourinia, "Small square monopole antenna with inverted Tshaped notch in the ground plane for UWB application", IEEE Antennas and Wireless Propagation Letters, vol. 8, pp. 728-731, July, 2009.

12. M. Azarmanesh,S.Soltani,P. Lotfi Design of an ultra-wideband monopole antenna with WiMAX, C and wireless local area network IET Microwaves, Antennas & Propagation, Vol. 5, no. 6, pp. 728 733,Jan., 2012.

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    ,March,2012.

14. I.S. Ja Constantine Balanis, Antenna Theory Analysis and Design, 2nd ed., pp. 722-730, 2009.

15. David M,Pozar,Microwave Engineering,3rd ed.,2010.

16. Ansoft High Frequency Structure Simulation (HFSS), Ver. 13, Ansoft Corporation, 2010.