A Compact 3.5/5.5 GHz Dual Band-Notched Monopole Antenna For Application In UWB

Communication Systems With Defected Ground Structure

Jyoti R. Panda and Rakhesh S. Kshetrimayum Department of Electronics and Communication Engineering

Indian Institute of Technology Guwahati, Assam 781039 INDIA

Abstract- A compact ultrawideband monopole antenna having dual band-notched characteristics with a defected ground structure (DGS) proposed. Two symmetrical L-shaped slots are created on the ground plane to generate the UWB characteristics in the proposed antenna. To generate the notch at 5.2/5.8 GHz band, a U-shaped slot is cut in the rectangular radiating element, which mitigate the potential interference with WLAN. To have another notch band simultaneously around 3.0/4.0 GHz, which is the operating band of WiMAX (3.3-3.6 GHz) and C-band (3.7-4.2 GHz), an inverted U-shaped element is printed on the opposite side of the substrate. By properly varying the dimensions of the U­shaped slot and the radiating element, not only two controllable notch resonances, but also a very wide bandwidth from 1.91 GHz to 3.91 GHz (152%) with two sharp notched bands covering all the 3.5/5.5 GHz WiMAX, 4 GHz C-band and 5.2/5.8 GHz WLAN, are achieved. The proposed antenna properly optimized and simulated providing broadband impedance matching, appropriate gain and stable radiation pattern characteristics.

I. INTRODUCTION

FEDERAL Communication Commission's (FCC)'s ruling in February 2002 [1] for the commercial use of huge band

from 3.1 GHz to 10.6 GHz has completely revolutionized the wireless and high speed data communication world. This huge bandwidth from 3.1 GHz to 10.6 GHz is known as the ultrawideband (UWB) spectrum. Shortly after the FCC's ruling in February 2002, research started throughout the world to design the various systems and components that will enhance the credibility of the UWB system and UWB antenna is one of them. Last nine years researchers all over the world designed and proposed many UWB antennas, which are compact, low cost, less fragile, light weight and easily incorporable in the portable and hand held devices in the UWB system. There are many challenges in the UWB antenna design and the notable among them are broadband impedance matching, appropriate gain characteristics and stable radiation pattern.

But along with the vast operating bandwidth of the UWB antenna (3.1-10.6 GHz), there exist some narrowband wireless

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services, which occupy some of the frequency bands in the UWB bands. The most well known among them is wireless local area network (WLAN) IEEE802.11a and HIPERLAN/2 WLAN operating in 5-6 GHz band. Apart from WLAN, in some European and Asian countries, world interoperability for microwave access (WiMAX) service from 3.3-3.6 GHz and 4 GHz C-band also occupy in the UWB band. In some antenna designs, the UWB antenna uses filters to notch out the interfering bands. But the use of filters increases the complexity of the UWB system and also increases the weight. Hence it is needed to design the UWB antenna with dual band­notched characteristics both in 3.0-4.5 GHz and 5-6 GHz to mitigate the interference between the narrowband wireless systems and UWB systems. Till now many designs of the UWB band-notched antennas are proposed [2]-[13] to alleviate the disturbance caused by the WLAN with the UWB system.

The simple and most commonly used approach is to incorporate various shapes and sizes of slots into the main radiator. In [2] a bell shaped patch with stair case structure is proposed. Apart from that two parasitic patched are printed on the substrate to provide the band-notch characteristics. A L­shaped slot [3] is cut in the edges of main radiator with a C­shaped in the main radiator. An arc shaped slot is cut in the elliptical shaped patch [4], with a rectangular slot is cut in the ground plane. A rectangular tuning stub [5] is embedded in the circular annular ring, which generates the band-notched characteristics in the UWB spectrum. A split ring resonator (SRR) [6] is used in the main patch for the band notched characteristics. A ring shaped parasitic patch [8] is printed within the central circular slot. The central circular slot is cut from the bell shaped patch. A cutting pie with the flare angle e is cut from the circular patch [9]. This antenna is consists of two monopoles with a small strip bar is located in between the two monopoles [10]. A simple arc [12] is cut in the circular radiating patch provides the band notch characteristic. An E­shaped slot is removed from the rectangular radiating element of the UWB antenna, which is fed by a triangular tapered feed line with the end having a trident shape. A U-shaped slot is cut

in the ground plane [13]. A square shaped SRR is printed on the circular shaped radiating element [14]. Two L-shaped slots are cut from the ground plane with an arc is cut from the circular radiating patch [15]. A deep almost full shaped arc is cut in the annular circular ring [16]. An E-shaped slot is cut from the rectangular radiating patch with a notched ground plane [17]. In this antenna the circular disc is used as the radiating element, which is fed by the papered microstrip line. The ground plane is round cornered, which enhance the impedance bandwidth especially at the high frequency. In one structure two L-shaped slot is cut from the ground plane and in another structure a pair of square ring resonators is created in the ground plane, which provides the required the band-notch characteristic to notch out the WLAN band from the UWB spectrum [18].

Based on the background of the structures of various UWB notch-antennas above, this paper proposes a simple and compact microstrip line fed planar UWB antenna with dual band-notched characteristics in 3.74 GHz (2.96-4.78 GHz) and 5.5 GHz (5.18-6.35 GHz). The dual band-notched characteristic in the proposed antenna can be achieved by creating two symmetrical L-shaped slots in the ground plane, then removing a U-shaped slot from the rectangular radiating element and followed by creating another inverted U-shaped radiating element on back side of the substrate. It is observed that by adjusting the total length of the U-shaped slots to be

approximately half the guided wavelength (Ag ) of the required

notch frequency, a destructive interference takes place making the antenna non-radiating at that notch frequency. The tuning of the central notch frequency can be done by suitably adjusting the total length of the U-shaped slots. The optimization of the design and the subsequent simulation is done by IE3D software [20]. The proposed antenna provides an

impedance bandwidth of 1.91-13.91 GHz with VSWR:::; 2, except the bandwidths of 2.96-4.78 GHz for WiMAX system, 4-GHz C-band and 5.18-6.35 GHz for IEEE802.11a and HIPERLANI2 WLAN systems. The appropriate gain and stable radiation patterns are also obtained.

In this paper, a compact UWB antenna of area 22 by 24 mm2

is first presented having symmetrical L-shaped slots in the ground plane. By removing a U-shaped slot from the rectangular radiating element, a single band-notched characteristic from 4.89 to 6.03 GHz is obtained. By creating an inverted U-shaped slot on the backside of the substrate, dual band-notched characteristic for the proposed antenna is created to reduce the potential interference between the narrowband system and the UWB system. Details of simulation results and the antenna designs are presented to demonstrate the performance of the proposed antenna.

II. ANTENNA DESIGN AND RESULTS

A. UWB Antenna Design and Results Side View Bottom Layer

\0--- 22 ---01 Fig. 1. Geometry and configuration ofUWB antenna.

Fig.l shows the geometry and configuration of a UWB antenna. The antenna was fabricated on an h=1 mm FR4 epoxy substrate with the dielectric constant 8r=4.4 and loss tangent tanO=O.002. As shown in the figure, the shape of the radiating element is rectangular. In the ground plane two symmetrical L­shaped slots are created to have the bandwidth for the usage in UWB communication systems. The radiating element is fed by 50-'0 microstrip transmission line, which is terminated with a sub miniature A (SMA) connector for the measurement purpose. The electromagnetic software IE3D is employed to perform the design and optimization process. The design parameters are given in the Fig.l.

Two symmetrical L-shaped slots are introduced in the ground plane. Hence in this way defects has been introduced in the ground plane. This defected ground structure (DGS) effects the current distribution in the ground plane, which helps in broadening the impedance characteristics of the antenna. Introducing two symmetrical L-shaped slots in the ground plane and carefully adjusting the parameters of Mg and Ng in Fig.l, extra resonances can be evoked and much enhanced impedance bandwidth may be obtained.

The proposed antenna is simulated using method of moments (MoM) based software IE3D [20]. The VSWR graphs for different values of Mg and Ng are shown in Fig.2 and Fig. 3 respectively. From the VSWR graph of Fig.2, it is clear that when there are no slots in the ground plane bandwidth is from 3 to 9 GHz, which is not adequate for the UWB communication systems. So, when there are no L-shaped slots in the ground plane, poor impedance matching occurred. As the length of the L-shaped slot Mg is increased, the impedance matching is improved quite a lot. At one point, when the length of Mg of the L-shaped slot on the both side of the 50-Q microstrip feed line is 7.3 mm, wide impedance bandwidth is obtained due to the proper impedance matching between the microstrip feed line and the rectangular radiating element. But when the length of Mg is increased further, again there is impedance mismatch between the feed line and radiating element resulting poor impedance matching, hence inadequate

impedance bandwidth for the UWB communication obtained. So, from the above parametric study, it is clear that there exist a optimum length Mg for the two symmetrical L-shaped slots in the ground plane, for which wide impedance bandwidth is obtained.

Another important parameter is Ng. which is the distance between the two symmetrical L-shaped slots in the ground plane, plays an important role in providing proper impedance matching resulting large impedance bandwidth. The distance between the two slots should be large enough to reduce the coupling between the two L-shaped slots, but by increasing the distance between the two slots Ng, the excitation due to the coupling between the two L-shaped slots is weakened, which disturbs the surface current density in the central portion of the ground plane exactly below the microstrip feed line. Increasing or decreasing the distance Ng between the two L-shaped slots, the impedance matching phenomenon of the antenna is degraded for the higher values of the frequency. The optimum value of Ng is adjusted to be 1.9 mm as shown in Fig. 3, for which the antenna provided the full band UWB characteristic from 3.44 GHz to 13.55 GHz. The optimum value of Ng (=1.9 mm) is exactly same as the width of the microstrip feed line. Fig.3 represents the gain in dBi verses frequency. The gain increases with the frequency and the maximum at 9.0 GHz. The maximum gain of antenna 1 is 3.16 dBi at 9.0 GHz.

4

3.5

� ! 2.5 · >

2

-Without Slot -Mg=5.50mm -Mg=6.20mm -Mg=6.45mm -Mg=7.30mm

Mg=8.00 mm --r-

, , 8 8 --;':1'0::----:':::--Frequency(GHz)

14

Fig. 2. Simulated VSWR for different values of Mg with fixed Values of Ng=1.9 mm ofUWB antenna.

4r-----.-----.-----.-----.-----.-----.

0:: �2.5 .

>

: -Ng=O.50 mm ".,." : ' -Ng=1.90 mm

: -Ng=4.00 mm : -Ng=4.50 mm

4 � --L- -1- L-

6 8 10 12 14 Frequency(GHz)

Fig. 3.Simulated VSWR for different values of Ng with fixed Values of Mg=7.3 mm ofUWB antenna.

4r------r------r------r------r-----�

2 ,, , ,,,,,,,, . ,,,, .. ,,j. .,,,, . .. ,,,,,,,,,, ... l.,,.,,,,.,,,,

! !

i 0 .................... � ........ ············�······················r····················· i .. ··················

� 1 : : ! � -2 ,""' ,." .,"",.' f""·" "·"""·,,·:·" ·""""·, .. ··,,·,

;·" "·,,·""" ·"'·

I"""·"""·"" ..

-4 """" " ""'''1' ''''''''''''''''''' :'''''''''''''''''''''+ '''''''''''''''''''1''''''''''''''"" ..

-"2 4 8 8 Frequency(GHz)

10 12

Fig. 4. Simulated gain (dBi) vs. frequency ofUWB antenna.

B. UWB Band-Notch Antenna Design and Results

Along with the UWB spectrum (3.1-10.6 GHz), some narrowband systems operate. Notable among them is IEEE 802.11a and HIPERLANI2 WLAN system. Hence, to mitigate the interference from the above narrowband system, band­notch function is desirable in the UWB system.

Fig.5 shows the geometry and dimension of the UWB antenna with band-notch characteristic from 4.89-6.03 GHz band. By removing an U-shaped slot from the rectangular­radiating patch having two symmetrical L-shaped slots in the ground plane of UWB antenna, a band notch function is created. It is noteworthy that when the band-notched structure is applied to the UWB antenna, there is no redesigning work needed for the previously taken dimensions. In general, the main aim behind the design methodology of the notch function is to tune the total length of the U-shaped slot approximately equal to the half guided wavelength (Ag) of the desired notch frequency, which provides the input impedance singular. At the desired notch frequency, the current distribution is around the U-shaped slot. Hence, a destructive interference for the excited

surface current will occur, which causes the antenna to be non­responsive at that frequency. The input impedance closer to the feed point, changes abruptly making large reflections at the required notch frequency.

Side Viow Top Layer Bottom Layer

Il ��'

I. 22 ----oi Fig. 5. Geometry and configuration ofUWB notch antenna.

The guided wavelength of a slot line is approximately given by [19]

it =it� 2 g 8 +1 r

(1)

where Ag is the guided wavelength, A is the free space wavelength and 8r is the permitivity of the substrate. According to Cohn [19], a half wavelength slot can be served as a resonator, because in the air regions of the slot, the magnetic field lines make curve and return to the slot region at half the guided wavelength interval. Hence to generate a resonance at a desired frequency, the physical length of the slot should be half the guided wavelength. In our case the shape of the slot is U and its physical length (L) should be approximately equal to

A-L == --2.... (2)

2 The guided wavelength (Ag) can be expressed in terms of the notch frequency (f,,) and is given by

it = .!:... � 2 (3) g

in 8r + 1 The notch frequency (f,,) can also be expressed in terms of the physical length (L) of the U-shaped slot and is given by

.f =

.::... � 2 (4) n 2L 8r + 1

where c is the speed of light in vacuum. Fig.6 depicts the simulated VSWR of the UWB notch antenna for the different N. As observed, the adjustment of the band-notched frequency can be done by varying the length (N) of the U-shaped slot. By decreasing N from 7.15 to 5.7 mm, the tip of the notched band shifted from 5.0 GHz to 6.0 GHz. The total simulated length L for the UWB notch antenna is denoted as L=2N+0.5+0.5+6.

The performance of the simulated VSWR of the UWB notch antenna, which provides the desired center notch frequency of 5.5 GHz, is shown in the Fig.6. From the figure it is very clear that, the desired filtering property is achieved by

introducing an U-shaped slot. Compared to UWB antenna design, the single band-notched UWB antenna effectively blocks out the 5-6 GHz and still performs excellent impedance matching at other frequencies of UWB band. The tip of the desired notch band is exactly at 5.5 GHz at the VSWR value of 6.89, which is the center frequency of the WLAN band. The notch band stretches from 4.89 GHz to 6.03 GHz, in which whole of the WLAN band is immersed. The antenna gain of UWB notch antenna, compared to UWB antenna in the entire UWB is presented in the Fig.7, which shows a sharp decrease in gain at 5.5 GHz, which is the center frequency of the WLAN band and good performances at other frequencies of the UWB band. The value of gain at 5.5 GHz is -7.83 dBi.

7�--�-a'---���==���

8 ················f········� : II : II : I. S ············ f . '/'

, � I

12 8 8 10 14 Frequency(GHz)

Fig. 6. Effect of length (N) on the VSWR of the UWB notch antenna.

4,r----r---r----r---r---�

_ :::t:���t:: -- "we Ante •• , iii : : -UWB Notch Antenna � -2 .......... ....... ! ... .... . ... . . .. y················l··················+··················· C) -4 ... ... .. . .. . j......... ... . ) ... _ ............ ) ...................• i .................. .

: � � : -8 · .. ········· .... · . . ·1 .. · · .... ······ ··

T .. · · · .... ··· .... · ··t .. ···· .. ··· .. ···· .... (··· · · ...... ···· ..

; : .8l---':-----'�-----::O----:'::-----:-'. � 4 8 8 10 12 Frequency(GHz)

Fig. 7. Simulated gain (dBi) vs. frequency of UWB notch antenna, compared to UWB antenna.

C. UWB Dual Band-Notch Antenna Design and Results Side View Bottom Layer

22

• Y .\' o.

......J W I . 1---- 22 ----I

Fig. 8. Geometry ofUWB dual band-notch antenna.

Apart from WLAN, WiMAX occupies band from 3.3-3.6 GHz and that is within the UWB frequency band of operation and this interferes with the operation of the UWB systems. Apart from WiMAX, 4 GHz C-band also operates at around 4 GHz. Hence to mitigate the potential interference between the narrowband systems, in this paper a new design of dual band­notched UWB antenna is presented. By introducing an inverted U shaped radiating element on the back side of the substrate of the UWB notch antenna, a dual band notch characteristic is obtained whose first notch band is extended from 2.96 GHz to 4.78 GHz having the center notch frequency at 3.74 GHz and the second notch band is extended from 5.18 GHz to 6.35 GHz having the center notch frequency at 5.5 GHz. Fig.8 depicts the geometry of the UWB dual band-notched antenna whose one U-shaped slots is created in the rectangular radiating element and other inverted U-shaped radiating element is introduced in the back side of the substrate. The U-shaped slot in the rectangular radiating element provides the band-notch whose center frequency is at 5.5 GHz. The inverted U-shaped radiating element in the backside of the substrate is responsible for crating a band notch whose center frequency is at 3.74 GHz, each of the total length of the U-shaped slot is obtained by using the expression (2).

Fig.9 shows the simulated VSWR of the UWB dual band-notch antenna compared to antenna UWB antenna. The VSWR graph of the UWB dual band-notch antenna provides two notches centered at 3.74 GHz and 5.5 GHz. The frequency band at center frequency at 3.74 GHz extends from 2.96 GHz to 4.78 GHz, which is the operating band of WiMAX and 4 GHz C-band. The frequency band at center frequency at 5.5 GHz extends from 5.18 GHz to 6.35 GHz, which is the operating band of WLAN. Hence it can be concluded that the two notch bands for the UWB dual band­notch antenna are created by two U-shaped slots, one on the rectangular radiating element and other at the back side of the substrate, effectively notch out the operation bands of WiMAX, 4 GHz C-band and WLAN respectively, mitigates the potential interference between the narrowband systems and UWB system. The overall UWB band extends from 1.91 GHz to 13.91 GHz.

9r--'�--�==�====c===�==� i --UWB Antenna 8 ............. . ... j -UWB i Dual Ba�d-notch �ntenna 7 ............ ... ............ ' �"""""'''''''''r ''''''''''''''''''; '''''''''''''''''' : '''''''''''''' ''

; ; ; ; 8 ....... .. .... ; ....... . . : : : ;

� 5 .... ......... .. 1. ... . .. . .. .. 1. ..... : .. :: ..... :.: ....... : .. : .... :.:1:..: ....... :. ::: .::.:::: .:: .... ::: .. � ! i i ! : 4 \�.. ..... "'--1-' ...... .. 1 .................. ;, ...... · ...... ·

.. [ ....

........... ..

. i .............. · ..

3 \�" ....

. + ... ..... . ... . ..... ... · .. . . . .. + .... . . ..... . . .. + .... .. · ..... ..... i ............... . .

·�',i��� .. .. ... + . ..

..

....

.. ...... �·���·�·�� .. C�=�·; .. ·� .. ·::���/ 8 8 10 Frequency(G Hz)

12 14

Fig. 9. Simulated VSWR of UWB dual band-notch antenna, compared to UWB antenna.

Fig.10 shows the variation of gain in dB with the frequency for the UWB dual band-notch antenna compared to UWB antenna. From the Fig.1 0 it is clear that there is sharp dip in the gain at around 3.9 GHz and 5.5 GHz, which confirms the effective operation of the UWB dual band-notch antenna in the two-narrowband systems. However, for the other frequencies outside the notched band, the antenna gain is appropriately varying and almost stable in the whole of the UWB band. The gain of UWB dual band-notch antenna at 3.9 GHz is -11.23 dBi and the gain of UWB dual band-notch antenna at 5.5 GHz is -11.75 dBi respectively.

4r------r------r------r------r-----� . .

: r.;;::�J:::::t�··::1

·· · -""

=- -2 .... ..... ........ �i.� .... ... . ...... ;. -UWB Dual Band-notch Antenna. In ,.' i i i i � ·4 .... .. , �� ... . . . . � .... ........... . . . � .... ·············· ··+············· · ··· · ··· i········ ······· .... .

a .; :+ ........... ·!1::1 -1� 4 6 8

Frequency(GHz) 10 12

Fig. 10. Simulated gain (dBi) vs. frequency of UWB dual band-notch antenna, compared to UWB antenna.

Fig.ll shows the E-plane (yz-plane) radiation pattern of the UWB dual band-notch antenna at 3.5, 5.5, 7.5 and 10.5 GHz respectively and Fig.12 shows the H-plane (zx-plane) radiation pattern of the UWB dual band-notch antenna at 3.5, 5.5, 7.5 and 10.5 GHz respectively The H-plane radiation pattern is purely omni-directional at all the simulated frequencies. In the E-plane, the radiation pattern is like a small dipole leading to a bi-directional radiation pattern. The E-plane radiation pattern is directional along _90° and 90° respectively

at simulated frequencies of 3.5 GHz and 10.5 GHz. At 5.5 GHz the E-plane is tilted left side upward and directed along _60°

and 120°. At 7.5 GHz the E-plane is tilted left side downward and directed along -150° and 30°. There is almost no change in the shape of E-plane radiation pattern at all the simulated frequencies. Hence the UWB dual band-notch antenna exhibits stable and constant radiation pattern at all the frequencies.

. ,,, ;r���.���------'���

«)

o

(d) Fig. 11. Simulated E-plane (yz-plane) radiation patterns of UWB dual band-notch antenna at (a) 3.5 GHz, (b) 5.5 GHz, (c) 7.5 GHz and (d) 10.5 GHz.

(,) o

(1J)

7�H-1"""''''''-''''''''_'''''�'''''. '_....,...--....... .,.,,17�O ) 1>;-. -+----+--:co1c:----+----+---:''J� «) (d)

Fig. 12. Simulated H-plane (zx-plane) radiation patterns of UWB dual band-notch antenna at (a) 3.5 GHz, (b) 5.5 GHz, (c) 7.5 GHz and (d) 10.5 GHz .

III. CONCLUSION

A compact microstrip line fed monopole antenna with single and dual band-notched function for the application in UWB communication system has been presented and analyzed. Two symmetrical L-shaped slots are introduced in the ground plane of the antenna, which are responsible for the proper impedance matching, thus provides the full operating band for the UWB systems. Creating a U-shaped slot in the rectangular radiating element and introducing an inverted U-shaped radiating element in the rear of the substrate, a single and dual band­notch characteristics are created, which alleviates the potential interferences with existing WiMAX, C-band and WLAN operating bands respectively. Stable radiation pattern and appropriate gain in the UWB bands are obtained. The antenna presented in this paper is expected to find future application in UWB system.

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[20] IE3D version 10.2, Zeland Corp., Freemont, CA, USA.