An Inset-fed Dual-Frequency Circular Microstrip Antenna with a Rectangular Slot for Application in

Wireless Communication Jyoti R. Panda and Rakhesh S. Kshetrimayum

Department of Electronics and Communication Engg Indian Institute of Technology

Guwahati, India E-mail :-{ j.panda, krs}@iitg.ernet.in

Aditya S. R. Saladi School of Electronics Engineering

Vellore Institute of Technology, VIT University Vellore, India

E-mail :- aditya.s.ram@gmail.com

Abstract— An inset-microstrip line-fed circular microstrip antenna with a rectangular slot on the radiation element for the dual-frequency operation is proposed. The circular microstrip antenna resonates at 1.58 GHz and 2.43 GHz, which enables it’s usage in the wireless communication domain such as in Wireless Local Area Network (WLAN). The two excited modes, which are responsible for the dual-band operation for the proposed antenna, are the TM11 and TM01. TM11 is the fundamental as well as the dominant mode for this proposed antenna. The two modes have the similar broadside radiation characteristics and same polarization planes. By varying the lengths of the rectangular slot, the proposed antenna can provide a tunable frequency ratio of 1.50 to 1.62 for the two operating frequency bands.

Keywords— Dual-frequency, inset-fed, circular microstrip antenna, rectangular slot, wireless communication, WLAN.

I. INTRODUCTION Microstrip patch antennas are attractive for their well-known efficient features such as compatibility with monolithic microwave integrated circuits (MMIC), light weight, less fragile, low profile etc. The main disadvantage associated with microstrip patch antennas is the narrow bandwidth, which is due to the resonant characteristics of the patch structure. But on the other hand modern communication systems, such as those for wireless local area networks (WLAN), as well as emerging applications such as satellite links (vehicular, GPS, etc.) often require antennas with low cost and compactness, thus requiring planar technology. Due to the light weight of the microstrip patch antennas, they are appropriate for the systems to be mounted on the airborne platforms such as synthetic aperture radars (SAR) and scatterometers. Because of these applications of the microstrip patch antenna, a new motivation is evolved for research and development on indigenous solutions that overcome the bandwidth limitations of the patch antennas. In applications in which bandwidth enhancement is required for the operation of two separate sub-bands, an appropriate alternative to the broadening of the total bandwidth is represented by dual-frequency microstrip antenna, which exhibits a dual-resonant behavior in a single radiating element.

The most well known technique for generating a dual-frequency behavior in the single-fed microstrip antenna is to introduce reactive loadings in the radiating element. The easiest way to do so is to create or etch slots on the radiating element of the microstrip patch antenna. The slot loading in the radiating patch creates a strong modification of the resonant modes, particularly when the slots are configured to obstruct and cut the current lines of the unperturbed modes. For example in [1], the simultaneous use of short circuit vias and slots allows a tuning frequency ratio from 1.3 to 3, depending upon the number of vias. The square microstrip patch antenna [2] provides dual band operation with a rectangular notch along the radiating edge. Two rectangular slots are created [3] in parallel with the two radiating edges of the rectangular patch for the dual-frequency operation. In [4], two rectangular slots are etched along the two radiating edges of the rectangular patch and were fed by an aperture, coupled to the radiating element. In [5], two rectangular slots are taken out from the rectangular radiating element, are placed in parallel to the two radiating edges of the patch and fed by a coaxial feed to have the dual-band operation of the microstrip patch antenna. The microstrip patch antenna [6] is fed by using an inclined slot in the ground plane of a microstrip line for dual-frequency operation. In [7], the reactive loading is provided by a structure of two cascaded microstrip-line sections embedded within a rectangular slot cut in the patch. By selecting suitable dimensions of the microstrip-line sections and the rectangular slot, dual-frequency operation can be obtained. In [8], the slots loaded in the patch are a pair of outward bent slots centered in the rectangular microstrip patch and oriented along the resonant direction, with the spacing between the two slots being about half the patch's radiating-edge length, a dual-frequency operation has obtained. The circular microstrip antenna is loaded [9] with a rectangular stub along its feeding axis for the dual band operation. Dual-frequency design [10] of a single-layer single-coaxial-feed circular microstrip antenna can be obtained with an offset open-ring slot embedded on the circular radiating elements. In [11], a v-shaped slot is created in the center of the rectangular patch, which provides dual-frequency operation. In [12], multi-band printed antenna for an application in WLAN

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domain is presented with a circle slot and a cross-shaped patch.

Based on the background of the structures of various dual-frequency microstrip antennas above, this paper proposes a simple inset-fed dual frequency circular microstrip antenna having a rectangular slot on the circular radiating element. The dual-frequency operation of the proposed antenna is achieved by etching a rectangular slot on the top of the circular radiating element. The proposed antenna is fed by 50-Ώ microstrip transmission line and the fed is inserted into the circular radiating element for the proper impedance matching. The proposed dual-frequency inset-fed circular microstrip antenna resonates at 1.58 GHz (TM11 Mode) and 2.42 GHz (TM01 Mode) having the percentage bandwidth of 2.5 and 2.07 respectively for the slot length of 25 mm. The frequency ratios of the two operating frequencies are tunable in the range of about 1.50 to 1.62. Since one of the resonant frequencies is at 2.42 GHz, this antenna can be used in the operating band of wireless local area networks (WLAN).

In this paper, a simple dual-frequency inset-fed circular microstrip antenna with a rectangular slot for the application in the WLAN is proposed. The radius of the antenna is 25 mm.

II. ANTENNA DESISN AND RESULTS

A. Single Frequency inset-fed Circular Microstrip Antenna

Fig. 1 shows the geometry and configuration of a single

frequency inset-fed circular microstrip antenna. The antenna (referred to as antenna 1 in this paper) was fabricated on an h=1.6 mm FR4 epoxy substrate with the dielectric constant εr=4.4 and loss tangent tanδ=0.002. As shown in the figure, a microstrip transmission line is used to feed the circular microstrip antenna. The feed is inserted deep into the circular radiating element for the proper impedance matching. This arrangement for the feeding a microstrip antenna is known as the “inset-feeding”. The electromagnetic software IE3D [15] is employed to perform the design and optimization process. The design parameters are W=75 mm, L=80 mm, n =37 mm, g =1.0 mm, wf=3.05 mm and R=25. The inset length of the microstrip feed line is fixed at n=37 mm, which is 1.48 times (1.48R) of the radius of the circular microstrip antenna.

The dominant and fundamental mode responsible for the first resonance in the circular microstrip antenna is TM11 mode. For the dominant and fundamental mode TM11, the expression for the resonant mode is given by [13]

0r 11

r

1.8412c(f ) =2πR ε

(1)

where c0 is the velocity of light in free space, R is the radius of the circular microstrip antenna, εr is the dielectric constant of the substrate. The resonant frequency in eq.(1) is derived without the fringing effect. Due to fringing, the patch looks electrically larger then the normal circular patch and it is taken

Fig. 1. Geometry of antenna 1.

Fig. 2. Return Loss (dB) vs. frequency of antenna 1.

into account by incorporating a radius correction factor. For the circular patch, the radius correction is introduced by using effective radius Re, in place of actual radius R, given by [14]

12

er

2h πRR R 1 ln( ) 1.7726πRε 2h

⎧ ⎫⎡ ⎤= + +⎨ ⎬⎢ ⎥⎣ ⎦⎩ ⎭ (2)

where h is the height of the dielectric substrate. After the radius correction, the resonant frequency for the dominant mode TM11 is modified by eq.(2) and is expressed as

0r 11

e r

1.8412c(f ) =2πR ε

(3)

Hence, putting all the numerical values of the design parameters into eq.(2) and eq.(3), the theoretical first resonance frequency(f1) was found to be 1.63 GHz. Fig.2 shows the return loss (|S11|) (dB) vs. frequency of the single frequency inset-fed circular microstrip antenna. The antenna resonates at 1.61 GHz (TM11 mode) at the return loss value of –25.96 dB, which is very close to the theoretical value of the first resonance i.e. at 1.63 GHz. The band extends from 1.59 GHz to 1.64 GHz at –10 dB having the percentage

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bandwidth of 3.09. Fig.3 depicts the VSWR characteristics of the antenna 1. The value of VSWR at the first resonance frequency 1.61 GHz is 1.10. This depicts that there is good impedance matching between inset-fed microstrip transmission line and the circular radiating element in the frequency band from 1.59 GHz to 1.64 GHz.

From the Fig. 4 the variation of the input impedance versus frequency of the inset-fed circular microstrip antenna can be seen. At 1.61 GHz, the value of imaginary part (reactance) of the input impedance is almost 0-Ώ and simultaneously the value of real part (resistance) is approximately 50-Ώ .Hence, from the graph it is clear that there is proper impedance matching occurs at the first resonance frequency i.e. at 1.61 GHz.

B. Dual-frequency inset-fed circular microstrip antenna with rectangular slot.

Fig.5 shows the geometry and dimension of the dual

frequency inset-fed circular microstrip antenna with a rectangular slot (referred to as antenna 2 in this paper). The

Fig. 3. VSWR vs. frequency of antenna 1.

Fig. 4. Antenna input impedance vs. frequency of antenna 1.

Fig. 5. Geometry of antenna 2

rectangular slot is etched at the tip of the circular patch of the microstrip antenna. The design parameters of the proposed inset-fed circular microstrip antenna with a rectangular slot are W=75 mm, L=80 mm, m=70 mm, n =37 mm, g =1.0 mm, wf=3.05 mm, w=25 mm and R=25. The length of the inset is n=37 mm which is 1.48 times the radius of the circular patch. The width of the rectangular slot is p=1 mm. Fig.6 shows the return loss (|S11|) (dB) vs. frequency of the dual frequency inset-fed circular microstrip antenna with a rectangular slot on the tip of the circular patch. The antenna resonates at 1.58 GHz (TM11 mode) and 2.42 GHz (TM01 Mode) at the return loss values of –17.21 dB and –41.67 dB respectively. The first resonance (f1=1.58 GHz) band extends from 1.56 GHz to 1.66 GHz at –10 dB having the percentage bandwidth of 2.53 and the second resonance (f2=2.42 GHz) band extends from 2.39 GHz to 2.44 GHz at –10 dB having the percentage bandwidth of 2.07. Fig.7 represents the effect of variation of slot length (w) on the two resonance frequencies. As the value of the length of the slot increases from 23.5 mm to 26.0 mm, the first resonance frequency (f1) is slightly affected but the second resonance frequency (f2) is considerably affected. The second resonance frequency (f2) shifted left i.e. decreased with the increase of the slot length (w). At w=25 mm the second resonance frequency (f2) is at 2.42 GHz, for which this antenna can be used in the wireless local area network (WLAN) domain. Fig. 8 depicts the VSWR vs. frequency of the two resonance frequencies. The values of the VSWR at 1.58 GHz and 2.42 GHz are 1.31 and 1.01 respectively. This depicts that there is good impedance matching between inset-fed microstrip transmission line and the circular radiating element at the two resonant frequencies of 1.58 GHz and 2.42 GHz. From the Fig. 9 the variation of the input impedance versus frequency of the inset-fed circular microstrip antenna at the two resonance frequencies can be seen. At 1.58 GHz, the value of imaginary part (reactance) of the input impedance is almost 0-Ώ and simultaneously the value of real part (resistance) is approximately 50-Ώ. Similarly at 2.42, the value

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of imaginary part (reactance) of the input impedance is almost 0-Ώ and simultaneously the value of real part (resistance) is approximately 50-Ώ. Hence, from the graph it is clear that there is proper impedance matching occurs at the first resonance frequency i.e. at 1.58 GHz as well as at the second resonance frequency i.e. at 2.42 GHz.

Fig. 6. Return Loss (dB) vs. frequency of antenna 2.

Fig. 7. Effect of variation of slot length (w) on the resonant

frequencies of the antenna 2.

Fig. 8. Simulated VSWR vs. frequency of antenna 2.

Fig. 9. Antenna input impedance vs. frequency of antenna 2.

Fig. 10. Variation of slot length (w) vs. resonant frequencies

of antenna 2.

Fig. 11. Variation of slot length (w) vs. normalized frequency

(f2/f1) of the antenna 2. Fig.10 provides the information regarding the effect of variation of the slot length (w) on the two resonant frequencies. As the value of the length of the slot increases from 23.5 mm to 26.0 mm, the first resonance frequency (f1)

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is slightly affected but the second resonance frequency (f2) is considerably affected. The second resonance frequency (f2) shifted left i.e. decreased with the increase of the slot length (w) from 2.54 GHz to 2.38 GHz. From Fig. 11, it is clear that the normalized frequency (f2/f1) decreases with the increase of the slot length (w). The frequency ratios of the two resonant frequencies are tunable in the ranges from 1.50 to 1.62. Hence, this proposed inset-fed circular microstrip antenna provides wide tunable range for operation of this antenna in different frequencies by changing the length of the slot. Table I provides the dual-frequency performance of the proposed antenna with the various slot length (w).

TABLE I

Dual-frequency performances of the proposed antenna with various slot length (w)

With the help of the electromagnetic software IE3D, the excited surface current densities for the two operating frequencies can be obtained. The surface current densities of the two resonant frequencies of f1=1.58 GHz and f2=2.42 GHz, has been shown in the Fig. 12. For the first resonant frequency f1, resonant mode is perturbed TM11 mode. It is found that, the excited surface current density is almost similar to that of the TM11 mode when there is no slot in the circular radiating element.. The main reason behind this behavior is that, the slot is etched very close to the patch periphery, where the excited patch surface current for the TM11 mode has a minimum value. Because of this reason the resonant frequency of the perturbed TM11 mode is slightly lower then the unperturbed TM11 mode in case of circular microstrip antenna without the slot.

Fig.12. Simulated current distributions for the proposed design

with slot length w=25 mm (a) at 1.58 GHz (b) at 2.42 GHz.

For the second resonant frequency f2=2.42 GHz, the resonant mode is found to be perturbed TM01 mode, Generally the null of the current distribution for the unperturbed TM01 mode is located at the center of the circular patch, when there is no slot on the circular patch. Because of the loading of the rectangular slot in the periphery of the circular radiating element, the null of current distribution, which is shown in the Fig.12 (b), is shifted close to the edge of the rectangular slot. The main reason for the shifting of null of the perturbed TM01 mode to the edge of the rectangular slot is to maintain the similarity of the current distribution of the TM11 mode at the center of the patch. Hence for the dual-frequency operation of the proposed antenna, the resonant frequencies are expected to be of same polarization planes and similar radiation characteristics. Fig.13 represents the simulated E-plane (y-z-plane) and H-plane (x-z-plane) radiations pattern of antenna 2 at 1.58 GHz for slot length w=25 mm and Fig.14 represents simulated E-plane (y-z-plane) and H-plane (x-z-plane) radiation patterns of antenna 2 at 2.42 GHz for slot length w=25 mm.. From the Fig. 13 and Fig.14 it is clear that the E-plane radiation pattern is broader than the H-plane radiation pattern, which is the usual case for microstrip antennas. It is seen that both operating modes have the same broadside radiation patterns and polarization planes.

Fig. 13. Simulated E-plane (y-z-plane) and H-plane (x-z-plane) radiations pattern of antenna 2 at 1.58 GHz for slot

length w=25 mm.

Fig. 14. Simulated E-plane (y-z-plane) and H-plane (x-z-plane) radiation patterns of antenna 2 at 2.42 GHz for slot

length w=25 mm.

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III. CONCLUSION An inset-microstrip-line-fed dual-frequency circular microstrip antenna with a rectangular slot for using in wireless applications such as wireless local area network (WLAN) has been demonstrated. The bandwidth around the two operating frequencies is sufficient enough for the dual-band wireless applications and similar broadside radiation characteristics are observed at both the resonant frequencies. By varying the length of the rectangular slot, the two operating frequencies of the proposed design can have a tunable frequency ratio from about 1.50 to 1.62. The obtained radiation patterns are suitable for the base-station wireless applications.

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