Design of Coaxial Fed Microstrip Patch Antenna for Wi-MAX/ IMT Applications

 

Jaswinder Kaur* and Rajesh Khanna

Department of Electronics and Communication Engineering, Thapar University, Patiala-147004, Punjab, India.

 

ABSTRACT:

In this paper, a single layer, single co-axial feed, broadband, compact rectangular microstrip patch antenna is proposed. This proposed antenna is constructed by cutting a Т-shape slot under a Pi-shape slot in the patch fabricated on a substrate with relative permittivity 2.33 and thickness 8mm using simple coaxial feeding technique. In addition, four circular slots have been cut into the radiating edges of patch for broadband operation to cover both Wi-MAX (3400MHz-3690MHz) & IMT (3900MHz-4400MHz) communication standards. Basically, the use of these circular slots has effectively excited multi-resonant modes together with good impedance bandwidth. A thick substrate helps broaden the bandwidth for covering the entire frequency range of Wi-MAX & IMT communication standards. The proposed slot loaded patch antenna resonates at 2.66 GHz and has a broadband from 3.4 GHz to 4.5 GHz with the corresponding bandwidth of 108 MHz and 1.14 GHz and return losses of -16.0 dB and -22.6 dB respectively. The antenna gives a stable radiation performance with gain greater than 7dB over the entire broadband. The simulation has been performed by using CST Microwave Studio, which is a commercially available full wave electromagnetic simulator based on the method of finite difference time domain technique. Meanwhile, the proposed antenna exhibit almost omni-directional radiation pattern, relatively high gain and low cross polarization. Results for reflection coefficient and far-field radiation pattern of the designed antenna are presented and discussed. The analysis of the simulated results confirms successful design of co-axial fed microstrip patch antenna (MPA).

 

KEY WORDS: Wi-MAX, IMT, broadband, impedance bandwidth, microstrip patch antenna (MSA), CST Microwave Studio

 

INTRODUCTION:

Slot loaded patch antennas are currently under consideration for use in broadband communication systems due to their attractive features, such as wide frequency bandwidth, low profile, lightweight, easy integration with monolithic microwave integrated circuits (MMIC), low cost, good impedance matching, appreciable radiation patterns and ease of fabrication.  Recent technologies enable wireless communication devices to become physically smaller in size. Obviously, antenna size is a major factor that limits miniaturization..

 

With the rapid growth of the wireless mobile communication technology, the future technologies need a very small size antenna and also the need of multi-band antennas is increased to avoid using two antennas and to allow video, voice and data information to be transmitted simultaneously

 

Recently, there is an increased demand for multi-frequency resonant and wide-band antennas that can be easily integrated with the communication system. In order to meet the above mentioned requirements, microstrip patch antenna (MPA) is one of the best candidates. The major drawback of microstrip antenna is their narrow impedance bandwidth [1]. Wireless local area network (WLAN) and Worldwide Interoperability for Microwave Access (Wi-MAX) technology is the most rapidly growing area in the modern wireless communication [2-4]. This gives users the mobility to move around within a broad coverage area and still be connected to the network. This provides greatly increased freedom and flexibility. For the home user, wireless has become popular due to ease of installation and location freedom. Naturally, these applications require antennas. This being the case, portable antenna technology has grown along with mobile and cellular technologies. It is important to have the proper antenna for a device. The proper antenna will improve transmission and reception, reduce power consumption, last longer and improve marketability of the communication device. Also, enlarging the frequency bandwidth of antennas is the major requirement, especially to avoid channel saturation at the receptor. In this paper, slot loaded broadband microstrip patch antenna for Wi-MAX & IMT applications is designed and simulated using CST Microwave Studio. The proposed patch antenna firstly resonates at 2.66 GHz and has another broadband (3.4GHz-4.5GHz) with impedance bandwidth of 1.14 GHz which is feasible for Wi-MAX (3400MHz-3690MHz) & IMT (3900MHz-4400MHz) applications. The large impedance bandwidth has been obtained by modifying the shape of radiating element which is based on the modification of surface current distribution which requires an intensive analysis.

 

ANTENNA DESIGN:

In this paper several parameters have been investigated using CST Microwave Studio software. The geometry of slot loaded patch antenna is shown in Figure 1(a).

The design specifications for patch antenna are: Substrate permittivity () = 2.33, Substrate thickness (h) = 8 mm, Length of patch (L) = 37 mm, Width of patch (W) = 24 mm, Feed point location = (0, 2.8), Dimensions of ground ( Lg x Wg ) = 90 mm x 80 mm. The longer dimension of the patch is along the x-axis while the shorter dimension is along the y-axis. The Pi-shape slot dimensions and T-shape slot dimensions are manipulated for optimum impedance matching and appreciable return loss. The width of each slot is 2mm. The antenna structure is fed with a co-axial probe of 50 ohms. The inner and outer radius of co-axial probe is 1.5 mm and 3 mm respectively. Also, outer radius for each of the circular slots being cut into the radiating edges of patch is taken to be 3mm with co-ordinates (x, y) as (15, 12.5) respectively. The structural view of patch antenna is shown in Figure 1(b).

 

Figure 1(a): Geometry of slot loaded single layer patch antenna

 

Figure 1(b): Structural View of Patch Antenna in CST Microwave Studio

 

RESULTS AND DISCUSSION:

The broadband characteristics of the proposed antenna are achieved by incorporating a Т-shape slot under a Pi-shape slot and four circular slots been cut at the radiating edges along the width of rectangular MSA. The frequencies of these bands are decided by the electrical length of these slots. In addition to other factors, the thick substrate (h) = 8mm, helps in achieving the required wide impedance bandwidth [5]. The feed location is moved from the centre of geometry along y-axis to get the best possible impedance match to the antenna. Simulation studies of proposed antenna reported here are carried out using CST Microwave Studio full wave EM simulator. The return loss of the slot loaded patch antenna is shown in Figure 2 which shows that it resonates at 2.66 GHz and has a broadband covering the frequencies from 3.4 GHz to 4.5 GHz. These resonant frequencies give the measure of impedance bandwidth characteristics of the patch antenna [6]. The impedance bandwidth for the proposed antenna is 108 MHz (from 2.61 GHz to 2.72 GHz) for the first band and 1.14 GHz (from 3.4 GHz to 4.5 GHz) for the second broadband at -10 dB return loss. From Figure 2 the return loss values at the two operating bands are -16.0 dB and -22.6 dB respectively. The achieved values of return loss are small enough and frequencies are closed enough to specified frequencies bands for Wi-MAX & IMT applications. These return loss values suggest that there is good matching at the frequency point below the -10 dB region.

 

Figure 2: Return Loss verses frequency graph

 

Figure 3 shows VSWR verses frequency graph for the proposed antenna, the value of VSWR at the resonating frequency of 2.66 GHz is 1.34 and also for the broadband from 3.4 GHz to 4.5 GHz, VSWR is below 2. As shown, VSWR is 1.39 for the central frequency of 4 GHz. The value of VSWR for both frequency bands is less than 2 which shows that there is an appropriate antenna impedance matching at these two frequencies.

 

Figure 3: VSWR verses frequency graph for = 2.66 GHz & 4 GHz

 

Figures 4(a), (b) & (c) show the simulated 3-D radiation pattern which is almost omni-directional with directivity 6.911 dBi, E-plane pattern (Elevation plane) and H-plane pattern (Azimuthal plane) of proposed antenna at the resonating frequency of 2.66 GHz respectively.

 

Figure 4 (a): 3-D Radiation Pattern of proposed antenna at 2.66 GHz

 

Figure 4 (b): E-Plane Radiation Pattern of proposed antenna at 2.66 GHz

 

Figure 4 (c): H-Plane Radiation Pattern of proposed antenna at 2.66 GHz

 

Figures 5(a), (b) and (c) show the simulated 3-D radiation pattern which is almost omni-directional with directivity 7.404 dBi, E-plane pattern and H-plane pattern of proposed antenna at the central frequency 4 GHz of the broadband satisfying Wi-MAX and IMT communication standards respectively.

 

 

Figure 5 (a): 3-D Radiation Pattern of proposed antenna at 4 GHz

 

 

Figure 5 (b): E-Plane Radiation Pattern of proposed antenna at 4 GHz

 

Figure 5 (c): H-Plane Radiation Pattern of proposed antenna at 4 GHz

 

Figure 6 shows the simulated 3-D broadband gain curve of proposed antenna. The marked points show the corresponding gain of 6.54 dB and 7.17 dB at 2.66 GHz and 4 GHz respectively.

 

Figure 6: Broadband gain curve of proposed antenna design

 

CONCLUSION:

A novel wideband antenna is constructed by cutting a Т-shape slot under a Pi-shape slot in the patch fabricated on a substrate with relative permittivity 2.33 and thickness of 8mm. Also, four circular slots are being cut at the radiating edges of the patch for proper impedance matching and broadband operation. Co-axial feeding technique is used for this design as the main advantage of this type of feeding scheme is that the feed can be placed at any desired location inside the patch in order to match with its input impedance. Also this feed method is easy to fabricate and has a low spurious radiation.  The proposed design is simulated using CST Microwave Studio software to cover both Wi-MAX and IMT frequency bands. This proposed design is operating in two bands viz band I (2.61 GHz to 2.72 GHz) and band II (3.4 GHz to 4.5 GHz) covering high speed wireless applications. The achievable bandwidths of the proposed antenna are obtained about 108 MHz and 1.14 GHz at -10 dB return loss which corresponds to 3.5 Wi-MAX and (3900-4400 MHz) IMT frequency bands. The achievable gain is 6.54 dB and 7.17 dB with the corresponding return losses of -16.0 dB and -22.6 dB at band I (2.61 GHz to 2.72 GHz) and band II (3.4 GHz to 4.5 GHz) respectively. Stable and good radiation pattern results have been obtained across the entire operating frequency bands which seem to be adequate for the envisaged applications. The impedance matching of the proposed antenna is achieved by adjusting the feed point of the co-axial feeding structure. The wide impedance bandwidth for upper operating frequency depends on the size of different slots being cut into the patch. Although this antenna is designed for Wi-MAX and IMT band applications, the design concept can be extended to other frequency bands of interest. With the simplicity of feeding, the investigated wide bandwidth antenna is a good candidate for fabrication to be used for many wireless communication applications. However, the size of microstrip patch antenna (MPA) reported here, is not very small because of the large size of ground plane. Also, the antenna is not so thin and compact because of the use of substrate material of thickness 8mm which is quite big. Work is going on to achieve even better results with good axial ratio and efficiency over a wide impedance bandwidth. Also, this work has not been yet fabricated and tested due to lack of fabrication facilities at our institute, but it will be soon fabricated and tested by the authors.

 

REFERENCES:

1.       Garg R, Bhartia P, Bahl I and Itipiboon A. Microstrip antenna design handbook. Artech House, Boston – London, 2000.

2.       Pigin P. Emerging mobile WiMax antenna technologies. IET Communication Engineer, October/ November 2006.

3.Kaur J, Khanna R and Kartikeyan MV. Comparison of Un-slotted and Slotted rectangular MSA for Wireless Applications. MIT Int. J. Electronics and Comm. Engg. 2011, 1, 57-59.

4        Ahuja N, Khanna R and Kaur J. Dual Band Defected Ground Microstrip Patch Antenna for WLAN/WiMax and Satellite Application. Int.J. Computer Applications 2012, 48, 22.

5.       Schaubert DH, Pozar DM and Adrian A. Effect of Microstrip Antenna Substrate Thickness and Permittivity: Comparison of Theories with experiment. IEEE Trans. Antennas and Propagation. 1989, 37, 667- 682.

6.       Balanis CA. Antenna Theory Analysis and Design. Third edition, Wiley, New Jersey, 2005.

 

 

 

Received on 11.01.2013                             Accepted on 22.01.2013        

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