Compact Circularly Polarized Patch Antenna for WiMAX Applications with Improved Impedance Bandwidth and Axial Ratio

This paper presents a circularly polarized microstrip patch antenna for WiMAX (Worldwide Interoperability for Microwave Access) application with improved impedance bandwidth and axial ratio compared to the existing designs. The antenna is designed at a resonant frequency of 3.56GHz on the FR4 substrate with 2mm thickness. The dimensions of the antenna are 0.35λo×0.35 λo×0.023λo and it is fed through a probe feed. An impedance bandwidth of 360MHz (10.11%) in the frequency range of 3.44GHz−3.8GHz, with a gain of 3.16dBi, axial ratio bandwidth of 1.9% (3.56GHz−3.63GHz) and VSWR<2 are obtained. Keywords-microstrip patch antenna; circular polarization; probe feed; WiMAX applications

INTRODUCTION Microstrip patch antennas are in great demand because of advantages such as their low profile, they are compact in size, the conformity to the supporting structure, and their flexibility and ease of fabrication. One of the important applications of this antenna is the use of transmitting or receiving systems that require circular polarization. Circular polarization is obtained by perturbing an unsymmetrical structure which results in orthogonal field components with quadrature phase-shift [1][2][3]. Circularly polarized antennas are more useful in wireless communications, RFID (radio-frequency identification), wireless LAN, and GPS. As of today these are the most opted due to the flexibility in the antenna positioning, the fewer misalignment losses, and the less interference between signals [4][5]. Circular polarization is classified by the number of feed points on the microstrip antenna. A single feed is more advantageous since it does not require an external polarizer [6]. Various techniques can be applied to the patch to create perturbation which in turn results in circular polarisation such as a capacitor loaded on an annular ring slotted patch [7], loading of shorting pins [8], capacitive and inductive loading techniques [9], and loading shorting elements in the patch [10]. Slits and slots formation on the patch [11][12][13][14], truncated corners [15], and capacitor loading produce very less impedance bandwidth and narrow axial ratio. The loading of shorting pins gives larger gain but little in bandwidth and is not compact. A new low cost structure is proposed in order to improve the impedance bandwidth and axial ratio when compared to the above techniques. For WiMAX application, according to the IEEE 802.16 standard, the frequency bands are 3.3GHz-3.4GHz, 3.4GHz-3.6GHz, and 3.6GHz-3.8GHz in various regions.

II. DESIGN OF THE PROPOSED STRUCTURE
The proposed Circularly Polarized Patch Antenna (CPPA) of circular shape with radius r=10.4mm is designed using FR4 substrate of thickness h=2mm and ε r =4.4. The dimensions of the patch antenna with the ground plane on the other side are shown in Figure 1. Triangular shaped narrow slits are cut on the four quadrants of the circular patch. One of the corner slits is cut deeper to create asymmetry in the structure which in turn results in circular polarization. Here x is the distance between the corner edge ends to the slit edge. The patch is fed through the probe feed at an optimal position.

A. Parametric Analysis
The performance of the proposed antenna structure is investigated using the IE3D 12.0 CAD tool which is based on the method of moments. As illustrated in Figure 2, the axial ratio for various values of x is simulated. As the value of x decreases, the axial ratio decreases. The return loss for various feed point locations is also simulated (Figure 3). The optimum feed point location is chosen to be (5, 0). The optimum value of x is chosen to be 6.58mm because if it is reduced below this value, the axial ratio bandwidth decreases again. The surface current distributions at the resonant frequency are shown in Figure 4.

B. Smith Chart
Smith chart gives the impedance of the antenna at the resonant frequency. Figure 5 shows that the impedance of the antenna is 50Ω. It clearly shows that impedance matching occurs at the resonant frequency, so the proposed antenna is suitable for the WIMAX application.  IV. MEASURED RESULTS The antenna, fabricated on low cost FR4 substrate, is shown in Figure 6.

A. Return Loss
To check the performance of the proposed design the return loss is shown in Figure 7. For the fabricated antenna, the measured return loss is -26.5dB at the resonant frequency of 3.56GHz with 10dB impedance bandwidth of 360MHz falling in the range of 3.44GHz−3.8GHz. This clearly shows that the designed antenna is suitable for WiMAX applications.

B. Axial Ratio
The axial ratio is a parameter that indicates the circular polarization of an antenna and is shown in Figure 8. For the proposed antenna, an axial ratio of 1.7dB and a bandwidth of 70MHz in the frequency range of 3.56GHz−3.63GHz are obtained. The gain graph is shown in Figure 9. The performance parameters are summarized in Table I. The radiation patterns in azimuth and elevation planes and VSWR (voltage standing wave ratio) are shown in Figures 10-12.

C. Discussion
The performance of the proposed antenna is better than the one of the existing designs. The inductive and capacitive loading technique to generate circular polarization yielded low return loss and compact size. The loading of shorting pins creates short from the ground to the patch producing a low impedance bandwidth. The formation of slits and slots on the patch varies the results a lot, which depend on the shape, size and position on the patch. A comparison of these techniques in terms of return loss, axial ratio and gain with the proposed design is made and listed in Table II. The proposed antenna's measured and simulated results show good agreement.

V. CONCLUSION
In this paper, a circularly polarized patch antenna is fabricated at 3.56GHz for WiMAX application. The proposed antenna's dimensions are 0.35λ 0 ×0.35λ 0 ×0.023λ 0 . Return loss of 360MHz, axial ratio bandwidth of 70MHz and a gain of 3.16dBi were obtained. Parametric analysis was done for various feed point locations, to obtain better return loss characteristics, and for different values of depth of one of the slits, to obtain a better axial ratio. Measured and simulated results show good agreement. The proposed antenna is of low cost, compact and better than the existing designs.