A Reconfigurable Radial Line Slot Array Antenna for WiMAX Application

WiMAX refers to interoperable deployments of IEEE 802.16 protocol, in similarity with wireless fidelity (Wi-Fi) of IEEE 802.11 protocol but providing a larger radius of coverage. WiMAX is a potential replacement for current mobile technologies such as Global System Mobile (GSM) and High Speed Downlink Packet Access (HSDPA) and can be also applied as overlay in order to enlarge the capacity and speed.

Another significant problem of conventional microstrip antenna is the narrowing of halfpower beamwidth (HPBW) which could only cover forward radiated beam from −50• to 50• [9]. This antenna also has another salient advantage where it can generate a broadside radiation pattern with a wider HPBW covering from −85° to 85°. Such wide HPBW is deemed as an interesting characteristic in which the antenna can function as WiMAX application.
As the proposed antenna is etched from FR4 substrate, it is inexpensive in terms of fabrication. Dimension wise, the proposed antenna length and width are 150 mm and 150 mm respectively, which is smaller than conventional microstrip antenna that could achieve the same function and performance [10]. In [3,8,[9][10][11][12][13], switching mechanisms are utilized to alter the radiation pattern efficiently. The antenna, proposed in this paper, can dynamically be used in a beam shaping and broadside radiation pattern for WiMAX application.
This chapter is organized as follows: In Section 2, the RLSA radiating surface, aperture slots and feed line designs incorporates with PIN diode switches are explained and the effects of different configuration of the switches are investigated. The measurement and simulation of beam shaping and broadside radiation pattern using PIN diodes switching results will be shown in Section 3. Finally, conclusion will be drawn in Section 4.

Antenna structure
The proposed antenna structure, as shown in figure 1, has the ability to exhibit two major types of radiation patterns; the beam shape and the broadside radiation pattern. The 'circular' and a 'bridge' feed line are interconnected by switches, which consists of end-fire beam-shaped reconfigurable switches (EBRS) and broadside reconfigurable switches (BRS). The EBRS are referring to the first up to the fifth switches while the BRS are the first, fifth, sixth and seventh switches as shown in figure 1(a).
Four aperture slots are used to couple the feeding line to the radiating surface as shown in figure 1(b). Inaccuracy of alignment between the layer of feed line and aperture slots to the radiating surface can significantly deteriorate the antenna's performance especially on the gain characteristic. The aperture slots determine the amount of coupling to the RLSA radiating surface from the feed line of the proposed antenna. Hence, the feed line must be aligned beneath the aperture slots accurately as shown in figure 1(c). The length of the four aperture slots are 40 mm while their width are 3 mm.
The RLSA pattern that is used as the radiating surface in the proposed antenna has the arrangement as shown in figure 1(d) in order to provide a linear polarization along the beam direction. There are 96 slots, with 16 slots in the inner-most ring, and 32 slots in the outer-most ring. The width and length of the RLSA slots are 1.5 mm and 15 mm respectively. The gaps between the slots are mostly 8 mm. The diameter of the circular radiating surface is 150 mm.
Generally, by turning the EBRS ON and the sixth and seventh of the BRS OFF, it will result in a beam shape radiation pattern. The pattern will becomes narrower with an increasing number of EBRS switches turned ON. While by turning ON the BRS and the second up to fourth of EBRS turned OFF, a broadside radiation pattern will be obtained.

Result and discussion
Measurement shows that four different types of beam shape radiation pattern can be well reconfigured with the configuration of the EBRS. Different activation of EBRS will results in different gain and HPBW. By turning ON the first switch of the EBRS, gain and HPBW of 4.85 dB and -65° to 70° are obtained respectively, as shown in figure 3(a). While in figure  3(b), turning ON the first and second switches of the EBRS will narrow the HPBW from -40° to 45° with a gain of 7.2 dB. Figure 3(c) demonstrates the beam shape of the radiation pattern with the HPBW from -15° to 20° and a gain of 9.9 dB by turning ON the first, second and third switch of the EBRS simultaneously.
www.intechopen.com The BRS configuration has the ability to turn the radiation pattern from the beam shape to broadside radiation pattern perfectly as shown in figure 6 and figure 7. Figure 6 shows a divisive broadside radiation pattern with a maximum gain of 10.8 dB and a wider HPBW of -85°-85° when turning all of the BRS ON simultaneously. Certain combination configuration between the BRS and EBRS are able to generate another radiation pattern which is a broadside single-sided radiation pattern. This kind of pattern is lean to the right with HPBW of -80°-80° when turning ON the sixth and seventh of BRS and the first up to fourth of EBRS concurrently with a maximum gain increased up to 12.8 dB as shown in Figure 7. Since in this switching configuration the direction of radiation pattern is focused on one side, it d www.intechopen.com achieves a higher gain in comparison to the divisive broadside pattern. Figure 8 depicts the 3D simulation of the far field radiation patterns of the proposed antenna which is aligned with the measured radiation patterns as shown in figure 6 and figure 7. However, the measured gain and HPBW are slightly less compared to the simulations due to CST simulation's ideal and free loss environment.

Conclusion
A novel reconfigurable radiation pattern microstrip antenna using RLSA is introduced in this paper. This chapter has taken advantages of the high performances of RLSA in terms of gain and less signals reflection, to make the proposed antenna becomes more efficient. This antenna is designed based on aperture coupled structure. The ability of the beam shape and broadside radiation pattern is attributed with the usage of PIN diode switches that integrated in the feed line of the proposed antenna. It is shown through the measurements that the radiation patterns can be well reconfigured through the assist of the orientation and geometry of the RLSA slots. The proposed antenna which has a dimension size of 150 mm X 150 mm, can be tuned to reach a high gain of 14.64 dB. The antenna can also provide wider value of HPBW that covered from -85° to 85° which is far better than -50° to 50° HPBW of a conventional microstrip antenna. The broadside patterns are achieved by turning ON selected configuration of the combination between the BRS and EBRS. The structure of the proposed antenna which is not bulky compared to the conventional microstrip antenna would be greatly suitable for beam shape and broadside radiation pattern application such as WiMAX. www.intechopen.com