Design of a Yagi-Uda Antenna with Gain and Bandwidth Enhancement for Wi-Fi and Wi-Max Applications

There are various patch antennas used for Wi-Fi and Wi-MAX applications. But, the problems with them are low gain, low power handling capacity and hence, conventional yagi-uda antennas are used. Conventional yagi-uda antennas are used in applications where high gain and directionality are required. Generally, numbers of directors are added to increase gain of these antennas. But, here we present modified yagi-uda antennas, in which the gain and bandwidth can be enhanced by not adding additional directors.<br><br>In this paper, we initially discuss the designs of various yagi-uda antennas with uniform, non-uniform spacings between directors and then we discuss their gain and bandwidth enhancement by various approaches. Simulation results show that, the proposed techniques can enhance gain as well as bandwidth when compared with the traditional yagi-uda antennas.


CONVENTIONAL YAGI-UDA ANTENNAS
Conventional yagi-uda antennas were invented in 1926 by Shintaro Uda of Tohoku imperial university, Japan along with his colleague Hidetsugu Yagi. These antennas are used in HF , VHF (30-300MHz), and UHF (300-3000MHz) ranges. Basically, yagi-uda antennas consist of three different types of linear dipole elements as shown in figure 2. The elements are active element, reflectors and directors [8,9]. An active element is the one to which the source or excitation is applied. Generally, length (Lac) of this element is slightly less than λ/2 i.e. ranges from 0.45λ to 0.49λ. Active element is also known as a feeder or a driven dipole. Length of reflector (Lr) is 5% greater than the length of active element. Having a length greater than the active element causes good reflections towards forward direction. Basically, more than one reflector can be used, but it does not add any advantage specifically. A reflector is located behind active element at a distance (Sra) of 0.25λ. In the designs of yagi-uda antennas, directors play a key role in achieving better gain and directivity. Usually, their length is 5% smaller than the active element i.e. lies between 0.4λ to 0.45λ. Generally, gain is enhanced by adding number of directors as well as by optimizing the spacing between them. In the standard designs, spacing between the directors and the spacing between an active element, directors varies between 0.35λ to 0.4λ. Radius (a) of each element is 0.00425λ [8].
antenna for WLAN and Wi-MAX applications is covering 10dB return loss frequencies ranging from 2.25GHz to 2.72GHz. The authors in [11], present a 10-element (with 8 directors) conventional yagi-uda antenna for WLAN applications. The proposed antenna is covering frequencies ranging from 1.8GHz to 2.4GHz with a gain of 13dBi. They carried out simulation using method of moments based EM simulation software called SuperNec v2.9. The proposed antenna in [11] is also covering bands other than WLAN and moreover, it uses 8 directors to achieve a gain of 13dBi.  [11]. In this paper, we discuss various yagi-uda antennas operating at 2.45GHz. Proposed antennas cover complete Wi-Fi 2.4GHz band along with few channels of Wi-MAX 2.3GHz and 2.5GHz bands. With the proposed gain enhancement approaches, we are able to achieve a maximum gain of 12.30dBi with 4 directors. Simulations of these antennas are performed on method of moments based EM structure simulator software called FEKO suite 6.3.

DESIGN OF CONVENTIONAL YAGI-UDA ANTENNAS
In this paper, we initially focus on the designs of conventional yagi-uda antennas with 4 directors as shown in figure 2. Here, we have selected 4 directors for better gain and bandwidth. First of all, we consider uniform spacing between the directors and then we focus on nonuniform spacing.

UNIFORM SPACING BETWEEN DIRECTORS
In this paper, for better understanding of the designs, we have mentioned all the dimensions in terms of 'λ'. Here, uniform spacing between the directors is denoted by 'Sad' and it is assumed as 0.35λ. Design of the proposed uniform spaced yagi-uda antenna with 4 directors in FEKO is shown in figure 4.  Table I.

NON-UNIFORM SPACING BETWEEN DIRECTORS
As discussed earlier, spacing between the directors and the spacing between a director and an active element lies between 0.35λ to 0.4λ. Previously, we focused only on uniform spacing between the directors. Here, we examine different possible non-uniform spacing combinations for our applications. Different non-uniform spacing outcomes are shown in table III. As per the designs with 4 directors (uniform spacing and non-uniform spacing) are concerned, design with table II case I dimensions is the best design for our application. So far, we have been dealing designs with 4 directors. In the next section, we observe the response of proposed design, when the number of directors is varied.

INCREASING NUMBER OF DIRECTORS
Here, our aim is to compare the results obtained in this section with the results obtained in the modified yagi-uda antenna design section. Design results of proposed antennas with a variation in number of directors are given in table IV.

DESIGN OF MODIFIED YAGI-UDA ANTENNAS
In this section, we discuss different approaches to enhance the gain by not disturbing the bandwidth obtained in table II case I. To obtain better performance in terms of gain and bandwidth, we replace reflector element by better reflectors like parabolic plate or rectangular plate of good conducting material.

REPLACING REFLECTOR ELEMENT BY A PARABOLIC PLATE
Here, we present designs of modified yagi-uda antennas, in which the reflector element is replaced by a parabolic plate. In this case, 'Sra' is a distance between the centre of the parabolic plate to the active element, which is also 0.25*λ. Proposed parabolic plate and modified antenna designs are shown in figures 7 (a) and (b).

(a)
Electronic copy available at: https://ssrn.com/abstract=3652951 To have improved performance than the conventional yagi-uda antenna, diameter (D) of the parabolic plate must be greater than the length of active element. In the initial design, we assume radius (r) as 'Lr/2' and then we make changes in depth. In the next level of the design, we increase radius (r) and then we vary the depth as shown in below tables. To avoid complexity, here also we have mentioned parabolic structure dimensions in terms of 'Lr'. Different combinations of radius 'Lr/2' and depth (d) results are presented in table V. There is one more point to observe is that, as we decrease the depth (d), gain decreases slightly but the bandwidth increases considerably. Modified yagi-uda antenna with radius 'Lr/2' and depth 'Lr/5' is achieving a maximum gain of 11.14dBi, which is higher than the gain of yagi-uda antenna with 5 directors (11.04dBi). But, the proposed antenna design is not covering Wi-Fi and Wi-MAX bands. Antenna design with radius 'Lr/2' and depth 'Lr/30' is also yielding higher gain but less bandwidth than the yagi-uda design discussed in table II case I. This design is providing better bandwidth than the design with radius 'Lr/2' and depth 'Lr/5'. Here, we need a modified yagi-uda antenna with higher gain and bandwidth than the antenna discussed in table II case I. To achieve the required goal, let us increase the radius of parabolic plate. Design results of the antenna with increased radius and variation in depth are shown in table VI. In this case, with the increment in radius, gain as well as bandwidth increases. Design with radius '(2*Lr)/3' and depth 'Lr/30' is giving a maximum gain of 11.53dBi, which is higher than the yagi-uda antenna discussed in

REPLACING REFLECTOR BY RECTANGULAR PLATE
Here, we discuss the response of modified yagi-uda antennas, when the reflector element is replaced by a rectangular plate having length (L) and width (W) as shown in figure 9 (a). In this case 'Sra' is simply a distance between the rectangular plate to the active element, which is also 0.25*λ. Arrangement of the conventional yagi-uda antenna with rectangular plate is shown in figure 9 (b).   If we look at the results shown in table VIII, we get a maximum gain of 11.61dBi, which is approximately equal to the conventional yagi-uda antenna with 6 directors (11.63dBi). In this case also, the design is achieving more bandwidth and almost maintaining the same 2.5GHz -10dB cutoff (2.557 GHz) with -10dB cutoff of the yagi-uda antenna (2.565GHz) discussed in table II case I. Various plots of the proposed modified yagi-uda antenna with rectangular plate having width 'Lr*1.5' and length 'Lr*1.5' are shown in figures 10 (a) and (b).

(a)
Electronic copy available at: https://ssrn.com/abstract=3652951 Here also, we can state that, the proposed antenna design with rectangular plate having length and widths as '1.5*Lr' can also be used for our applications. Hence, we can conclude that, the proposed designs in table VII and the design with rectangular plate having length and widths as '1.5*Lr' are most suitable antennas for Wi-Fi and Wi-MAX applications.

CONCLUSION
In this paper, initially we discuss conventional yagi-uda antennas by considering uniform, nonuniform spacing between directors. Here, we observed the problems associated with nonuniform spacings and concluded that, the antenna design presented in table II case I is suitable for our dual applications. Next, we varied the number of directors and noted that, as we increase number of directors, gain as well as size of the antenna increases. After that, we replace the reflector element by various structures. Out of this examinations, parabolic plate designs discussed in table VII and the design with rectangular plate having length and widths as '1.5*Lr', are giving increased gain and bandwidth than any other yagi-uda antennas. Finally, we can state that, proposed designs in table VII and table VIII are most suitable antennas