244 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 6, 2007 Design and Analysis of Microstrip Bi-Yagi and Quad-Yagi Antenna Arrays for WLAN Applications Gerald R. DeJean, Member, IEEE, Trang T. Thai, Student Member, IEEE, Symeon Nikolaou, Student Member, IEEE, and Manos M. Tentzeris, Senior Member, IEEE Abstract—In this letter, the design of a microstrip bi-Yagi and microstrip quad-Yagi array antenna is presented. These designs are a derivative of the original microstrip Yagi antenna array and can achieve a high gain and a high front-to-back (F/B) ratio in comparison to the conventional microstrip Yagi structure first pro- posed by Huang in 1989. The proposed bi-Yagi and quad-Yagi an- tenna arrays can also achieve a higher gain (3–6 dB) than the con- ventional microstrip Yagi array. Simple fabrication techniques can be used with these designs due to the placement of the feeding net- work on the same layer with the antenna elements. Furthermore, simulations and measurements demonstrate with very good agree- ment that the proposed arrays can achieve a gain as high as 15.6 dBi (compared to a gain of 10.7 dBi that is achieved by the mi- crostrip Yagi antenna array) while maintaining an F/B ratio that is relatively high. Index Terms—Bi-/quad-Yagi array, capacitive coupling, front- to-back ratio, microstrip Yagi antenna array, quasi-endfire. I. INTRODUCTION T HROUGHOUT the last several years, many contributions have taken place in the design and optimization of printed microstrip Yagi antenna arrays [1]–[8]. Huang introduced the first standard design in 1989 for mobile satellite (MSAT) ap- plications, which required a low-cost low-profile antenna that covers a 40 beamwidth [1]. This design consisted of four ele- ments of different sizes that were capacitively coupled to each other to produce a fixed beam between 20–60 . One of the major limitations of this design was the low front-to-back (F/B) ratio (as low as 5 dB), where the back radiation considered in this design was the radiation in the elevation angles between . Another limitation in the design of microstrip Yagi array antennas is the necessity of low dielectric materials because the center-to-center spacing between elements is a function of the free space wavelength , not the guided wavelength , which depends on the substrate dielectric con- stant. Conversely, the size of the elements is dependent on . This means that high dielectric constant materials will result in a large center-to-center spacing between elements as their size decreases; hence, the patch elements will not be sufficiently cou- pled to each other. Since Huang’s initial design, there have been Manuscript received October 17, 2006; revised February 6, 2007. G. R. DeJean was with the Georgia Institute of Technology, Atlanta, GA 30332 USA. He is now with Microsoft Research, Redmond, WA 98052 USA (e-mail: dejean@microsoft.com). T. T. Thai, S. Nikolaou, and M. M. Tentzeris are with the Georgia Institute of Technology, Atlanta, GA 30332 USA (e-mail: gth116a@mail.gatech.edu). Digital Object Identifier 10.1109/LAWP.2007.893104 some modifications to this antenna design process and configu- ration to address these limitations. Padhi and Bialkowski devel- oped 10 dB and a high F/B ratio (as much as 15 dB) that can be used for ISM, HIPERLAN, and millimeter-wave applications above 30 GHz [3]. In order to meet the challenges of the design of printed mi- crostrip Yagi antenna arrays with quasi-endfire radiation (radi- ation between broadside and endfire ) that can achieve a high gain ( 12 dBi) while maintaining a low cross-polarization and a high F/B ratio, two new designs are proposed in this letter, which are derived from the microstrip Yagi array presented in [3]. The first structure is called the mi- crostrip bi-Yagi array and the second is called the microstrip quad-Yagi array. The microstrip bi-Yagi and quad-Yagi arrays can achieve gains of 13.0 and 15.6 dBi, respectively, while a high F/B ratio is maintained. These qualities are essential to the design of planar antenna geometries that require high-gain quasi-endfire radiation patterns with F/B ratios around 8–10 dB that can alleviate propagation loss effects through line-of-sight reception of waves at angles off broadside for applications such as wireless video transfer, millimeter-wave ad hoc sensor net- works, and point-to-multipoint wide-band links. II. ANTENNA DESIGN AND PRINCIPLES OF OPERATION The proposed microstrip bi-Yagi and quad-Yagi array antenna designs are displayed in Figs. 1 and 2, respectively. These de- signs are a derivative of the original microstrip Yagi antenna array [3] in which a high gain is obtained through the construc- tive interference of two individual microstrip Yagi structures (R-D-D1 -D2 and R-D-D1 -D2 ) that maximally radiate at and , respectively. (An illustration of this antenna is shown in Fig. 3.) The operational frequency is around 5.2 GHz, but frequency scaling of this de- sign is quite simple since the only manufacturing tolerance is the minimum trace of the metals (no via processing required). The major benefit of these structures, in comparison to the structure in [3], is the increased gain (by 3–6 dB) that can be achieved based on the design of the antenna. The common dimensions of the antennas in Figs. 1 and 2 are as follows: the length and width of the reflectors (R) is ( mil, the length and width of the driven element (D) is mil, and the lengths and widths of the director1 (D1) and director2 (D2) elements are mil. The distance between the elements along the -axis (g) is 35 mil. These values were chosen to optimize the 1536-1225/$25.00 © 2007 IEEE