662 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 53, NO. 2, FEBRUARY 2005 Development of a Wide-Band Short Backfire Antenna Excited by an Unbalance-Fed H-Shaped Slot RongLin Li, Senior Member, IEEE, Dane Thompson, Student Member, IEEE, Manos M. Tentzeris, Senior Member, IEEE, Joy Laskar, Senior Member, IEEE, and John Papapolymerou, Senior Member, IEEE Abstract—The short backfire antenna (SBA) has been widely used for mobile satellite communications, tracking, telemetry, and wireless local-area network applications due to its compact struc- ture and excellent radiation characteristics. The most common excitation topology for the SBA is a balance-fed wire dipole, which has the disadvantage of a narrow frequency bandwidth for the input impedance. In this paper, an H-shaped slot is employed to excite the SBA for the first time. The H-shaped slot is unbal- ance-fed from a coaxial line. It is demonstrated that the H-shaped slot-excited SBA can achieve a bandwidth for input impedance of more than 20% (VSWR ) while maintaining good radiation per- formance. The antenna structure is described and the simulation and experimental results are presented. The operating principle is investigated to explain why the slot-excited SBA can result in good impedance and radiation characteristics. A parametric study is conducted for the use of practical engineering design. Index Terms—Short backfire antenna, slot-fed antenna, wide- band antenna. I. INTRODUCTION T HE short backfire antenna (SBA) was first developed by Ehrenspeck in the 1960s [1], [2]. This type of antenna usu- ally consists of a half-wavelength ( 2, is the free-space wavelength at operating frequency) dipole exciting antenna, a pair of planar reflectors, and a rim. The exciting dipole is cen- tered between the two reflectors which have a diameter of about 2.0 and 0.4 , respectively, and are separated by a spacing of 0.5 . The rim with a height of about 0.25 surrounds the larger reflector. The SBA can achieve a gain on the order of 13–15 dBi, with sidelobes of at least 20 dB and a backlobe level lower than 30 dB [3]. In addition to its excellent radi- ation characteristics, the SBA is very attractive to mobile/mar- itime satellite communications, tracking, telemetry, and wire- less local-area network (WLAN) applications [4], [5] due to its compact structure (0.5 in height) and simple feed configura- tion (a single dipole rather than an array). Since it is essentially a leaky cavity structure for which the excitation is provided by the primary dipole source, the dipole- excited SBA has a narrow frequency bandwidth for its input impedance. The natural impedance match bandwidth is only 3–5% for the voltage standing wave ratio (VSWR) under 1.5 Manuscript received January 21, 2004; revised June 13, 2004. This work was supported in part by the Georgia Electronic Design Center, in part by the Na- tional Science Foundation (NSF) under CAREER Award ECS-9984761 and Grant ECS-0313951, and in part by the NSF Packaging Research Center. The authors are with the Georgia Electronic Design Center, School of Elec- trical and Computer Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0250 USA (e-mail: rlli@ece.gatech.edu). Digital Object Identifier 10.1109/TAP.2004.841291 [6], [7]. This situation becomes even worse as the height of the rim increases for suppressing the sidelobe level [8]. Our investi- gation shows that the bandwidth of a dipole-excited SBA is less than 1% for VSWR when the rim height rises to 0.5 if without a matching circuit. In addition, a balun is necessary for the balance-fed dipole “exciter.” An alternative excitation configuration is the waveguide feed system, such as coaxial or rectangular waveguide feed [9], [10]. Unfortunately, the waveguide-excited SBA requires a bulky feeding structure. Another candidate for the SBA excitation may be the patch antenna. But there are two problems that need to be resolved before this approach can be used to excite the SBA. First, if the patch antenna is located too low, the bandwidth for impedance may be very narrow and also it is more difficult to excite the leaky waves required for the backfire radiation. On the other hand, if increasing the height of the ex- citing patch, the feed probe needs to be longer, which results in an increased inductive component in the input impedance due to the parasitic inductance from the feed probe, thus causing a matching difficulty. Furthermore, a longer feed probe will produce a higher cross-polarized component, degrading the excellent radiation performance for the SBA. Recently, it has been demonstrated that a center-fed slotted patch antenna can considerably enhance the impedance and ra- diation performances even though the patch is suspended about 0.1 high over a ground plane [11]. Essentially the enhanced performances are mainly attributed to the unbalance-fed slot that is formed at the center of the patch antenna. In this paper, we propose to make use of the configuration of unbalance-fed slot on a suspended patch for the SBA excitation for the first time. Instead of using a rectangular slot as in [11], we employ an H-shaped slot since it has more degrees of design freedom for antenna performance optimization [12]. Also the slotted patch can be positioned much higher than 0.1 (about 0.32 ) re- quired for the impedance match of the slot-excited SBA. As will be discussed later, the unbalanced feed acts as a two-wire trans- mission line, hence avoiding the difficulties for a longer feed probe, such as the increased parasitic inductance and the higher cross-polarized component. The antenna structure for the pro- posed SBA is described first, followed by simulation and experi- mental results. The mechanism of operation for good impedance and radiation performances is investigated comprehensively. It will be clarified why the unbalance-fed slot can excite the back- fire radiating mode for the SBA and how the exciting slot can be matched to a 50- feed. Finally, a parametric study is presented for the potential need in practical engineering design. 0018-926X/$20.00 © 2005 IEEE