Four-Port Microstripline Antenna Integrated With a Distributed Amplifier K. W. Eccleston* and S. Bommana Dept of Electrical and Computer Engineering, University of Canterbury, Christchurch, NEW ZEALAND *Email: kim.eccleston@elec.canterbury.ac.nz Abstract: In this paper, we propose a 4-port edge-fed rectangular microstripline antenna that is integrated with a 4-FET dual-fed distributed amplifier. The 4-port antenna is directly connected to the FETs and has the additional roles of power combining, harmonic suppression and FET biasing. This approach eliminates transmission lines and other elements that would otherwise be needed to realize the amplifier and connect it to an antenna, thereby saving circuit area and minimizing losses. I. INTRODUCTION Conventional microwave output stages may typically comprise power amplifier modules, a power combiner, harmonic filters, and an antenna that are separately designed and interconnected with transmission lines. These interconnecting transmission lines add extra losses that degrade overall performance. An alternative to is integrate these functions within one circuit [1]. In addition to eliminating the interconnections, the antenna and amplifier are designed to be mutually compatible with each other, and the antenna can have multiple roles such as harmonic tuning and suppression [1] - [3], power combining [1][3]-[6], and dual mode [7]. The integrated antennas that have appeared in the literature have predominantly been edge-fed microstripline patches [3]- [6], or microstripline fed slot [2][3], though other geometries have also been proposed [1][7]. As far as integrated power combining is concerned, edge-fed microstripline patches [3]-[6] and microstripline fed slot [3] have been used, and in either case, due to their feeding and resonant behavior, harmonics can be suppressed [3]. N-way combining requires an antenna with N ports. On the other hand, integrated antennas described in the literature that perform power combining [3] - [6] have been restricted to two ports. In this work we describe a 4-port edge-fed rectangular microstripline patch antenna which can be integrated with a dual-fed distributed amplifier [8]. II. INTEGRATED 4-PORT MICROSTRIP ANTENNA The 2-port edge-fed microstripline patch antenna has been demonstrated as a feasible method of 2-way combining [3]-[6]. The ports are located at each end of a wide microstripline whose length is λ/2 and these ports are driven anti-phase and this achieves even harmonic suppression in class-B push-pull amplifiers [3]. Moreover, the differential mode excitation radiates power [5][6]. This concept can be extended as shown in Fig. 1(a) to contain additional RF ports (ports 1 to 4) and a dc feed port. The RF ports are equally spaced and run down one edge of the patch to facilitate convenient connection to transistors. L is designed to be 3λ/2 at the antenna centre frequency and hence the RF ports are spaced λ/2 at the centre frequency. Since L is 3λ/2, the antenna will be resonant and when the RF ports (1 to 4) are fed with equal level but alternating relative phases of 0° and 180°, the mode profile shown in Fig. 1(b) results. Such excitation can be achieved by driving the RF ports with the drains of FETs of a dual-fed distributed amplifier (DFDA) [8] as shown in Fig. 2. Or moreover, the patch forms the output transmission line of a DFDA [8]. For the above mentioned resonance mode, the electric field will be zero half-way between ports 2 and 3 and this means that a dc voltage source connected to port 5 (dc port) does not have a significant effect on the antenna radiation properties. 1 2 3 4 W L E 5 (dc port) (a) (b) Fig. 1 Proposed four-port microstripline patch antenna: (a) outline, and (b) resonant mode profile. It is important that the antenna RF port excitations have equal amplitude for the above mentioned mode to be faithfully excited. Since the drain currents of the FETs of a DFDA have identical amplitude (but relative phase alternating between 0° and 180°), the antenna RF port impedances need to be identical 0-7803-9433-X/05/$20.00 ©2005 IEEE. APMC2005 Proceedings Authorized licensed use limited to: University of Canterbury. Downloaded on November 29, 2009 at 21:26 from IEEE Xplore. Restrictions apply.