Wideband Microstrip 90° Hybrid Coupler Using High Pass Network

A wideband 90° hybrid coupler has been presented and implemented in planar microstrip circuit. With similar structure of conversional 2-section branch-line coupler, the proposed coupler consists of a lumped high-pass network but not the quarter wavelength transmission at the center. The values of all lumped elements were optimized to replace a quarter-wavelength transmission line with a phase inverter. To demonstrate the proposed concept, a 1-GHz prototype was fabricated and tested. It achieves 90% impedance bandwidth with magnitude of S11 less than ？10？dB. Within this bandwidth, more than 13？dB port-to-port isolation, less than 5.0 degree phase imbalance, and less than 4.5？dB magnitude imbalance are achieved, simultaneously. The proposed coupler not only achieves much wider bandwidth but also occupies less circuit area than that of the conversional 2-section branch-line coupler. 1. Introduction Branch-line coupler is one of the fundamental components in RF/microwave front-end systems. It is a 4-port device that divides and/or combines power simultaneously with port-to-port isolation and 90° phase shift. These features make the branch-line coupler extremely useful in various applications such as antenna feeding network, phase shifter, balanced amplifier, and mixer. Bandwidth and compactness are two important design issues to meet the demand of the high performance systems. The bandwidth can be enhanced by increasing the number of sections, but the size will be increased. With the restriction of the line width, the characteristic impedance of the transmission line is limited to about 400？Ω; hence, the number of sections is limited to 3 or 4. Recently, various wideband and/or compact couplers have been proposed. Multisection technique is conventional and effective way to enhance bandwidth of the branch-line coupler. Several multisection branch-line couplers integrated with pattern ground structure [1], coupled line [2], and loaded line [3] were demonstrated. These works achieve more than 70% bandwidth. Series stub matching presents a simple way to broaden bandwidth with limited improvement [4]. A quadrature hybrid coupler using metamaterial transmission line, which achieves 67%, was proposed in [5]. However, a crossover is required for the coupler and the coupler introduces 6？dB insertion loss instead of 3？dB for the signal division. Figure 1 shows the schematic diagram of the wideband 90° hybrid coupler using phase inverter, which was firstly proposed in [6] and analyzed in [7]. The physical size of the proposed design is close to a conventional