| Microwave passive couplers are widely used in microwave and millimeter-wave applications and communication systems. Common examples are branch line coupler, rat race coupler, power divider, and crossover junction. They are used for the dividing, combining and re-directing of signal power.;Very often, a passive coupler utilizes simple quarter-wavelength transmission lines for implementation which will lead to narrow-band operation. Therefore, it is difficult to deploy such circuit for wide-band or multi-band applications. Multi-section topologies may be used to broaden the operating bandwidth, with which the major drawbacks are enlarged circuit size and the requirement of extreme high (or low) branch-line characteristic impedances. Both are not attractive for mass and low cost production. Conventional design approaches are, therefore, not suitable for modern communication systems with multi-band operation.;For size miniaturization and cost reduction, the design of dual band devices has become an emerging research area in recent years. A desirable dual-band solution should offer size compactness, high performance (e.g. low insertion loss) and compatible with conventional printed circuit broad (PCB) technology, especially microstrip lines.;In this research, several new devices, including rat-race coupler, power divider and crossover junction, capable of operating at dual frequency bands are proposed. These structures involve only simple branch-line sections and a minimal number of shunt stubs. All characteristic impedances are ranged from 20 O to 100 O. Most designs can operate with wide frequency spacing between the two bands. These designs offer low insertion loss as well as good return loss performances, and are small in size, in compared to the broadband approach. For design purposes, explicit closed-form equations are derived for the evaluation of circuit parameters. In addition, the usable range of these devices with respect to frequency band separation is examined. For verification, various prototypes are constructed by using microstrip technology and in-house fabrication facilities. Both simulated and measured results are presented and compared with state-of-the-art examples. |
