Design and Fabrication of a Compact Branch-Line Coupler Using Resonators with Wide Harmonics Suppression Band

The branch-line coupler (BLC) is an important device in radio frequency (RF) and microwave (MW) circuits. The main drawbacks of the conventional BLC are as follows: first, the four long quarter-wavelength (lambda/4) transmission line sections occupy a large size, especially at the low frequencies, and second, the presence of unwanted harmonics. This research paper presents a compact 750 MHz BLC with harmonics suppression using resonators. The typical BLC consists of four lambda/4 branches, two series arms of 35 omega and two shunt arms of 50 omega impedances. In the proposed BLC, these long branches are replaced with two types of compact resonators. The proposed resonators have the same responses at the operating frequency of 750 MHz and suppress higher frequencies. The designed BLC is simulated, fabricated and measured. The results show that the proposed BLC has good performance at 750 MHz with a bandwidth of 200 MHz, which provides more than 26% fractional bandwidth (FBW). It has a very compact size, about 84% size reduction, as compared with the typical BLC. Moreover, the fabricated BLC suppresses the 2nd up to 7th unwanted harmonics with a high suppression level.