An Ultra Compact Microstrip Branch Line Coupler with Wide Stopband Using LCL Filter and Meandered Stubs for Microwave Applications

Abstract

A branch line coupler (BLC) with ultra-compact size and harmonic suppression ability using an LCL filter and meandered stubs is proposed in this paper. There are some important factors in microstrip coupler design, including size reduction, harmonic suppression, and low insertion loss. Thus, improving each of these factors will contribute to a more efficient design. In the proposed circuit, for the first time, LCL filters, including four T-shaped circuits and four meandered line open-ended stubs, were used together to reduce the circuit size and suppress unwanted harmonics. The proposed LCL filters, incorporated in the BLC branches, resulted in superior size reduction and harmonic suppression for the presented BLC. The proposed BLC correctly worked at 900 MHz with 300 MHz operating bandwidth, which showed 33% fractional bandwidth (FBW). Additionally, a wide suppression band from 1.4 GHz to 8.8 GHz, with more than 20 dB attenuation level was obtained, which suppressed the second to ninth unwanted harmonics. The overall size of the proposed 900 MHz coupler was only 11 mm × 10.4 mm (0.044 λ × 0.042 λ) while the size of the conventional 900 MHz coupler was 61.5 mm × 62.5 mm (0.25 λ × 25 λ). The proposed BLC had a very small size and only occupied 3% of the size of the conventional coupler, which corresponded to a 97% size reduction. To the best of the authors’ knowledge, to date, the best size reduction has been obtained among the published couplers. Furthermore, the experimental results verified the simulated and analyzed results of the proposed technique and demonstrate its potential for improving the performance and miniaturizing the size of other similar BLCs.