Chebyshev-Response Branch-Line Couplers with Enhanced Bandwidth and Arbitrary Coupling Level

Robert Smolarz,Krzysztof Wincza,Slawomir Gruszczynski

doi:10.3390/electronics9111828

Abstract

A new approach to the synthesis of broadband branch-line couplers with arbitrary coupling level was investigated in this paper. It was shown that the operational bandwidth of a classic branch-line coupler can be increased by utilizing N-section impedance transformers added to each of the coupler’s ports. Furthermore, the obtained response can be approximated by the Chebyshev polynomial. Moreover, it was proven that for such couplers a range of coupling coefficient values can be obtained by the modification of classic branch-line topology. The analysis of the electrical parameters of the proposed branch-line couplers was comprehensively investigated. To verify the correctness of the proposed design procedures, 3-dB, and 6-dB broadband branch-line couplers operating at the center frequency of 2 GHz having return losses greater than 20 dB were designed, fabricated, and measured.

Highlights

Directional couplers are popular and commonly used components in microwave electronics.A significant number of known solutions is based on coupled-line sections [1,2,3]
Rat-race, and branch-line couplers can be successfully implemented in the microstrip technique due to the simplicity of their circuit topologies
A new design method of broadband arbitrary coupled branch-line couplers has been comprehensively investigated in this paper

Summary

Introduction

Directional couplers are popular and commonly used components in microwave electronics. To enhance the operational bandwidth novel rat-race coupler’s topologies have been proposed, where a classic 270◦ -long transmission line section is replaced by a shorted-coupled-line section [13], which in turn, can complicate the fabrication process. In [16,17] the authors reported that additional quarter wave sections connected to each of branch-line ports can be used to achieve broadband response. Apart from the broadband operational bandwidth of branch-line couplers, their second crucial parameter is the coupling coefficient, which can be related directly to a power division between output ports. To verify the correctness of the proposed design methodology, a complete synthesis has been shown and electrical parameters of II, and III—order broadband branch-line couplers have been calculated. The applicability of the proposed couplers has been confirmed by the fabrication of 3-dB and 6-dB broadband components operating at the center frequency of 2 GHz, and having return losses greater than 20 dB

Theoretical Analysis

Broadband Branch-Line Coupler Design

Conclusions