Multisection Couplers With Coupled-Line Sections Having Unequal Lengths

A new type of broadband multisection coupled-line directional couplers is proposed. The investigated directional couplers are composed of coupled-line sections having unequal electrical lengths and a transmission-line section. The presented couplers feature similar bandwidth to the standard multisection directional couplers with significantly reduced maximum coupling coefficient, and the overall size smaller than in the case of tandem couplers. Moreover, it is shown that in the presented couplers an ideal 90 ° differential phase characteristic can be achieved when such couplers are solely composed of coupled-line sections having appropriate electrical lengths. Additionally, it is shown that the proposed couplers´ bandwidth can be increased by increasing the number of quarter-wave-long coupled-line sections. Theoretical analysis has been validated by three designs of broadband directional couplers operating at a frequency f₀=1.8 GHz. Measurements of the manufactured directional couplers have been shown to experimentally confirm their properties.