A 18:1 Bandwidth 180º Coupler with Wideband Common-Mode Rejection Balun

In this paper, a 180º coupler achieving 18: 1 operating frequency band is presented. The coupler is composed of three wideband multi-section Wilkinson power splitters connected by a wideband RF balun, where the balun is designed based upon a slot-crossing structure on the ground plane to achieve inherent wideband common-mode rejection. To verify the design concept, a prototype operating from 1 GHz up to 18 GHz is designed and measured to verify the design theory. For both common port (Σ) and differential port (Δ) inputs, the measured magnitude imbalances are smaller than ±1 dB while the phase imbalances are smaller than ± 4º. In addition, the proposed design achieves figure-of merit (FoM) that is better than 0.22dB/GHz by calculating the in-band insertion losses over entire operating bandwidth.