Multisection Vialess Baluns with Coupled-Line Impedance Transformers

Multisection vialess baluns are proposed by cascading several lambda/4 edge-coupled lines. Without needing any via-hole or ground connection, these vialess baluns are simply implemented on microstrip structures. With the backside aperture, the even-mode impedance of coupled microstrip lines is raised beyond 280 Omega. Taking advantage of multisection coupled line approach, the eventual even-mode impedance can achieve as small as 2 Omega, thus satisfactorily meeting the requirements of lambda/4 coupled line baluns with open-circuited terminals. Three- and five-section vialess baluns are designed and the predicted performances are validated by experiment. In a wide frequency range (1.8-3.6 GHz), the measured amplitude imbalance is less than 0.4 dB and phase imbalance is within plusmn5deg