Analysis of a Multi-Octave Co-Axial Balun Cascaded With Harmonic Termination

Wideband high efficiency power amplifiers are required in tactical communication systems. Ferrite loaded coaxial baluns are well proven impedance transformation elements used in multi-octave V/UHF push-pull power amplifiers. Differential baluns meet the multi-octave bandwidth requirement conveniently. In this paper, we explore possibility of incorporating the efficiency enhancement technique - harmonic impedance tuning into the balun. A co-axial balun encompassing a ferrite core with frequency dependent complex permeability is modeled in circuit simulator. The modeled balun is analyzed in terms of even and odd mode impedances to draw inference on harmonic impedances presented to a non-linear push-pull transistor. Third harmonic impedance tuning through balun has been demonstrated by reactively loading (filter) the unbalanced port of balun appropriately. With this harmonic tuning method it is possible to maintain fundamental impedance unaltered by the harmonic impedance network.