Analysis of suspended microstrip taper baluns

The baluns used in broadband microwave mixers are often in the form of suspended microstrip circuits with tapered conductors. These structures have been analysed using TEM theory of the even and odd modes and results are presented for typical cross-sections. Tapers were approximated by cascading short lengths of uniform line and calculations and measurements were carried out on various tapers. The performance of the baluns is poor at frequencies where the even mode resonates. Between the resonances, the tapers perform better as a balun than a similar structure using uniform strips of equal width. The resonances can be damped by suitably loading the even mode.