Optimum design for a broadband microstrip balun

The dependence of impedance bandwidth on internal impedances is examined to indicate a design strategy and performance potential for the balun design originally proposed by Marchand (1944). The balanced port impedance and its VSWR when loaded with equal balanced and unbalanced load or port impedances as seen by the balun are calculated for an unbalanced load or source impedance of 50 ohms and various values of the characteristic impedances of short- and open-circuit stub lines in order to determine true bandwidth-maximizing values for the characteristic impedance of open-circuit (phase-compensation) stub lines. The initial design characteristics necessary for successful empirical bandwidth maximization are clarified, and preliminary experimental results are dicussed which show clear optima in the impedance and length of a phase-compensation line for a given balanced line geometry. It is concluded that optimally designed microstrip Marchand baluns have the bandwidth potential to meet most balanced antenna-feed requirements in the VHF band and above.