Design of a New Type Planar Balun by Using Trans-Directional Couplers

This paper presents a new type of planar balun implemented by trans-directional (TRD) couplers. The formulation of the proposed balun is derived and the characteristics of the balun are analyzed. This balun requires no via-hole connection to ground. Thus, the phase errors caused by the inductive effect of via-holes are reduced. The configuration of TRD couplers also helps to reduce the complexity of dc biasing of active circuits in the next stage. Two prototypes are designed and built to verify the design concept. The first one, operated from 1.94 to 2.69 GHz, is realized by 3-dB TRD couplers. The second prototype, which is operated from 376 to 458 MHz, is constructed by two 4.8-dB TRD couplers. The dimension of this prototype is only 16.8 mm × 39.4 mm. This small area is achieved by meandering the coupled-line sections because its coupling effect is purely contributed by periodical shunt capacitors on the microstrip lines. Measured results are in good agreement with simulated results.