By considering the high flexibility of uniplanar technology to design complex multifunction subsystems, a design procedure based on a subsystem repartitioned into elementary blocks and very simple electrical modeling was proposed and applied to a uniplanar biphase(0° or 180°)modulator/mixer, which was intended to be used as a building block in microwave uncontact impact card of Intelligent Transportation System(ITS) at 5.8GHz. Theoretical electromagnetic field calculation methods were combined with conventional transmission line and computer aided design calculations to analyze and design the critical parts of the subsystems. The subsystem designed in this paper has the advantages of small size, compact structure, high performance and low cost.