Radiation suppression for microwave microstrip impedance transformers

This paper proposes new design principles to reduce the radiation from microstrip impedance transformers using shunt-series networks, which are widely used and spread everywhere in modern microwave circuits. These new principles are developed based on a newly simplified analytical expression for the radiation from a microstrip line. The principles suggest that using dual shunt open stubs with low characteristic impedance can reduce the undesired radiation, but the use of dual shunt short stubs should be avoided due to the extra via radiation. To further suppress the radiation, the series line before the load and shunt stubs should have high characteristic impedance. According to the proposed principles, the radiation from a well-designed impedance transformer can be suppressed by more than 5 dB.