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In this paper, new principles for designing low-radiation microwave circuits with single-stub impedance transformers are developed and a design flowchart for perfecting these principles is provided. The single-stub microstrip impedance transformers are widely used in microwave circuits and usually become the main radiation sources of these circuits. The developed principles show that the characteristic line impedance of a short-circuited (open-circuited) shunt stub should be high (low). In addition, if the shunt stub is open-circuited, using dual shunt open stubs can further reduce radiation; nevertheless, using dual shunt short stubs should be avoided due to additional via-current radiation. In most transformers, a high-impedance series line is recommended. Obeying the proposed principles, the radiation from a well-designed impedance transformer can be suppressed for more than 5 dB in contrast to that from the transformer using only 50-Ω microstrip lines. Six microstrip amplifiers are demonstrated, adopting various representative impedance transformers, but with the same RF performance at 1.5 GHz. The measured and simulated radiation characteristics all support the proposed principles. This work is the first study on the radiation minimization of such microwave circuits.