High-performance microstrip directional coupler for radio-astronomical receivers at cryogenic temperature

A microstrip directional coupler with high directivity is proposed for microwave applications requiring a weak coupling factor. This component has been used in the L-band radio-astronomical receiver of the Sardinia radio telescope. To improve the performance of the receiver, the operating temperature is lowered to 20 K. An in-house cryostat has been manufactured to perform measurements on the directional coupler, and very good agreement with the design specification is achieved.