Cryogenic hybrid coupler for ultra low noise Radioastronomy receiver

Modern ultra low noise receivers used for Radio astronomy have evolved to provide very wide instantaneous bandwidth. Some of the configurations used in present cryogenic Front Ends, like sideband separating mixers and balanced amplifiers need 90deg hybrids at the IF, typically in the 4-12 GHz band. There are devices commercially available covering this band with good ambient temperature characteristics, but their cryogenic performance drops to unacceptable levels. This paper describes the design, construction and measurement of a multioctave stripline hybrid for the 4-12 GHz band specially conceived to survive and perform well when cooled to 19 K (-254degC). The materials and mechanical construction have been carefully selected and the result is a very compact, reliable and low thermal mass device, able to survive extreme thermal cycling. The coupling and reflection show very low temperature dependence. The comparison of the results of a typical commercial unit with the new design presented clearly shows its advantages at cryogenic temperature.