A 10‐GHz superconducting niobium re‐entrant cavity parametric transducer was developed for use in a cryogenic 1.5‐tonne Nb resonant bar gravitational radiation antenna. The transducer has a very high electricalQ(6×105at 4.2 K), and was operated at high cavity fields without degrading theQ. A very high electromechanical coupling between the antenna and the transducer was therefore achieved. The highest coupling attained, constrained by the available pump power, was 0.11. If the transducer were to be operated in conjunction with a wideband impedance matching element, an antenna bandwidth comparable to the frequency of the antenna would be attained. The temperature dependence of theQof the transducer was in good agreement with theory. At temperatures above about 6 K theQwas degraded by the increase in the BCS surface resistance, while at lower temperatures theQwas limited by radiative losses.