We describe modifications to an Oxford Model TLE 400 dilution refrigerator permitting measurements of microwave response at X-band. Coupling to the cavity is accomplished with a Gordon coupler involving a 1/4-in. access hole and a 1/4-in. dielectric rod running coaxial with the circular waveguide which feeds the microwave cavity. The rod extends through a vacuum load lock and may be removed. A stainless-steel sample insertion rod may then be inserted allowing top loading of specimens directly into the cavity; the samples themselves are attached to a copper fixture, involving a combination of left- and right-hand threads which may be reversibly screwed into and out of the bottom of the cavity. Preliminary surface impedance studies of s-wave superconductors (Sn, Zn, and In) and the heavy fermion UPt3 superconductor have been performed with this spectrometer.
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