We discuss a SQUID holder designed for high magnetic shielding. We show how to estimate the attenuation of the magnetic field from the normal magnetic modes for an approximate geometry. The estimate agrees satisfactorily with the attenuation measured with a commercial rf SQUID installed in the holder. The holder attenuates external magnetic fields by more than 109at the SQUID input. With the SQUID input shorted, the response to external fields is 10minus;5Fgr;0/G.
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