  We fabricated compact low-pass stainless-steel powder filters for use in
low-noise measurements at cryogenic temperatures and investigated their
attenuation characteristics for different wire lengths, shapes, and preparation
methods up to 20 GHz. We used nominally 30-micrometer-sized SUS 304L powder and
mixed with Stycast 2850FT by Emerson and Cumming with catalyst 23LV. A 0.1 mm
insulated copper wire was wound on preformed powder-mixture spools in the shape
of a right-circular cylinder, a flattened elliptic cylinder and a toroid, and
the coils were encapsulated in metal tubes or boxes filled with the powder
mixture. All the fabricated powder filters showed a large attenuation at high
frequencies with a cut-off frequency near 1 GHz. However, the toroidal filter
showed prominent ripples corresponding to resonance modes in the 0.5-m-long
coil wire. A filter with a 2:1 powder/epoxy mixture mass rate and a wire length
of 1.53 m showed an attenuation of -93 dB at 4 GHz and the attenuation was
linearly proportional to the wire length. As the powder-to-epoxy ratio
increased, the high-frequency attenuation increased. An equally-spaced
single-layer coil structure was found to be more efficient in attenuation than
a double-layer coil. Geometry of the metal filter-case affected noise ripples
with the least noise in a circular-tube case.
