  We have constructed several impedance-matched low-pass filters using a
stripline geometry with a dissipative dielectric. The filters are compact,
simple to construct, and operate in cryogenic environments. The dissipative
dielectric consists of magnetically-loaded silicone or epoxy, which are
commercially available under the trade name Eccosorb. For a stripline length of
32 mm, the filters have a passband that extends from dc to a 3 dB bandwidth
between 0.3 and 0.8 GHz. The 3 dB bandwidth can be adjusted beyond this range
by changing the filter length. An extremely broad stopband at higher
frequencies, with attenuation exceeding 100 dB, is achieved along with a return
loss greater than 10 dB measured up to 40 GHz. This combination of high
attenuation and low reflected power across a broad stopband ensures that
spurious or unwanted signals outside the passband do not reach or return to the
device under test. This type of filter has applications in microwave frequency
measurements of sensitive non-linear devices such as superconducting heterodyne
mixers, quantum tunneling devices, and quantum computing elements.
