  The design and validation of a dual polarization source for waveguide-coupled
millimeter and sub-millimeter wave cryogenic sensors is presented. The thermal
source is a waveguide mounted absorbing conical dielectric taper. The absorber
is thermally isolated with a kinematic suspension that allows the guide to be
heat sunk to the lowest bath temperature of the cryogenic system. This approach
enables the thermal emission from the metallic waveguide walls to be
subdominant to that from the source. The use of low thermal conductivity Kevlar
threads for the kinematic mount effectively decouples the absorber from the
sensor cold stage. Hence, the absorber can be heated to significantly higher
temperatures than the sensor with negligible conductive loading. The kinematic
suspension provides high mechanical repeatability and reliability with thermal
cycling. A 33-50 GHz blackbody source demonstrates an emissivity of 0.999 over
the full waveguide band where the dominant deviation from unity arrises from
the waveguide ohmic loss. The observed thermal time constant of the source is
40 s when the absorber temperature is 15 K. The specific heat of the lossy
dielectric MF-117 is well approximated by $C_v(T)=0.12\,T\,^{2.06}$ mJ g$^{-1}$
K$^{-1}$ between 3.5 K and 15 K.
