  Future measurements of the millimeter-wavelength sky require a low-loss
superconducting microstrip, typically made from niobium and silicon-nitride,
coupling the antenna to detectors. We propose a simple device for
characterizing these low-loss microstrips at 150 GHz. In our device we
illuminate an antenna with a thermal source and compare the measured power at
150 GHz transmitted down microstrips of different lengths. The power
measurement is made using Microwave Kinetic Inductance Detectors (MKIDs)
fabricated directly onto the microstrip dielectric, and comparing the measured
response provides a direct measurement of the microstrip loss. Our proposed
structure provides a simple device (4 layers and a DRIE etch) for
characterizing the dielectric loss of various microstrip materials and
substrates. We present initial results using these devices. We demonstrate that
the millimeter wavelength loss of microstrip lines, a few tens of millimeters
long, can be measured using a practical aluminum MKID with a black body source
at a few tens of Kelvin.
