Transmission losses in Nb superconducting coplanar waveguide (CPW) and microstrip (MS) line losses were investigated by frequency, temperature, signal power, and magnetic field for 2 mm waveguide at 3.3 K using a network analyzer. From these measurements, loss was separated into the most significant mechanisms including quasiparticle loss, two-level system dielectric loss, vortex flow loss, and radiation loss. These loss mechanisms were extrapolated to a wide THz frequency range in this study. Understanding these loss mechanisms is crucial for high performance high-sensitivity MMIC-based detector design and other applications of low-dispersion superconducting transmission lines.