A 1.3 GHz phase averaging reference line has been installed for Fermilab’s NML ATSA accelerator. The reference line is composed of directional couplers and 7/8” heliax cable. The reference line is shorted at one end of the line to provide reflected signals that are summed and phase averaged with the forward signals at each directional coupler. The phase drifts of the 7/8” heliax are compensated for by the phase averaging at each coupler. A method is also outlined to minimize phase deviation caused by impedance mismatches and directivity of the directional couplers. Simulation results of the reference line, results of a scaled version of the reference line built in the lab, and results of the installed reference line are presented in this paper.