Meaning of S11 performing a clockwise arc circle about the smith chart origin?
Your reflection factor has constant magnitude at 50kHz...600MHz but its phase angle changes.

Guesses: One possibility is that you have microwave quality cable and a same quality resistive load at the end of the cable. Or the end is open and you have an attenuator at the input. 

120 degrees curve suggest your line is a 6th part of the wavelength at 600MHz.