I've tested a device with my VNA. It displays S11 in a Smith chart as a plot x(w) + iy(w) (w the frequency). Curiously, as w increases from 50KHz to 600MHz, the plot is an almost perfect 1/3 circle whose center is the origin of the Smith chart. In particular, this implies that the absolute value of S11 remains constant in the frequency range. This is not to be confused with the constant VSWR circles since these circles relate the plots to the length of a transmission line at fixed frequency. I am curious to know what is the physical meaning of these circles (how to interpret them).
This is the behaviour of a signal traveling through an (almost) lossless system with constant group delay (linear phase shift).
In reality this can be a short cable or something like this. The actual fraction (1/3 in the example) tells you something about the time how long the signal needs to travel through the cable (or more abstract "system"). If you know the epsilon of the cable (or respective the speed of light within the cable) you can calculate the length of the cable.
In reality this wont be a circle if the cable is longer the losses will show itself as a spiral more than a circle.
Just ask further question and i update my answer.
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.