Looks rather broadband RF ground if I did the measurement correctly for this 50 ft wire. QRM at low frequencies probably from AM stations. The wire has two 90 degree bends and last few feet at hot end are coiled.
Edit: This is probably not correct since it is dominated by the coax between VNA and counterpoise wire (coax used for “matching” here). Those large impedance peaks are due to coax length. It might be better to use impedance |Z| (Ohm) instead of loss resistance Rs. But the wire resonance can be seen from the plot and it changes with the length of the wire as expected.
73, Jaakko ac1bb/oh7bf