The SWR for a 50 Ohm cable will depend on the feedpoint impedance at the end of the cable. The impedance will depend on the antenna dimensions, its height, and its proximity to the environment. If the antenna is supported horizontally, its impedance at resonance could vary from a few Ohms to a maximum around 90 Ohms and then down to about 72 Ohms as the height is increased. If you are seeing a minimum SWR at your operating frequency, the length of the antenna is at or very close to a half-wave. Sometimes you can bend the ends of the antenna towards the ground and reduce their length to bring the impedance to 50 Ohms and an SWR of 1.0.
Ken