What must be done to prevent standing waves on a feed line connected to an antenna?
Why Standing waves exist because part of the forward wave is reflected back toward the transmitter. A reflection happens only when the load impedance differs from the line's characteristic impedance, so when the antenna feed point impedance equals the feed line's Z0 (commonly 50 ohms for coax) all the power is absorbed by the antenna, nothing comes back, and the SWR is 1:1. Line length, line type, and grounding do not change this: only the match at the load does.
Watch out The choices about the line being some number of quarter or half wavelengths are tempting because line length does matter for impedance transformation tricks like quarter-wave matching sections, but a length alone cannot stop a mismatched load from reflecting power. DC ground potential at the feed point relates to static and lightning protection, not to SWR.
No reflection, no standing wave. Match the load to the line's Z0 and SWR is 1:1, whatever the line length.