There’s a persistent myth that feedline length needs to be some magic multiple of a wavelength to work correctly. It doesn’t, for a properly matched coax feedline, but feedline length does interact with common-mode current problems in ways worth understanding before you run a cable.
Coax length and SWR: mostly a non-issue
If your antenna presents a reasonable match at the feed point and your coax has low loss, the actual physical length of coax between your radio and the antenna has minimal effect on the SWR your radio sees, beyond the small additional loss longer runs introduce. The old advice about cutting feedline to specific electrical lengths mostly applies to systems using open-wire ladder line without a balun, or to specific antenna designs like the G5RV that rely on the feedline itself as part of the matching system, not to a standard coax-fed dipole or vertical.
Where common-mode current actually comes from
The real problem worth caring about is common-mode current: RF flowing on the outside of your coax shield instead of staying confined to the intended signal path inside the cable. This typically happens when an antenna isn’t perfectly balanced relative to the feedline, which is nearly always the case in the real world, letting some RF leak onto the shield exterior. That stray RF can cause RFI in nearby equipment, distort your antenna’s radiation pattern, and even bite you if you touch the coax while transmitting.
The fix is a choke, not a specific length
A properly placed 1:1 common-mode choke, built from coax wound on a ferrite core, right at the antenna feed point is the actual fix for common-mode current, and it works regardless of the total feedline length you’re running. I’d focus your effort on installing a good choke rather than chasing feedline length superstitions. Once the choke is in place, run whatever length of coax actually gets you from the antenna to the shack, and don’t lose sleep over the number.
Related reading: Loading Coils and Shortened Verticals · Trap Dipoles and Verticals · Diagnosing and Killing HF Noise