The real simple answer about the coax length making a difference in the SWR is that if it does, then there's a impedance mismatch usually with the antenna, that it hasn't been tuned correctly for whatever reason. If the radio is 50 ohms, the feed line is 50 ohms and the antenna is 50 ohms, there's just no SWR that will amount to much. Finding out where thing s are not 50 ohms is the way to correct things, right?
So, tell me how I'm misunderstanding what's happened, or what I'm missing.
I'm taking for granted that the antenna tuning was done with an external meter (and jumper). That jump in SWR to 2:1 was found with an internal SWR meter, or did you put that external meter back in-line after "going directly to the antenna"? I'm also assuming that there wasn't any huge changes with something else, like moving the antenna to another spot, slamming the coax in the car door, something like that? Very simplified, what changed? For an SWR change like that, something had to have changed.
- 'Doc