From my experience, take this for what it’s worth cause I’m just here for comic relief and to provoke thought or just provoke depends on who you ask.
Anyway, my experience is make that antenna longer, the SWR reading decreases. Put it back to its original length and do proper grounding, that SWR reading will go down like it did when you made the antenna physically longer. It’s absolutely not the coax, as CK has said thousands of times, there is no magic length of coax.
Sometimes, I’ll get a good pair of insulated pliers and take the end of that whip and drag it around under hood. I’ll cram it and hold it to the positive battery terminal. Then, after the fire department has left, I’ll stand there with my arms crossed just looking at the antenna. I’ll say “well, did you learn anything from all this?” and then just turn and walk away.