Some great information about coax lengths and swr

[BOOTY MONSTER]

it's "usually @ 36 ohms" because of the shape of the ground element (which is the vehicle) so that will vary from vehicle to vehicle as well as where its placed on that vehicle . FWIU , a 1/4 wave ground plane antenna (which is basically what %99 of all mobile antennas are) needs ground elements sloped down at a certain angle to tune to 50 ohms .... along with the proper length radiator .

here's a link that explains it without me doing a bunch of typing .

http://www.comportco.com/~w5alt/antennas/notes/ant-notes.php?pg=20

BTW a 36 ohm antenna is about a 1.3 vswr which is completely usable .