Ok.. I know this is an old thread but there is a bit of mis-information that I feel needs to be corrected.
18' of coax is NOT required. If you have a properly mounted resonant antenna with a 50 ohm impedance (most single vertical mobile antennas) and a 50 ohm feedline (RG58 or RG8 typically) any length of coax will work and you will get the same SWR reading anywhere along that length because your antenna and coax are matched.
If your SWRs are off the charts, fix your antenna don't adjust your coax. By adjusting your coax you are hiding the issue from your radio (and yourself). If the SWR is off the charts you likely have a ground plane issue. Your antenna is probably mounted to your tailgate or spare tire rack or your antenna mount is powder coated/painted and not making sufficient metal to metal contact with the body of the vehicle. When this happens your antenna is using the coax as it's ground plane (wrong) and you see high SWR. By making the coax 18' it is in the null of the wave so you see a low SWR. It's a bandaid like replacing a steering stabilizer to fix death wobble. You could also have extremely high SWR if you have a bad piece of coax.
If your antenna and coax are matched (1:1 SWR or damn close) your will have no/little reflected signal and therefore nothing to hide from your radio.
Saying you need a certain length of coax for a certain frequency is almost like saying you need a certain length hose for a certain PSI or a certain length wire for a certain amperage.
Also.. coiling the coax, it's called an RF Choke, Choke Balun, or Ugly Balun though not really a balun at all but indeed a choke they are common and won't hurt your signal and on't fix your SWR but it will "choke" off stray RF.
