FYI From FireStik:
About the length: This is a testy subject with many engineering types. They have argued with us on many occasions regarding this matter. They say that if your system is set-up properly that the length of the coax is irrelevant. We agree! However, mobile installations have so many variables that a perfect set-up is the exception, not the rule. One guy has a pick-up and another has a fiberglass motorhome. One wants the antenna on the bumper, another on the hood and a third on the roof. Few people want to drill holes in their vehicle so quality grounds are always a consideration. Because of the imperfect world, we almost always recommend 18' (5.5m) when our products are used. We do so with good reason too! At 18' the voltage curve has dropped back to the zero voltage point where the cable meets the antenna which reduces the reactance within the cable itself (a null cable if you would). It has been our experience that if the antenna location makes it somewhat out of sync with its surroundings, cable lengths that are not multiples of our 18' suggestion adds to the problem.