according to the widely accepted theory of smokey cb swr equation, you must multiply the height of your vehicle including all ancilliaries ie : roofracks and boxes, by the average distance you travel every week. then work out your average mpg and divide that by the width of your vehicle. take the two numbers and add them together and that should give an idea ( in millimetres ) of how high you need to be mounting your aerial, although obivoulsy if it is an aluminium defender you need to get a special ali magmount for your roof...