CHOOSING A MOBILE ANTENNA
Mobile antennas come in all shapes and sizes. By convention, most HF antennas have a 3/8 inch male thread whilst VHF & UHF have male PL-259 type. So make sure you have the right base fitting fitted to yuor car.
Magnetic mounts are great for mounting VHF and UHF as the capacity between car body and antenna base is more than enough to give the same rersults as if you drilled a hole in the body. Boot mounts and hatch mounts are the common alternatives and work equally as well, but do offer the advantage of being able to adjust the angle to take into account the surface shapes and curves of a modern vehicle. If you choose a magnetic mount for HF work, then choose one of the three-legged models. They are much stronger and are the only ones that have enough capcity to reproduce a normal electrical earth.
VHF and UHF antennas come pre-tuned and require no adjustment normally. High VSWRs are almost always caused by poor mounting positions, cable problems or badly fitted connectors. Anything below 2:1 is quite acceptable as cable losses are really tiny on the normal short length of cable involved.
HF antennas are an entirely different consideration. The antenna is extremely small relative to its normal electrical length. So get a good one and do not skimp. Single band models may be inconvenient, but they normally offer some of the best performances. The larger the diameter of the loading coil, the better the efficiency. The Americam screwdriver antennas are probably the best if you want automatic multi-band operation. The problem here is how to mount them safely. The best option is to get a local car repair shop to weld up a bracket that attaches directly to the car chassis and place the mount on it. 4 x 4 drivers may consider the spare wheel attachment on the rear door, but there may be RF earth issues as the door itself does not have a great deal of metal in it and may not be well bonded to the chassis. Avoid mounting HF antennas on roof racks or roof bars. It seems a great idea at first, but they offer poor earths and are offten a form of series inductance that can detune the antenna. Many HF installtion problems arise from poor choice of mounting. Also, the longer the antenna the better. However at the end of the day the installtion will be a compromise, so do not be put off if you cannot achieve the optimum requirements. Ham radio is always a compromise somewhere along the line and often the challenge is part of the fun.