So here's another question that I need answering. What do the inductors you see on a lot of antennas do? Almost all mobile antennas have one or two inductors on them, so I was just wondering how they contributed to the performance of the antenna.
On lower bands, particularly HF, these inductors are actually called loading coils. Basically, the wavelength is long enough that a full size 1/4 wave or 5/8 wave vertical would be impractical. The loading coil is what makes up the difference. It enables you to use a shorter antenna. HOWEVER, there are some issues to be aware of:
First, the point where the antenna attaches to the coaxial cable is a current node. Even very small resistances can cause significant losses. Bond the coax to the antenna and to the vehicle with care and seal it up so that dirt and corrosion can not take hold.
Second, the coil usually sees significant current, so coil Q matters. The greater the Q and lower resistance you can get from the coil, the better off your antenna performance will be. This is why you see those ridiculous looking bug-catcher antennas on many ham vehicles. They do indeed look ridiculous, but because of the Q, they work surprisingly well. On some antennas, people have learned to put the loading coil up as high as possible to keep it away from the current node at the base. This improves performance and durability a little, but it does get rather ridiculous mechanically.
Basically, unless you're designing an antenna for 40 or 80 meters, there usually isn't much to be gained by putting the loading coil in such a precarious position.
Third, this does limit your bandwidth compared to a full sized antenna. Those who claim that you can use a shortened antenna with wide bandwidth are usually selling a lossy antenna. You will get the wide band SWR match, but it won't radiate well. You'll have a dummy load that looks like an antenna.
Clearly this advice becomes more and more extreme as you go lower in to the HF bands. The practical limit is on 75/80 meters. Anything below that is mostly a waste of time, unless you have a very long antenna stick on a boat. On 10 Meters, this issue isn't particularly critical because the loading coil isn't making up for that much of the wavelength. On Six meters you may be able to get a full 1/4 wave whip, but it will be big. A loading coil won't hurt too much, so use one if you need it.
In general, for two meters, I suggest a plain quarter wave vertical antenna. It is simple, stupid, and as long as you keep connections clean it will work pretty well. If you have a perfect groundplane, the impedance might be as low as 36 ohms, but you can usually match to that anyway, and your coaxial losses will be minimal.
For 70 cm, some like to use a 5/8 wave vertical. Frankly, it hardly matters. You can buy better coaxial cable to your radio and better connectors and make up for any supposed gain that you might get with a 5/8 wave vertical.
Remember that when going mobile, your primary concerns will be durability, corrosion, and dirt. You'll lose more signal to those problems than you might get with a fancy design. Keep everything well bonded, clean, and simple.
Good Luck!
