There's plenty of information out there about the merits of various mobile antenna designs: base, center, or top loading; big coils vs. skinny coils or helically wound; long or short masts; funny shapes; matching gimmicks; and tuning schemes. It's a matter of deciding what blend of efficiency, bandwidth, and operating convenience will be mixed with aesthetics and physical vulnerability. Once decided, the antenna can be chosen, somehow attached to the vehicle, coax run from it to the rig, and off we go mobiling—right?

Well, perhaps, but all the factors and decisions mentioned concern only half of the antenna. What about the other half?

To make sure we understand each other, let's review how the mobile antenna works. Mobile antennas for 160 meters through 10 meters, with a few obscure exceptions, are end-fed quarter-wave resonant conductors working against a ground plane.

"Quarter-wave resonant" means that although it may not actually be 1/4 wavelength long, loading devices like coils in series with the conductor make it electrically appear to be that length.

The ground-plane antenna may be a pole, a dipole, or a shortened version, replaced by a flat plane perpendicular to the remaining half. Usually, the remaining half of the dipole is mounted vertically, or nearly so, and the plane is therefore horizontal. (But this is not a requirement for it to radiate.)

Dipoles—and therefore ground planes—are capacitive devices, essentially big resonant capacitors. We connect the output of an alternating current (AC) generator (that we call a transmitter) to the two terminals of these capacitors, and AC current flows.

The result is an electrostatic field between the two elements of the capacitor, oscillating at the frequency of the AC supplied by the transmitter. Perpendicular to this electrostatic field, an electromagnetic field is formed.

If the capacitor is well-designed, a portion of the energy in the electromagnetic field is "lost" or radiated on each current reversal. If everything works as intended, this energy can be detected at a distance from the source—and radio communication is possible.

One thing that should be obvious from all this is that current can only flow in capacitive devices that have at least two elements. No one has figured out how to get sustained current flow in a one-terminal device. That "ground plane" element is just as important. No ground plane → no current → no field → no signal.

In mobile installations, the ground plane is usually the metal skin of the vehicle. If the vehicle doesn't have a metal skin, either the frame and/or undercarriage must do the job, or a quarter-wave resonant ground radial must be constructed. Now we've returned to a dipole.
Even with a conductive-skinned vehicle, several "truisms" determine whether or not a mobile signal will "get out." These fall into three major categories:

1. Size

2. Positioning

3. Connections

SIZE

1. The typical passenger car, family van, or pickup truck represents a fairly good ground plane on higher bands such as 10, 12, 15, 17, and 20 meters.

2. Below 20 meters, these vehicles introduce significant ground resistance, reducing efficiency.

3. If we have a choice, bigger is better, especially at lower frequencies.

4. Operating 75-meter mobile from a motorcycle would be like beating your head against a wall.

POSITIONING

1. To create the most uniform radiating field, the antenna mast should be mounted at the center of the ground plane, with the entire plane at or below the level of the base (feed point) of the antenna mast.

2. On a passenger car or van, the best location for performance is the center of the roof.

3. Mounting the mast at one end of the vehicle produces a lopsided radiation pattern, since the ground plane is all on one side of the mast.

4. Any mount position that puts part of the ground plane above and/or parallel with the base reduces efficiency.

5. The worst case is a bumper-mounted vertical antenna mast on the back of a van. In this setup, the high-current portion of the field is entirely enclosed, severely limiting radiation.

6. Solutions for the van problem include mounting the mast up front, on the roof, or as high as possible and tilting it away from the vehicle to "open" the field.

CONNECTIONS

1. In order for the ground plane to function as the other half of the antenna, it must be directly connected to the other terminal of the generator—meaning the shield braid of the coax must be attached at the base (feed point) of the antenna mast.

2. This connection must be low impedance—it should offer no inductance or resistance. Thin wires to the body or a mast-to-bumper connection introduce losses.

3. If a body mount is not used and the coax shield is not directly connected to the vehicle's metallic skin, then the connection should be made using "fat" or broad conductors, such as:
* Large braids (1"–2" wide)
* Flashing metal
* Several large braids from the base mount to different points on the frame or skin of the vehicle

4. The connections between panels on the vehicle should also be reinforced with fat braids. Hinges and latches on hoods, trunk lids, and doors do not ensure a good RF connection.