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Mounting Height and Optimal Antenna Length
In general, height is the primary factor that will affect your antenna's performance. To achieve greater height, mount the antenna on your vehicle's roof. A CB antenna is considered full sized if it is 8 feet 6 inches long, which is one quarter of the radio wave's wavelength. For many mobile applications, however, this height can restrict your vehicle's ability to pass through tunnels and under bridges, so a shorter length can be achieved without a significant loss of performance by employing a partially coiled design.
Shorter Antennas With Coils
Adding a coil of wire to your antenna will compensate for its shorter length. The coil can be added near the top of the antenna or near the base of its length. A base-loaded antenna incorporates the coil into its base. Base-loaded antennas are a common type of portable antenna for cars and trucks. Center-loaded antennas are common on large commercial trucks, and make use of a coil located between the antenna's base and center. Top-loaded antennas feature a coil at the top of a mostly fiberglass antenna. These are considered more versatile than other types of antennas, and can be effective at smaller sizes and lower wattages than their base- and center-load counterparts.
Antenna Bandwidth
A CB antenna should meet or exceed the bandwidth requirements of a 40 channel CB. The frequency range in which these channels transmit and receive is 26.965 to 27.405 MHz.
Counterpoise
CB antennas require a ground plane, or counterpoise, to act as a reflective unit. While the antenna itself serves as the radiating unit, the roof of your car or truck will act as the counterpoise.