What is Antenna Ground Plane?
A ground plane for a CB radio antenna is the metal surface required below the antenna. In mobile applications, the vehicle body/frame acts as the ground plane (reflective unit). The metal surface enables the signal generated when transmitting to reflect off the surface and travel into the atmosphere.
An antenna ground plane is not to be confused with an electrical ground. A typical mobile CB antenna does not transmit horizontally, it radiates in a downward pattern. A metal surface (the larger and flatter, the better) is required underneath the CB antenna to launch the signal out into the atmosphere. Without an adequate ground plane surface the signal will not travel out into the atmosphere which results in limited transmission range. Damage to your CB radio may also occur if you do not have a proper ground plane because this generally causes a high SWR and the signal may back feed into the transceiver, causing internal damage to your radio.
It is worth noting that a poor ground plane or radiating surface may not affect radio reception. You may still receive transmissions from other radios with or without a ground plane. Basically, you can receive transmissions on a clothes hanger!
It is vital to understand that you must check and set your antenna (tune your antenna) to ensure it is functioning properly.
If you have a fiberglass vehicle or boat you will need a No Ground Plane Antenna. In No Ground Plane systems, the coax shield is used for counterpoise (substitute for the ground connection).
CB Antenna Placement
The ground plane surface is a major consideration when deciding where to place your CB antenna. Ideally, the best place is in the center of your roof. Roof mounted antennas provide the best ground plane and a 360 degree radiating surface to transmit your signal. However, most people don't want to drill holes in the center of their roof. The next best location is to mount the antenna on the fender or trunk. Examine your vehicle to determine the best location for the antenna to be mounted where it will have the most metal surface to radiate from. Keep in mind that when you mount your antenna on the edge or side of the vehicle a 360 degree radiating pattern will not be achieved. The transmission of the signal will be limited on that side of the vehicle. Example: If you mount an antenna on the front driver side fender, your signal will not transmit well to the front right of the vehicle where the antenna has no ground plane. The signal will transmit very well to the back right of the vehicle. To achieve the best performance an antenna mounted in the fender should be at least six inches over the roof, which enables additional ground plane surface for transmission.
No Ground Plane CB Antenna
A no ground plane CB Antenna is a CB antenna that doesn't require a ground plane (large metal surface under it) to operate. Ground Plane antennas are specially designed to work on fiberglass vehicles and boats. A No Ground Plane antenna installed on a vehicle or boat constructed of metal will not operate correctly. In most cases you will not be able to tune the antenna which will result in a high SWR that could damage your radio. A no ground plane antenna is designed to use the ground or water to transmit horizontally. A No Ground Plane antenna will not perform as well as a traditional CB antenna of the same length, but in situations where you do not have an adequate ground plane such as on a boat, RV or motorcycle, they do the trick. Make sure to read the instructions thoroughly before installing the antenna.
There are some devices available that can enhance your ground plane. This is accomplished by normally adding a 3 point radial to the bottom of the antenna. Ground Plane enhancers are not designed to be the sole ground plane for an antenna, they just help achieve a better ground surface.