Chassis Ground versus Ground-Plane

Compliments of Firestik® Antenna Company Technical Support Team

Copyright © 2023 Firestik® Antenna Company

Antenna installers need to understand the difference between chassis ground and ground-plane. They are quite different in what they mean to a mobile antenna installation.

Chassis ground is an electrical path on a vehicle that emanates from the negative battery post/connection of the vehicles battery. A more descriptive term would be “DC Ground” or “Direct Current Ground”, as it does refer to the negative side of the vehicles battery. Electrical components require a path (connection) to both the positive and negative connections of the battery. Without having both, electrical components will not work.

The term “ground-plane” is a common description used in the antenna business to describe the more accurate term “counterpoise”. Antennas, especially those that are required to transmit (versus just receive) need counterpoise (ground-plane) in order to function, When it comes to transmitting, ground-plane is every bit as important to the antenna as it becomes the reflective element for the antenna.

All common ground-plane dependent transmitting antennas need to be installed to a mount that is chassis grounded. Some people think that means a wire needs to go from the mount to the frame of the vehicle, which is also referred to as the chassis. However, chassis ground can be any conductive part of the vehicle where there is an electrical path back to the battery. For the most part, all of the metal on a vehicle is chassis grounded. There are some cases where the bed of a pickup is isolated from chassis ground due to rubber mounts. Also, on some modern vehicles, metal panels are glued to other components and the glue acts as an insulator. Plastic and/or fiberglass is not conductive and will not act as a DC current carrying material. And while carbon fiber is conductive, its conductivity is very low and cannot be relied on to act as a path to DC ground. If at anytime a mount needs a chassis ground connection, we recommend a braided grounding strap or a 10ga or heavier wire that is as short as possible to reach the nearest DC ground point. Grounding straps or wires that are long will more than often lack the requirements needed by the antenna.

Counterpoise (ground-plane) is provided by metal in the horizontal plane .. such as the hood, roof, trunk lid or bed of a pickup. The metal sides of a vehicle that has a composite material roof will be insufficient for a roof mounted antenna since it is on a vertical plane and not a horizontal plane.

Standard transmitting antennas require both a ground-plane (counterpoise) and a chassis ground (DC Ground) in order to function properly, or at all.

If your vehicle does not have the available counterpoise (ferrous metal) that will act as the effective element of the antenna system, all is not lost. Firestik manufactures a non-ground-plane (NGP) antenna system that does not require the vehicle to supply the counterpoise, and a chassis (DC Ground) is not required. A special coaxial cable and a specifically designed antenna is provided in the NGP kits. There is a dedicated section in the Firestik Tech Help directory that has information on the NGP applications. Go to Firestik Tech Docs to learn more.

Updated Dec. 27, 2023

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