On standard mobile antenna systems (those NOT designated as no-ground-plane), it is imperative that the antenna mount be grounded to the vehicles chassis. Ungrounded mounts will cause high SWR readings and/or low output power readings. There are three common situations when the mount may be inadvertently affixed to an ungrounded fixture.
For proper performance the mount must be grounded. It should be a standard part of the installation procedure to verify that the mount is grounded. It is an easy test to perform with either a VOM meter set in the Resistance (Ohms) mode or in the audible diode testing mode. Another method of checking is with a self-powered continuity tester. These test need to be made with the coax disconnected from the antenna mount or the radio. If the coax is fully connected and the radio is grounded you will get the false sense that you have a sufficient ground at the mount. Grounding the mount via the coax cable to the radio is not good enough. It requires a direct ground. Either the mount has a good direct chassis ground or it is grounded via the object it is mounted to.
The SWR readings on an ungrounded mount will almost always be 2.0:1 or higher on all channels. You may actually see the SWR dip during the testing procedure (i.e. 3.5:1 on channel 1, 2.5:1 on channel 20, 3.5:1 on channel 40). But, anytime it stays high you can be 90% sure that the mount is ungrounded, there is insufficient ground plane and/or you have low quality or the incorrect coax.
Also, keep this in mind. If it becomes necessary to run a ground wire or braid to a chassis ground point, use a wire of at least 12 gauge. We have reviewed installations where the installer used a small gauge wire (20ga) and when the SWR was tested it fell from over 3:1 to below 1.5:1. However, the power out put was very low due to the undersized path to ground provided by the thin wire. The path to ground must be sufficient enough to carry the power being applied to the antenna.