I have a radiodditty GMRS mobile radio that I built into an ammunition box for portability and connected to a lithium battery. The battery box is plastic/nonmetal. When I checked the SWR of the antenna, I get a very high reading, between five and 10. Is there any way to reduce the SWR? Do I need a ground plane? Is there a way to add a ground to the system, such as a wire connected to a post in the ground? The radio box was intended to be used as a mobile unit to coordinate multiple smaller GMRs radios. Thank you