High swr on GMRS box

[hmurmur]

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 

[tweiss3]

Do you have information on that antenna? Have you tried adjusting the length to see if you can "tune" it into a good SWR? Your problem likely is no ground plane. Oddly enough, if your faceplate of the go-box was aluminum not plastic it might be enough.

[KAF6045]

Concur: need antenna details.

That looks like a rather thick whip, and almost looks like it is extendible -- if it is, you'll need to adjust its length (and maybe mark the final point).

Most whips are 1/4 wave and require a ground plane to provide the "mirror" image to make a pseudo-dipole.

[Borage257]

Could you make a post about your box build?

[JLeikhim]

You would have had better results with a metal ammo box. An ideal ground plane woukd be an aluminum disc about 13 inch diameter. You can try lining the box you have with aluminized HVAC duct tape.

Sent from my SM-T350 using Tapatalk

[WRTY227]

You might be able to use a Nagoya UT-72 antenna NMO type that has a kit for adding ground plane radials.

I currently use one for my base station that is elevated at approximately 35 feet and it works great.

[WRUE915]

I have never used one of these setups, but you probably need a metal ground plane (even if it a 1/2 wave, ground plane still helps). It looks like your antenna attaches directly to the top plate which looks like plastic (maybe make that top cover oit of metal?).

The Midland "Ammo Can Base Station" recommends using the supplied mag mount antenna on the ammo can lid (separated from the can) as a ground plane.

As suggested above, maybe try the UT-72G or similar mag mount antenna for GMRS on the lid.