High SWR - Cobra 29LX w/ magnetic mount antenna

The toolbox needs to be grounded to the body and frame of the vehicle.
Antenna should still work even if you peel off the foil.

 

I would try sticking the antenna to the roof of your truck and see what that does. The coating on the toolbox may be too thick.

 

That's the best place to test and use it.

 

It looks like the coax and antenna stud assembly are separate from the base. Is there continuity (0 ohms) from the antenna stud to that bottom stud that attaches it to the base? Is the base made out of metal or plastic?

 

I'm just guessing that the top stud is connected to the center conductor of the coax and the bottom stud is connected to the outer shielding. If there is a dead short between the two, that will cause the SWR to be off the charts. A slight short with some resistance may cause it to just be kinda high.

 

Got any soldering skills? If not, buy yourself a cheap, battery powered continuity tester that has a small incandescent 194 or flashlight bulb. It must be incandescent and not LED.

Take the antenna into a slightly darkened room. Connect the alligator clip of the tester to the barrel nut on the coax and touch the probe to the center conductor. The light should not light at all. Even a faint glow means you have a short somewhere.

Try it the other way around... connect the alligator clip to the center conductor and touch the tip of the probe to the mag base. It should not light, even a little bit.

 

And I just looked back up at your original post. Even at 3.5 SWR, it may be that the whip is too long for 27.185.

Is the SWR lower on channel 1 vs channel 40?

Maybe there aren't any CB shops in your area but what about a shop that sells HAM radio gear? Hooking your antenna to an analyzer is going to be the easiest way to see what's really going on if you can't find anything obvious like a shorted coax.