SWR Through the roof and a grounded SWR meter

[TheBasseteer]

I have a 2022 Ford Maverick. unibody construction, with a folding aluminum tonneau cover over the bed. I installed a Cobra 75 All Road into one of the storage boxes built into the bed sides. I have a Hustler RQM magnetic antenna that sits on the cab center front and off center to the right due to re-enforcing ridges in the cab roof. the coax runs between the inner and outer body panels and into the plastic storage box via a hole I drilled in the box. 12V power comes from a 12V line that is built into each side of the truck bed; I cannot get a good SWR to save my life. I installed grounding strap from each of the aluminum plates on the tonneau cover and grounded those to the side of the bed via pre threaded holes and the "BlackBox/Base unit of the Cobra 75 is ground strapped to the bed sides by threaded machine screws, testing ground with a volt-ohm meter indicates I have good ground all around. Because the AllRoad uses a blue tooth mic/handset to control the black box, I have to sit at the back of the truck with the tailgate down to check my SWR. Now the fun part I was in the parking lot of a shopping center 2 mornings ago trying to figure out what was wrong used the built in SWR meter on the handset first. Readings. were all above 4. I then hooked up my Workman SWR meter between the transmitter and the antenna, set it on a plastic shoe box that I keep in the truck and took reading a little bit better but not much, comparing them to the built in meter readings they were similar. I had to get something out of the shoe box so I set the SWR meter on the tailgate of the truck effectively grounding it I took another reading; 1.1 on channel 1 and 1.2 on channel 40 with the handset SWR reading similar. Can somebody here give me any idea what the --------is going on here?
Thanks in advance.
Jim

 

[RFI-EMI-GUY]

1) Magnetic mount antennas are horrific on CB frequencies. They are ungrounded and get only minimal coupling to the vehicle body via a small amount of capacitance between the magnet , its protective film and the vehicle body. There are add on gizmo's to improve that capacitance (I am sure PRC Guy will weigh in) but CB and mag mount antennas do not work together happily. They work for the vast majority of folks who do not own a VSWR meter and just slap the antenna on and are happy hearing Bubba a block away, but no better than that. What is happening is that 1/2 of the radio signal is radiating in a sloppy way via the coax and when you ground the coax, it is magically happy. But that happiness may not last as you move the cable around.

2)You should install a permanent antenna like an NMO27 , and tune for best VSWR.

3) You need all vehicle doors, tail gate and hatches closed to set VSWR properly. It will otherwise be way off when driving.

 

[mmckenna]

testing ground with a volt-ohm meter indicates I have good ground all around.

Your multimeter is showing you DC ground.

DC Ground and RF ground plane are two different things.

You have a physically small roof top trying to act like a ground plane on 27MHz. It's not going to work very well.
Your SWR being high could be a number of things, bad connectors, RF on the outside of the coax (as RFI-EMI-GUY said).

Taping into existing wiring isn't a hot idea, either.

Here's what I'd suggest...
Put the radio inside the cab.
Run the positive lead for the radio to the positive post on the battery. Do NOT tap into existing vehicle wiring.
Run the negative lead from the radio to a body ground point as close to the radio as you can get it. Do NOT run the negative lead to the battery, no matter what the internet tells you.
Get a permanent mount antenna on the roof so you have an actual ground connection at the base of the antenna.
Tune the antenna for low SWR, but don't get hung up on a perfect 1:1 SWR. That is ideal, but it may be hard to get on a smaller vehicle like that.

 

[W9WSS]

For the OP, the above suggestions are sage and should be taken seriously. Both RF-EMI-GUY and mmckenna are professionals and know what they're talking about. My IMMEDIATE advice is to get rid of that magnetic-mounted antenna. My new car was less than a week old and had NMO-mounted antennas (for Ham Radio) installed by a professional technician. My radios work flawlessly, and I have no issues with power, range, output, or poor signals when I transmit.