2018 Freightliner coax issue. Any idea?

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Toyman, from what you discribe your truck has multiple signals from the antennas and the Cb radio signal is split off to your radio. This is a cheap design for the truck manufacturer but lousy for your Cb radio setup. This is a common design and common problem for Cb radio usage on newer trucks.
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.I suggest that you disconnect the coax from the driver's side antenna and use your own short length of coax from the driver's side antenna directly to your Cb radio. You can run the coax through your driver's side window just for testing purposes and find out what your SWR reading is on an external meter. This will provide a starting point to determine just what is going on with your setup.

 

To much reflect on back of cab..

 

The 18's came with built in antenna anyways didnt it so cant unhook ant from any side..
Installing a pole then ant on top is not the way to go....Moves your ground point away from antenna....

 

My thread on the same problem is long. And going thru overhead with newa bad idea (tried it) BUT

the coax can be run from the new Pro Comm Antenna Mirror Arm Mount thru the door wiring seal behind the exterior panel. A few bolts to remove. Work it around base and then thru lower panels under steering column (a few screws). Lots of space behind dash.

Have it come up the dash on exterior to top on passenger side. Mount CB on dash. (Velcro, etc).

Fused 12-ga power leads POS and NEG from main circuit panel behind passenger side access to fuses. (This is the way a Qualcomm or other would be powered). Put in a DC Quik connect to tie to a HD radio power cord. (Use ANCOR for all terminals, butt connectors, etc. NOT auto parts chain store crap.

With ring terminations just fasten POS and NEG with another nut atop each lead (fine thread; take one into hardware store and buy several).

A 1/4” socket driver with extension makes that easiest.

 

Every time I read about the problems of the hidden antennas in the new trucks I am reminded of the issue with the older Frightliners. That hidden in the roof antenna is going to cause issues with any antenna you mount if it remains in the vehicle I believe. Our trucks had the factory antennas mounted on the side of the sleeper behind the doors. When I mounted my Cascadia mirror mount antenna, if I didn't remove the factory antenna on that same side, my swr would be high. Remove the antenna and it would drop back down. In 2015 I believe, they moved the antennas to the back of the cab (automatic truck wash kept ripping them off the side of the truck). I did not have any interference from those antennas on the back of the cab. I don't see how one embedded in the roof could not interfere no matter where you mount your own antenna. Maybe not on the back of the cab, but unless you are pulling a FB or tank, that isn't a very good place to mount an antenna.......FWIW

 

I don’t know where they hide the Thin Film Antenna (pics online available; Hirschmann Corp), but it’d be fine to be allowed to remove it.

Maybe ground the factory coax? Never had to consider this before.

 

Been there done that on back of the cab..No range,swr was fine but i dont remember my other #'s..Performance was so so at best pullin a dry box..

 

Yeah, I wondered if grounding it or shorting it would change anything. Somebody more educated in these matters than me will hopefully answer that.