Thanks for all the quick replies. The antenna is mounted via the mag mount base in the center of the roof. Coax from the mount runs back to the rear door which swings out along a horizontal plane, like normal passenger doors. The cable is routed down the door channel in the body then forward through the cargo area. It is tucked in beside the rear seats, then along the carpet edge in the rear footwells. It was then pulled up between the front seats and the center console. It arrives on the floor in the front footwell on the passenger (right) side.
So, this raises a few possible issues…
Running the coaxial cable through a door/hatch is going to result in eventual damage to the cable. Damaging the cable can result in higher SWR, water intrusion, or just outright breaking it. More on solutions later.
The other issue is that running coaxial cable alongside other vehicle wiring is a great way to pick up noise from the existing wiring. Keeping coaxial cable away from existing wiring is important, if you must cross existing cable, cross at 90º.
The Midland 75-822 comes with a special harness which replaces the battery back on the bottom of the unit. It is a radio designed for both mobile and portable use. When the radio is equipped with the car adaptor, it has a preconfigured power cord with cigarette lighter plug. This harness also contains the antenna lead. You can find pictures of the radio here if you wish to have a look
https://midlandusa.com/product/75-822cb-radio/
Cigarette lighter sockets are great for charing your cell phone, lighting cigarette, etc.
They are awful places to pick up power for radios.
The wiring for the cigarette lighter sockets is bundled in with all the other under dashboard wire, which often includes CAN buss data, blower motors, sensors, HVAC, etc. Great sources of noise. Noise from outside the cabin can get in following existing wiring.
One thing you can try is to feed the radio off batteries. That would allow you to determine if the noise is coming in via the power.
Getting clean power in to the radio is important. If you are serious about getting your amateur radio license down the road, you are going to want a good power feed for your radios. That means fed right off the battery. Cigarette lighter sockets can rarely supply enough current for any serious radios. The noise issue also becomes pretty big.
I'd strongly suggest running a dedicated power feed from the battery into your cabin. Eventually you are going to need it anyway, and it can solve a lot of issues.
Chasing ignition noise when you already have a compromised power feed is sort of pointless.
I haven't yet tried the antenna in other locations on the vehicle. I had not yet considered the idea of running a separate power line directly from the battery, but will certainly keep that idea in mind. The vehicle is reasonably new so I don't suspect faulty ignition components, but will certainly use the AM radio trick to check for RF from the ignition. Many years ago I had a car which had ignition noise on the radio, so I understand that problem.
Don't rule out anything yet, even if it's new.
Could just be a noisy vehicle, some are. Some take a lot of work to make them quiet.
Try moving the antenna around to various points on the roof and see if one works better than the other. That's about the only benefit to magnetic mount antennas, the ability to move them around.
I am curious if possibly using the trunk mount option instead of a mag mount on the roof might work. The back door of the vehicle does have a lip on the upper edge which might serve the same function as a trunk on a sedan type car. The trunk mount option on the K40 requires 2 screws to be used which bite into the metal. In theory, this should then ground the antenna to the vehicle. Any thoughts on this idea would also be appreciated. I am new to CB and also studying to get a HAM license. All ideas and the benefit of those with more experience is greatly appreciated.
Unlikely that is going to fix anything.
DC grounds and RF Grounds/RF Ground Planes are different beasts. They can be the same thing, but a good DC ground doesn't automatically make for a good RF ground.
Also, putting the antenna all the way at one end of the vehicle is going to cause some issues with your ground plane. Your ground plane will be
VERY lopsided. Most of the sheet metal will all be in front of the antenna base, which will make the antenna directional.
Sure, CB'ers do it, but it's a lousy way to run an antenna, especially if you are serious about it working well.
The ideal location for any mobile antenna is dead center in the vehicle roof (centered front/rear and left/right). That gives the antenna a nice even ground plane in all directions. Anything else is a compromise.
If you are serious about making the CB work well, you are serious about getting your amateur radio license, and you want all this stuff to work well, then you really must get serious about antennas. Mag mounts are great for temporary use. They are lousy for long term use. The only benefit to them is ease of install/moving. That's pretty much where it ends.
Installing a proper NMO mount in the center of your vehicle roof will give you the ideal ground plane and good ground connection. While it does involve drilling a hole in your roof, it's easy and will not leak if installed per directions.
A NMO mount will give you the best overall performance and the widest selection of antennas. Installing an NMO mount with a CB antenna will work well now. When you get your amateur radio license, you can remove the CB antenna and install an amateur radio antenna, scanner antenna, etc.
The key to all this is making sure all your equipment is installed correctly. Unfortunately for the consumer, the CB radio industry has done a disservice by making users think that plugging into the cigarette lighter plug and dropping a mag mount antenna on your roof is all you need to make it all work.
It's not.
Radio, especially if you want it to work well, requires some more work. Clean power, properly installed antenna, etc. are all key. Plug-n-play works for cell phones, but not real radios. Cutting corners on the install is going to result in poor performance. You can't do the quick/easy install and expect it to work well. There is a reason that public safety agencies spend a lot of money on radio installs in their vehicles. If you want it to work well, you do it right. If lives depend on it, you do it right.
You need to make that decision.