I'm confused

Sounds like coax, antenna cable is not grounded / hooked up

 

Always eliminate the mic first....i.e. try a different one.

 

Try flat braided ground strap

 

First try a known good mic.
You stated that you are using RG58 coax. That is one of the correct coax cable types for when you are using a single antenna setup. You are using a single antenna, right? You are not trying to use RG58 with a dual antenna setup, right?
. If a good mic does not fix the problem, can you borrow some known good coax cable and use that temporarily to check if you can get the CB radio setup working correctly? You can use RG58 or RG8. For CB radio it makes very little difference.
.Do a physical check of the length of the coax for breaks and/or odd lumps that would indicate a break in the coax.
.If you could not get different coax to try out, you need to get someone to check the coax cable with a multimeter. Check the resistance of the center wire from one end to the other end. It should read zero. Then check the resistance of the outer shield from end to end. It should read zero. Check the center wire to the outer shield at the other end. It should read maximum resistance.
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For an antenna ground plane problem there are two options.
.Option 1 install flat braid straps. Do a search for antenna ground plane for the specifics on what to do.
.Option 2 install another additional CB radio antenna mount facing downward. Install a second antenna only this one facing downward and check your SWR reading with an external SWR meter.

 

Don't use the plastic washer on the antenna pointed down with no coax....must be completely grounded to the mount.