mounting a cb antena

For a magnetic antenna to work it has to be mounted on metal and the metal has to be a size big enough to become the other half of an antenna. Idealy, it should be a 9' circle. Obviously thats not practical or necessary. But a 2 or 3 ft circle of metal will give adequte CB performance for the average CBer.
On the flip side, a magnetic antenna will never give performance anywhere close to a quality, hard wired installed antenna. But you didn't ask about that.