RF on outside of coax
Without going into too much detail it is accurate to say that current flows out of the center conductor of the coax and the same exact amount of current, in the ideal conditions, will flow back to the source and should be inside the coax or on the inside of the shield. Essentially the shield can be viewed as two conductors or wires. When the current starts to flow it has a choice to return to the source over either the inside wire or the outside wire. That is inside the coax shield or outside the coax shield. The factor that determines where the current will flow depends on the ratio of impedances looking down both paths from the antenna connection. If the outside path has a high impedance relative to the inside path then no current or an insignificant amount of current will flow on the outside of the coax. The length of the coax becomes extremely important here as the impedance varies from high to low as the length of the coax is varied. If the impedance is low looking down the c...