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Coax is a funny thing. The center conductor is considered the hot. The shield is considered the cold or ground. The currents in each should be equal and opposite canceling radiation from the inside of the coax.
An electrical circuit generally requires a path out and back, i.e. lamp cord, battery, solar cell, phone line. So the RF goes out from the transmitter on the center conductor and returns on the shield.
But the interesting thing is the return current flows on the inside of the shield. If the impedance of the coax and the load at the end of the line (antenna) match, all of the power is dissipated by the antenna.
However, if the load at the end of the line (antenna) does not match the coax impedance some of the power is not dissipated by the antenna, is reflected back and flows on the outside of the coax shield referred to as common mode current. This common mode current on the outside of the shield is what causes radiation of the signal from the coax.
So, I suppose if the ATU is designed properly and the antenna matches the coax, there should be no reflected power, no common mode current, no radiation from the outside of the coax shield. This would be why it shouldn’t matter how long the coax is or how high the ATU is mounted.
The preceding explanation source is from various writings on the subject I have read over the years. I’m sure someone here will clarify/correct my explanation if my memory was lacking.