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…an unbalanced line must be shielded, and connected in the proper “polarity” to prevent leakage (out of a transmitting line, or into an audio or receiving line).
But an a-c waveform has no particular “polarity,” as the amount of energy in its positive-going alternations is the same as in its negative-going alternations.
When an r-f (a-c) waveform is applied to a coax cable, it doesn’t matter to radiation from that cable, the power available at the far end of that cable, and personnel safety whether the center conductor is driven by the source and the outer conductor is connected to ground, or vice-versa.
There are coaxial cables designed to be “leaky,” such as RADIAX®. They have a series of gaps in the construction of their outer conductor, which enable the inner conductor to radiate a small amount of the energy flowing along it into space, at each gap. However their application is designed for higher frequencies, generally above 50 MHz.
… Most transmitters, however, have an UNbalanced 50 ohm output,…
Many transmitters are able to drive a 50-ohm, unbalanced load, but that doesn’t imply that their output impedance is 50 ohms. If it was, then when that transmitter was connected to a 50-ohm load, 1/2 of the output power it produces would be dissipated within the transmitter.
For highest power delivered to the load, the output impedance of the transmitter itself must be as low as possible — less than a few ohms, typically.