There are many kinds of power loss associated with power transmission lines:
Resistive - due to the current in the wires heating them up, and also induced currents in nearby conductors.
Leakage - current flowing in places that it shouldn't, such as contaminated insulators, corona discharge, etc.
Radiation - electromagnetic waves carrying energy away from the wires.
The last one is the one you're asking about, and it is by far the least significant of the three.
No, you can't treat the wire as a "dipole". You have to treat it as an actual transmission line, because the spacing among the conductors and between the conductors and ground is very much less than a wavelength. This is what limits the ability to couple electromagnetic energy to "free space" — the electrical and magnetic fields are almost entirely canceled out right at the source.