Current Flowing in Antennas for Radio Signals
There IS a load.  Energy radiates from an antenna, so the equivalent circuit for that antenna has an energy-losing element, a resistance.   It doesn't represent an Ohm's Law compliant resistor with heat generation, in this case.   You have to consider that an antenna is a kind of coupling with a waveguide, and then (because it's in open space) remove the 'guide' bits.   A cable, or a waveguide, has a characteristic impedance, and so does an antenna in air.