What % of electrical energy does an transmitter antenna convert into radio waves ( at say 1 Mhz) ?
Electrical energy in an antenna is converted to the equivalent of a resistor (known as the Radiation resistance). While the energy lost by ohmic resistance is converted to heat, the energy lost by radiation resistance is converted to electromagnetic radiation. Losses to Ohmic resistance are usually very small and depend on the materials and cross section of the elements and connections. The radiation resistance of a quarter wave monopole over a ground plane is 36.5 ohms. The radiation resistance of a half wave dipole in free space is 73 ohms.