Why Antenna current flows through two kinds of resistance: radiation resistance, which represents power actually launched as radio waves, and loss resistance from conductor resistance, ground losses, loading coil losses and nearby lossy objects. Efficiency is the fraction of input power that becomes radiation, so it equals radiation resistance divided by total resistance (radiation plus loss), usually expressed as a percent. A short 160 meter vertical might have 5 ohms radiation resistance and 15 ohms of loss, giving 5/20 = 25 percent efficiency.
Watch out The choice comparing effective radiated power to transmitter output is not efficiency because ERP already includes antenna gain over a dipole and feed line loss, so a high-gain array can show ERP far above transmitter output. Dividing total resistance by radiation resistance just inverts the formula and would give a number greater than 1.
Efficiency is a fraction less than 1, so the smaller piece (radiation resistance) goes on top of the total.