 The disadvantage of this is that the efficiency of an antenna drops as its size is reduced below a wavelength.An antenna's radiation resistance, and the amount of power it radiates, is proportional to where is its length and is the wavelength.So even physically large ELF antennas have very small radiation resistance, and so radiate only a tiny fraction of the input power as ELF waves; most of the power applied to them is dissipated as heat in various ohmic resistances in the antenna.