Small Dipole with No Heat LossEditar
For a small uniform current element the available output power is equal to the induced voltage squared divided by four times the radiation resistance. Thus
E=effective value of the electric field of the wave.
a=length of the current element.
Rrad. = radiation resistance of the current element
Since the power flow per unit area is equal to the electric field squared divided by the impedance of free space,
i.e., , we have
The effective area of a half-wavelength dipole with no heat loss is only 9.4 per cent, 0.39 decibels,2 larger than the effective area of the small dipole. Therefore
The area of a rectangle with one-half wavelength and one-quarter wavelength sides is and it is, therefore, a good approximation for the effective areas of small dipoles and half-wavelength dipoles.