The radiated power is composed of the output power of the transmitter fed into the antenna, the standing wave ratio, and the antenna gain. Thus, the radiated power is higher than the supplied transmit power by the antenna gain. The radiated power is always less than the RF active power supplied by the transmitter.
In the case of the lambda-half dipole, the field strength maxima are perpendicular to the dipole plane, whereas the isotropic antenna radiates with the same field strength in all directions. If the same RF powers are fed into both antennas, the half-wave dipole has higher field strengths in the main radiation directions than the isotropic radiator. There is a direct relationship between the two values: the radiated power of a half-wave dipole, the ERP power, is higher by a factor of 1.64 than the Equivalent Isotropically Radiated Power (EIRP) of the omnidirectional radiator: 'ERP = EIRP* 1.64'. This value of 2.15 dB corresponds to the difference in radiated power between the two types of antennas.
The ERP value of the half-wave dipole is the reference for comparison with other antennas.