To use antenna factors most effectively in either EMI measurement or the reference antenna method, the antenna factor is more accurately derived than the conventionally obtained value when a half-wave resonant dipole antenna with Roberts balun (ANSI C63.5 Specification) is placed on the ground plane. In the calculation, the antenna is treated as the boundary value problem of Maxwell's equations and is analyzed by the Galerkin moment method. The balun is treated using circuit theory based on power transmission mismatch. To confirm the validity of these theoretical calculations, the computed value of the antenna factor in free space is compared with the measured results by the standard site method and the standard antenna method. It is confirmed that the tendency of the numerical results agrees well with that of the measured values.