The far field of an antenna is generally considered to be the region where the outgoing wavefront is planar and the antenna radiation pattern has a polar variation and is independent of the distance from the antenna. In this paper, the starting distance for the far field is defined for antennas operating in free space and over an imperfect ground plane. First, this paper intends to illustrate that 2D2 / λ formula, where D is the maximum dimension of the antenna and λ is the operating wavelength, is not universally valid, it is only valid for antennas where D >> λ. Second, this paper intends to compute a more specific constraint so instead of D >> λ we compute a threshold for D after which the 2D2 / λ formula applies. Third, this paper intends to properly interpret D in the formula 2D2 / λ when the antenna is operating over an imperfect ground plane. In this paper, we do not use 2D2 / λ for antennas operating over an imperfect ground instead we use a formula which depends on the transmitting and receiving antenna's heights over the air-Earth interface.