Near field to far field transformation by using equivalent sources in HF band

Initially designed for long distance communications, HF band (3–30 MHz) antennas are nowadays mainly used in over-the-horizon radar applications. The performances of such devices depend on the knowledge of the electromagnetic field radiated by the antennas. The “sky wave” and “surface wave” modes radiated by these HF antennas are drastically influenced by their environment and particularly by the electromagnetic properties of the ground upon which they are located. The conventional methods of radiation pattern computation, like full-scale radiation pattern measurement and mode expansion of the near field measured in an anechoic chamber, are, in that case, neither convenient nor suitable, respectively. Thus, this paper describes a new way to perform the near field to far field transformation by replacing the antenna under test by a set of equivalent sources for which the ground is taking into account.