Freq - Frequency
C - Speed Of Light ( 3 x 108)
Aperture - Opening Area of the horn antenna
The meaning of aperture is having an opening so that aperture antennas have an opening or hole. These opening of aperture antennas are covered with dielectric material in order to prevent the antenna from environmental effects. Due to the complexity of the mathematics, the far-field region observations will be restricted. These are essentially set up with metal or dielectric walls. For excitations feed pins or waveguide ports are used.
Horn antennas are popular in the microwave band (above 1 GHz), high gains are provided by the horn. There are three types of horn antennas basically 1. Pyramidal horn 2. E-plane horn 3. H-plane horn. When frequency increases the gain of the horn antenna increases. There is a fixed physical size in horn antenna, there is also a very little loss in horn antenna.so the gain and directivity are roughly equal in horn antenna Horn antennas have been widely used for space applications from the very beginning due to their capability of being best operation from Megahertz to Gigahertz to Terra hertz range.
Far field Distance - The far-field distance is the distance from the transmitting antenna to the beginning of the Fraunhofer region or far-field. In the far-field, the radiation pattern does not change shape as the distance increases.