Horn antennas are widely used in different practical applications. They serve as feed antennas in many types of more complex constructions of antennas. The horn antennas are powerful means of high energy technology, they are used as applicators in medical purposes. The horn antennas are the most popular probing device in non-destructive testing. The horn antennas are also important tools of standards of power gain. In all applications the information about of space distribution of electromagnetic field is very important. For instance in medical and technological applications knowledge of field distribution allow ones to control the depth of heating that is very important for success of operation. The feature of these applications is use of near-field wave. Information about longitudinal distribution of field serves for increasing accuracy in non-destructive testing due to more accurate calibration at the same distances for structure under test and metallic standard mirror. Usually they cannot be positioned in the same place under technological process and displacement courses error in calibration and therefore errors in dielectric constant and thickness. Power gain can be measured using metallic plate but this approach is simple only for far-field zone, for Fresnel zone a few sets of measurements at a few distances are need. This circumstance determines importance of the field description.