Tham khảo tài liệu 'electronic navigation systems 3e episode 2', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Radio wave propagation and the frequency spectrum 19 Because the effects of ground wave reflected waves are unpredictable some antenna arrays are constructed with a ground plane. Reflections from the ground plane are to some extent predictable and may be compensated for in the receiving system. Satellite navigation antennas and VHF RDF fixed antennae often use a ground plane to improve sensitivity and limit signal reflections. Antenna efficiency Antenna efficiency is of particular importance in all communications systems. If the efficiency of an antenna drops to 50 the maximum radiated signal also drops resulting in a consequent loss of range. It would be rare indeed to find any system that is 100 efficient and antennae are no exception. However antenna losses are well documented and consequently the effective isotropic radiated power EIRP figure for a system is usually calculated with reference to known efficiency figures. The losses leading to inefficiency in an antenna system may generally be classed as dielectric losses affecting the transmission properties of the antenna. Such losses in a transmitting antenna may be produced by arcing effects and corona discharge and in a receiving antenna they may be produced by bad connections or damaged wiring. Most of these losses can be controlled by careful installation good positioning of the antenna and diligent maintenance. . Antenna feed lines Whilst the connection between the transmitter output and the antenna input appears to be made by a simple wire it is in fact made by a balanced transmission line that possesses impedance. Usually the feed line is a correctly terminated coaxial cable specifically designed for the purpose. For most transmitting and receiving antenna systems the feed line possesses an impedance of 50 or 75 Q Because of its need to handle more power a transmitter coaxial cable will be physically larger than a corresponding receiver coaxial line unless of course both use the same line. .
