Interaction of antennas with masts under special consideration of the sensitivity and nonlinear effects in ship borne receiving systems

The minimum height of active and passive rod antennas is presented as a function of the environmental noise. A high linearity of the system is necessary to suppress intermodulation and crossmodulation effects generated by high level signals radiation from the transmitters on the ship. In many cases this problem only is solved by selective means at the front end of a receiving system. An appropriate solution is found in a fast tunable active receiving antenna. The effective height, the antenna impedance and the vertical pattern of an antenna mounted on top of a mast are influenced in an undesired way. The available dynamic of the receiving system is reduced considerably by the interaction of the mast and the antenna. Due to the resonant effects of the mast-antenna structure this reduction factor is frequency dependent and increases with increasing height of the mast. In addition there is a severe disturbing influence of the mast on the vertical diagrams of the rod antenna on this mast. Passive broadband rod antennas should not be mounted on a mast at all. With an active antenna however the effect of the mast on the antenna properties are reduced. In contrary to rod antennas loop antennas are advantageously decoupled from the mast by symmetrical mounting on top of a rotationally symmetrical mast.