Tham khảo tài liệu 'electronic navigation systems 3e episode 13', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Radio direction finding 349 Figure Signal currents induced in the vertical arms of a loop antenna produce a resultant potential difference across the input coil to a receiver. flowing through the inductor. For convenience we shall consider the plan view of the loop and the wavefront of the propagated signal. Figure a shows that when the wavefront is parallel to the plane of the loop the induced in both arms will be of equal amplitude and the same polarity. The two will therefore cancel producing no resultant current flow in the inductor and hence no input to the receiver. This is called a null position because at this point the audio output from a receiver drops to zero. Clearly there will be a second null position 180 away from the first. If the loop is turned so that its plane is now 90 with respect to the wavefront two will again be induced in both vertical arms but they will be of equal amplitude but opposite polarity. This causes a maximum circulating current to flow through the coil and a maximum output from the receiver Figure b . This situation corresponds to a maximum input to the receiver. Once again there will be a second maximum 180 away from the first the only difference being that the resultant current will flow in the opposite direction through the coupling coil. The AGP produced by such a rotating antenna is shown in Figure and for obvious reasons is called a figure-of-eight diagram. A transmitter bearing north or south produces a resultant null output. A transmitter bearing east or west produces a resultant maximum output. A fixed loop antenna system At the heart of this system are two permanently fixed loop antennae mounted on the same mast or base at 90 to each other one on the fore-and-aft line and the other on the port-and-starboard line of a vessel. An early manual RDF input system is shown in Figure to illustrate the principle. In this case each precisely mounted loop antenna is connected to a .
