The requirements for coupling systems are:
(1) The system must efficiently conduct the
energy picked up by the antenna to the radio. If this requirement is not met the direction
finder will lack sensitivity.
It must not pick up or otherwise add
additional energy from the wanted signal. Failure to meet this requirement results in bearing
(3) It must not introduce unwanted signals or
noise. These effects, if present, produce interference and impair the bearing readability.
With rotatable antennas the two most common types of coupling are DIRECT, and
Direct coupling is the simplest form of coupling, directly coupling the antenna terminals to the
receiver input. It is used in those few cases where it is practicable to design a DF system in
which the radio receiver is located at the antenna terminals, and the antenna and receiver
rotate together as a unit. Some directly coupled DF sets are small, hand-carried,
transistorized receiving sets used by counterinsurgency personnel. The loop is generally the
carrying handle, and the entire set is rotated to produce line bearings in the direction of the
Transmission line coupling is used in its simplest form when the antenna system is placed
some distance above, but is rotatable with, the DF receiver. Because of its length and
position the transmission line will, if its shielding or balance are not perfect, introduce
unwanted energy, causing bearing errors or impaired readability. Additionally, if the length of
the transmission line is an appreciable fraction of a wavelength or more, its characteristic
impedance must accurately match the impedance of the receiver and the antenna, or a
considerable loss of sensitivity will occur.
The direct and transmission line systems of coupling the antenna to the receiver may be used
when it is desired to rotate the antenna without rotating the receiver. This is accomplished by
adding a rotatable coupling element. Some of these elements are:
Slip rings. These are insulated metal
rings in contact with sliding fingers or brushes, which permit rotation without interrupting the
circuit. By mounting the rings on a shaft and providing fixed brushes, it is possible to
conduct the antenna current to a stationary receiver while permitting the antenna to turn at
coupling device consists of a transformer whose primary and secondary windings are
coaxial. Under this condition one winding may be rotated with respect to the other without
changing the coupling which exists between them.