Four or eight omnidirectional antennas are arranged in a
circular array and are connected to an RF combining or
"summer" circuit. The summer combines the antenna
signals in a way which simulates the continuous rotation of a
single antenna element about the axis of symmetry of the antenna
array.
In the simplified figure, a single antenna element is shown rotating in a circular path. As the element approaches the incoming rf wavefront, a frequency increase occurs, and when the element travels in the same direction as the wavefront, the frequency decreases. This up-down frequency shift or modulation occurs at the rotational frequency of the antenna.
Applying the modulated RF signal to the input of a narrow band fm receiver produces a component in its audio output at the antenna rotation frequency (sometimes called the commutation or sweep frequency). This tone is superimposed on the normal audio output and the phase of the tone (relative to the clock reference used to commutate the antenna) is the bearing angle. The direction finder processes this audio signal to calculate and display the bearing angle.
There are two connections to the receiver. The antenna output connects to the receiver's antenna jack and the audio output at the receiver's loudspeaker is taken to the direction finder audio input. Typically, the external speaker jack is used for this connection.
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