Principle of Operation
Four or eight omni-directional 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 figure to the right, a single antenna element is shown rotating in a circular path. As the element approaches the incoming RF wave-front, a frequency increase occurs, and when the element travels in the same direction as the wave-front, 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 tone at the audio output of the receiver 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 sweep or rotate the antenna) is the bearing angle. The direction finder processes this audio signal to calculate and display the bearing angle.
A simplified diagram of a radio direction finder using the Doppler principle is shown below.
