Principle of Operation

Doppler Systems radio direction finders utilize the Doppler Effect first described by the physicist Christian Doppler in 1842.  Doppler discovered frequencies increase as two objects move toward each other and decrease as they move away from each other.  In the radio frequency spectrum the motion of one object relative to another can be achieved by a rotating antenna as illustrated in the figure.  As the antenna moves toward the signal source the received frequency increases, and as it moves away from the signal source, it decreases.

Frequencies in the Doppler Effect

As a result the received signal is FM modulated at the frequency of antenna rotation.  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. 

In order to produce a usable signal the rotational rate of the antenna must be rotated at greater than 20,000 rpm.  Clearly this rate of rotation is impractical so instead several antennas (usually 4, 8, or 16) are arranged in a circle and electronically rotated.  Direction finders using this technique are sometimes referred to as pseudo-Doppler or synthetic Doppler.

In Doppler Systems Direction finders 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.

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.

Block Diagram of Simple Doppler Direction Finder