It is best suited to applications in VHF and UHF frequencies and takes only a short time to indicate a direction.
This makes it suitable for measuring the location of the vast majority of commercial, amateur, and automated broadcasts.
As long as the switching occurs rapidly, the Doppler effect will be strong enough to determine the direction of the signal.
[2] The operator rotated the antenna, looking for points where the signal either reached a maximum or, more commonly, suddenly disappeared or 'nulled'.
[4][5] A great advance in RDF technique was introduced in the form of the Bellini-Tosi direction finder (B-T) system, which replaced the rotation of the antenna with the rotation of a small coil of wire connected to two non-moving loop antennas.
The loop antennas were similar to those used in earlier systems but fixed in position, set at right angles to each other to form a cross-shaped arrangement.
In effect, it recreated the traditional technique at a much smaller scale, allowing the main antennas to be built at any size.
The Bellini-Tosi system still has moving parts, albeit small ones, but has the more major limitation that it requires the operator to hunt for the signal, which may take several minutes.
[11][13] The magnitude of the shift is a function of the wavelength of the signal and the angular velocity of the antenna: Where S is the Doppler shift in frequency (Hz), r is the radius of the circle, W is the angular velocity in radians per second, λ is the target wavelength and c is the speed of light in meters per second.
For this reason, Doppler DF systems normally mount their antennas on a small disk that is spun at a high speed using an electric motor.
By 1943, examples were available that worked in the UHF region, used to find the German Würzburg radars operating at 560 MHz.
[15] A significant advantage of this technique is that it requires only a single receiver, amplifier, and the appropriate FM demodulator.
In contrast, HF/DF and B-T systems require two closely matched receivers, one for each antenna pair, and often a third for a sense channel.
[7] Widespread civilian use of the technique did not start until the introduction of practical circuits for the quadrature detector and phase-locked loop, both introduced after the war, which greatly simplified the reception of FM signals.
[13] The signal from the antennas is sent into a single receiver, resulting in a series of pulses, whose amplitude depends on the phase at the instant of sampling.
Because the "movement" in Pseudo-Doppler proceeds in steps, the resulting signal is not as smooth as it is in the case of a moving antenna.