Patent GB-623635-A (GB)
Title: Improvements in and relating to radio direction-finding equipment
IPC: G01S1/02
Filed: 1945-10-18
Abstract: 623,635. Radio direction-finding. KEMP, C. G. Oct. 18, 1945, No. 27305. [Class 40 (vii)] A radio direction-finder comprises an aerial system with a rotating field pattern having two minima and indicating means to indicate the direction in which alternate minima coincide with the received signal wherein the alternate minima are selected by a rectifier which is sensitive to the phase reversal of the received carrier wave at each minimum and which provides a positive or negative output according to the phase of the carrier wave. In one embodiment (Fig. 1), the aerial system comprises a rotatable cross-arm carrying at its extremities two vertical dipoles DFA, DFB, coupled to form an H aerial and a vertical sense aerial DSA. The cross-arm may make 30 revolutions per minute and the output of H aerial system is fed to two valves V1 and V2 in push-pull. These valves are alternately rendered effective by applying an A.C. modulating voltage of 3 kc. from a local oscillator, Fig. 4 (not shown), to them over leads L2 and L3. The outputs of the two valves are fed to tuned circuit L-C5 which is adjusted by remote control and thence to a conventional receiver. In addition, the modulated signals from the directional aerial system have added to them at the point P amplified signals from the sense aerial. After detection in the receiver, the signal obtained comprises a 3 kc. waveform varying in amplitude as the aerial rotates and abruptly reversing in phase at each minima. This signal is applied (Fig. 3) to a phase adjusting network PA, narrow pass filter FR, amplifier AR and phase sensitive rectifier PSR. This rectifier receives over leads L12 and L13 a 3 kc. signal from the oscillator controlling V1 and V2 and is arranged so that the reversals in phase of the 3 kc./sec. signal at the rotation frequency of the aerial system reverses the polarity of the D.C. output. The waveform from circuit PSR which is roughly sinusoidal in form is squared by the circuit of V13, amplified, and differentiated by C21 and R28 into negative and positive " pips." These are applied at G to modulate a cathoderay tube indicator and also to a cathode follower stage CFS for operating a remote indicator if desired. The indicator preferably includes an electromagnetic deflecting coil energized from the 3 kc. source and biassed to provide a radial trace which is rotated about the neck of the cathode-ray tube in correspondence with the rotation of the aerial arm. On application of the positive " pip " from G, a radial trace is made on the tube indicating without ambiguity the direction from which the signal is received. Other intelligence, e.g. speech, on the received signal may be obtained by passing the receiver output through a low-pass filter removing the 3 kc. note. A further embodiment may use spaced frame direction'-finding aerials in place of the dipoles with a central screened loop as a sense aerial. A stationary aerial system comprising two mutually perpendicular horizontal arms carrying H aerials may also be used, feeding a goniometer. In such a system, the speed of rotation of the goniometer search coil may greatly exceed that of the aerial system in the previously described embodiments.