Patent GB-626224-A (GB)
Title: Improvements in or relating to radio direction finding installations
IPC: G01S3/46
Filed: 1946-10-31
Abstract: 626,224. Radio direction-finders. CAPLIN, F., and BAGLEY, J. H. Oct. 31, 1946, No. 32367. [Class 40 (vii)] A radio direction-finding installation comprises an aerial array in the form of a plurality of directional aerial elements, means for deriving a signal output from each of the aerial elements in turn, means for deriving a signal output from an omnidirectional aerial, means for modifying one of said signal outputs to change its frequency by an amount equal to a presentation frequency, means for deriving from the modified and unmodified signal outputs presentation signals at the presentation frequency and means for applying said presentation signals to a direction indicating device to produce an indication of the direction of approach of the received signals. In the arrangement shown in the drawing, signals from a pair of directional aerials N-S, E-W (e.g. Adcock type) arranged with their directions of maximum sensitivity perpendicular to one another, are applied alternately by a switching device S1 (e.g. an electronic switch) to an amplifying channel 1. and signals from an omnidirectional aerial 2 are fed to a second amplifying channel 3, a local oscillator stage 4 feeding mixing stages in both channels so that the amplified outputs from the two channels are at the same intermediate frequency. The output from channel 3 is applied to a second mixer stage 6 which supplies one side-band of the mixed signals of a frequency which differs from the intermediate frequency by an amount equal to the presentation frequency (i.e. the frequency at which the signals are ultimately presented on the cathoderay oscilloscope 9). Mixer 6, which may comprise a goniometer, the two field coils of which are energized in quadrature by the signals from channel 3 and the output from which. is taken from a search coil rotating at the presentation frequency, feeds a second mixer 7 to which is also applied the signal output from channel 1, the output from 7, comprising signals at the presentation frequency, being applied to a second switching device S2 operated synchronously with S1 under the control of the switching frequency of the latter switch, filtered out by means of a filter 8, and applied through switch-frequency filters 10 and 11 to the oscilloscope 9, the trace on the screen of which gives a direct indication of approach of the received signals. In a modification, the directional aerials may be in the form of loops or doublets and the aerial 2 may be omitted, the omnidirectional signals being obtained from centre tappings on transformer windings included in both directional aerial circuits. In another modification, a directional aerial system comprising four separate aerial elements spaced apart (N, S, E, W) is used, a switching device (corresponding to S1) being employed to apply signals from each aerial in turn to a radio receiver, a second switching device (corresponding to S2) distributing the output from the radio receiver to four channels corresponding to the four aerials, the channel outputs being then compounded in parts (e.g. by applying the two signals from a pair to a push-pull circuit to effect substraction) to provide two output signals corresponding to the N-S and E-W signals of the described embodiment. An advantage of the latter modification is that the sense ambiguity may be resolved by adding the signals to produce a combined signal, in phase with the presentation signals, shifting the phase of the produced signals by 90 degrees, and applying the phase-shifted signals to the cathode-ray oscilloscope to black out one half of the produced trace. In a further modification, the channel 3 may be dispensed with and the output signals from mixer 6 fed to amplifier channel 1.