Patent GB-604227-A (GB)
Title: Improvements in direction finders
IPC: G01S1/02
Filed: 1945-09-05
Abstract: 604,227. Radio direction-finding; aerials. WESTERN ELECTRIC CO., Inc. Sept. 5, 1945, No. 22915. Convention date, Feb. 17, 1942. [Class 40 (vii)] In a direction-finder comprising a pair of receiving channels connected to a common indicator, each channel has a separate automaticgain control device and components of a control signal, of a frequency within the pass-band of the channels including the incoming signal frequency, are fed to the channels to control the respective gain control devices so that the gains in the channels are substantially equal and independent of the strength of the incoming signals. The control signal may be of large amplitude compared with the incoming signals and differ in frequency therefrom by an audiofrequency. According to a different aspect of the invention a, control wave within the normal pass-band is supplied to the two channels of an amplitude comparison type direction-finder, the channels having frequency-phase angle characteristics with equal slopes over a frequency band width at least as great as the difference between the frequencies of the incoming and control waves, the output of each channel including a detector supplying current of the difference frequency between the incoming and control waves to the indicator, these currents representing the intensities and relative phase polarity of the incoming waves in each channel. In an amplitude comparison direction-finder combining both aspects of the invention, four spaced antennae 1, 2, 3, 4, consisting of wooden framed towers covered with sheet copper form the Adcock pairs 5 and 6 of the direction-finder. Pair 5 feed a receiver 21 through a balanced to unbalanced transformer 14 and pair 6 feed a similar receiver 22 through transformer 13. Each receiver consists of an attenuator 23, R.F. amplifier 24, frequency changer 25, I.F. amplifier 26, detector 27 and audio amplifier 28. Both frequency changers are fed from a common oscillator 29 and automatic gain control is provided by units 30 for each channel. The local control signal differing from the incoming signal by an audiofrequency is generated by adjustable oscillator 34 feeding modulator and amplifier 36, adjustable attenuator 37 and antenna 39 which is arranged fairly close to antennµ 1 to 4 and on a line making an angle of 45 degrees to those joining the pairs so that the energy picked up by the two channels is equal. Attenuator 37 is adjusted to maintain the control signal in the two channels about twenty times the strength of the incoming signal so that the gain control circuits 30 are operated almost. entirely by the equal control signal components in the two channels. Each receiver operates as a single side-band receiver regarding the injected wave as the carrier and the audio output is proportional to the amplitude of the incoming signal. The outputs of the receivers are applied to deflection plates 31 and 32 of a cathode-ray oscillograph 33 to form a straight line indication of bearing. It is stated that, previously, to secure a linear trace it was necessary for the phase-angle shifts in the two channels to be equal, but in this system in accordance with the invention it is only necessary for the slopes of the frequency-phase shift characteristics of the two channels to be the same for the two frequencies passing through them; i.e. the control and incoming signal frequencies. This is a considerably less stringent requirement; the phase-shifts in the two channels after the detector must, however, still be equal. An audio oscillator 40 may be used to modulate the control signal in order to set up and check the apparatus. The amplification of each channel is adjusted so that the outputs from them are equal in amplitude on the modulated checking signal. The slopes of the gain control (A.V.C.) voltage curves for the two channels should be equal. The invention in its first aspect set out above is not limited to direction-finders depending on amplitude comparison, but may also be used in systems comparing the phase of a pair of currents derived from spaced non-directional antennae. In such a system it is the shape of the trace which is indicative of direction, but equal amplitudes of compared signal, obtained by equal channel amplification, enable sharp traces representative of the deviation from the in-bearing position to be obtained.