1,055,575. Radio navigation. STANDARD TELEPHONES & CABLES Ltd. Aug. 14, 1964 [Aug. 19, 1963], No. 32676/63. Heading H4D. The Specification describes in detail a radio direction finding receiver; and broadly a receiver for a commutated aerial direction finding system (C.A.D.F.). In all the described systems the bearing &c. information is obtained by use of the phase angle between a received carrier wave and the resultant of upper and lower sidebands of the carrier wave received at a different point. In the direction finder, Fig. 1, which works on the interferometer principle, three horizontal dipole aerials 1, 2 and 3 are provided in a line at right angles to the middle of the required bearing sector. This arrangement receives horizontally polarized waves, but vertically polarized waves can be received using Adcock pair aerials. Signals at frequency F are received by all the aerials, differing in phase according to the incoming direction of the signals. The signals received on aerial 1 are modulated in balanced modulator 4, by a signal of frequency f 1 to produce sideband signals at frequencies FŒf 1 . Sideband signals at frequencies FŒf 2 are similarly produced by modulator 6 from the output of aerial 3. The carrier from aerial 2, together with the two sideband pairs are passed, via superhet receiver 8, to a demodulator 10. The phase of the resultant of the two sideband signals F+f 1 and F-f 1 is the same as that of the signal received by aerial 1. Thus by demodulating the sideband signals with the signal from aerial 2, two signals at frequency f, are produced respectively representative of the upper and lower sidebands, and having phases differing by twice the phase difference between the signals received by aerials 1 and 2. The demodulator 10 thus produces two signals at frequency f 1 and two at frequency f 2 , which signals are selected in filters and passed to a phase intercept resolver 11. By deriving beat signals at frequency f 1 - f 2 from the four input signals, fine and coarse bearing indications may be obtained, by comparing the phases of various pairs of the beat signals, and displayed at 12. Part of the demodulator 10 concerned with the sidebands FŒf 1 is shown in detail in Fig. 2 (not shown). It comprises a conventional phase shift discriminator 16 having two coupled input tuned circuits to which is coupled by winding 15, the two sidebands FŒf 1 and the signal F from aerial 2. When combined, these signals produce a signal which is amplitude modulated at frequency f 1 and is also phase modulated at frequency f 1 , the two modulations being in phase, and having relative depths, depending on the phase difference between the signal F and the resultant of the two sidebands. For example, if the phase difference is zero then there is only amplitude modulation. The amplitude modulation produces equal signals at points P and Q having in phase A.C. components at frequency f 1 . The A.C. components of these signals pass through respective halves of the centre tapped primary of tuned transformer T, and appear across resistor R. The phase modulation is equivalent to frequency modulation in quadrature therewith which produces signals at points P and Q having A.C. components in antiphase at frequency f 1 . The A.C. components of these signals pass along the primary of transformer T, and induce a signal at frequency f 1 in the secondary thereof. This signal is in quadrature with that across resistor R which is added to the said quadrature signal by a centre tapping connection, such that sum and difference products of the two signals appear at outputs A and B having frequencies f 1 and phases differing, as previously mentioned, by twice that between signal F and the resultant of the sideband signals. If the frequency F is not known, such that transformer T cannot be tuned (20), the signal F and the sidebands are frequency shifted by an amount F 0 , by means of frequency changer 22. Fig. 3A (not shown). and oscillator 23 and are then delayed by a period 1/4f 1 before being heterodyned with the unaltered signal F and sidebands in detector 25, to produce a signal at frequency F 0 , together with sidebands at frequency F 0 Œf 1 . When these signals are combined, they produce a signal amplitude and phase modulated at frequency f 1 , the phase of the amplitude modulation being 45 degrees and that of the phase modulation being- 45 degrees, i.e. the modulations are in quadrature. The combined signal is fed to a demodulator, Fig. 3B (not shown), similar to that shown in Fig. 2, excepting that the secondary of transformer T is provided with a C/R network conpensating for the quadrature relationship of the amplitude and phase modulations. The phase intercept resolver 11, Fig. 1, is shown in detail in Fig. 4 (not shown). The filters shown in Fig. 1 separating out the signals at frequency f 1 and f 2 and also separating out those due to the upper sidebands and those due to the lower sidebands, are shown in Fig. 4 as 30, 31, 32 and 33. The upper sideband signal at frequency f 1 from filter 30 is mixed with the upper sideband signal at frequency f 2 from filter 32, in detector 34, to produce a first signal at frequency f 1 - f 2 . The two lower sideband signals are similarly mixed in detector 35 to produce a second signal at frequency f 1 - f 2 . The phase difference between these two signals varies rapidly with change of bearing. The second signal is phase shifted in sin/cos potentiometer 36 before being compared in phase with the first signal in phase resolver 37. The resulting phase difference controls a motor M, which in turn controls the sin/cos potentiometer 36 via gears 44. The position of the shaft of motor M then gives a fine but ambiguous indication of the bearing. A coarse but unambiguous bearing indication is similarly produced by comparing the phases of two signals at frequency f 1 - f 2 obtained by mixing a lower side band signal of frequency f 1 or f 2 with an upper side band signal of frequency f 2 or f 1 respectively. A switch S is normally so positioned that the coarse indication is normally shown, a switch to the fine indication only being made on receipt of a definite signal. In the C.A.D.F. system receiver, the phase of the signal received by the commutated aerials is continually changing with respect to the signal of like frequency from the central or fixed aerial, the phase of the phase change depending on the bearing of the incoming signal. The signal received from the commutated aerial is subjected to balanced modulation to produce two sidebands. The sidebands are then detected by the signal from the fixed aerial to produce two modulation signals, the phases of which vary in accordance with the aerial commutation, i.e. they are frequency modulated. The phase of the phase variation of a selected one of the modulation signals is therefore used to indicate the bearing of the incoming signal.