Patent GB-621733-A (GB)
Title: Improvements in radio beacons
IPC: G01S1/02
Filed: 1945-04-23
Abstract: 621,733. Phase-difference measurements ; cathode-ray oscillographs. PRINGLE, J. W. S. April 23, 1945, No. 10216. [Class 37] [Also in Group XL (c)] A rotating-beam radio beacon transmits a carrier modulated with two sub-carriers, which are each modulated by a further source but with a varying phase difference related to the instantaneous position of the beam with reference to a datum direction. The directive aerial 1, 2, Fig. 6, is continuously rotated or oscillated about axis 21 by the motor EM and gearing 22, 25 or a cam mechanism, and is fed from the transmitter 3, which is modulated by unit 4 with sub-carriers of 2 and 3 mc/s. from oscillators 5 and 6. Oscillators 5 and 6 are modulated by the 20 kc/s. outputs of unit 7 which contains a phase-shifting device driven by shaft 26 and gearing 27 at the same speed as the aerial rotation, whereby the phase displacement in degrees between the two 20 kc/s. modulations corresponds with the bearing of the transmitted beam. In the mobile-craft receiver the sub-carriers, after detection, are separately amplified and detected to derive the two 20 kc/s. modulations, which after amplification are fed to a phase comparator unit to produce on a cathode-ray tube an indication comprising a lobe which is representative of the polar diagram of the transmitting aerial, and has an angular position dependent upon the phase displacement between the 20 kc/s. modulations. The phase comparator, Fig. 3, comprises two balanced-bridge demodulators in which the two 20 kc/s. signals which are themselves modulated with an envelope produced by the rotation past the receiver of the transmitted beam, are employed to demodulate one another. The modulation from the 3 mc/s. channel is fed directly to one input of each demodulator, while the output from the 2 mc/s. channel is fed directly to the second input of one demodulator and through a 90 degree phase shifter to the corresponding input of the other demodulator. The demodulator outputs comprise voltages proportional to E<SP>2</SP> cos # and E<SP>2</SP> sin 6 respectively, where E is the received signal strength and # is the phase displacement between the two 20 kc/s. modulations. These voltages when fed to the X and Y-plates of the cathode-ray tube result in the production of the lobe pattern which serves as a pointer indicating the bearing of the mobile-craft with respect to the beacon. In order to identify individual ones of a number of beacons operating on the same radio frequency the transmission may be suppressed during certain cycles of rotation to give a " group, flash " characteristic. This suppression may be produced by a switch 35, operated by a cam 32, driven by reduction gearing 33 from the aerial shaft. A second method of identification is the production of a " hieroglyphic " on the cathoderay indicator in place of a simple lobe, by the production, by means of gearing 28, cam 29, and phase-shifting control element 31, of a periodic departure from the normal law relating the rotation of the aerial system and the phase displacement between the modulation signals. Various frequencies and waveforms for the periodic variations will produce lobe patterns, such as those indicated in Fig. 5, which may be identified with particular beacons.