Patent GB-587389-A (GB)
Title: Improvements in or relating to electrical distance or height measuring systems and instruments
IPC: G01S1/02
Filed: 1939-12-18
Abstract: 587,389. Radiolocation. POCOCK, A. G. Dec. 18, 1939, No. 33604. [Class 40 (v)] In order to measure distance and height, periodically pulse varied high-frequency signals are continuously transmitted and the reflected signals are received back and set up a beat frequency, which, together with the pulse frequency, is fed to a cathode-rav tube so that each will cause displacement in the electron path, the distance between these displacements being an indication of distance which may be read against a scale. When applied to aircraft, the combined transmitter and receiver may be connected to. either of two aerials, one directed downwardly and the other forwardly to indicate. alternately its height above ground and the distance to an object in front. A short time unmodulated pulse of low potential is applied to the grid circuit of a pair of oscillator valves 20 and causes a momentary change of frequency. The oscillations appearing are led by a coaxial cable 21 to a directional aerial. Reflected waves of pulse frequency are heterodyned with the oscillator frequency in the plate circuit comprising a coil 22 and condenser 23, producing a beat frequency. The same beat frequency is produced when the pulse is sent out, producing a zero point in the cathode-ray tube 24. The beam of the cathode-ray tube 24 is rotated into a circular path by phase-splitting an A.C. voltage across a resistance 29 and condenser 30. The A.C. voltage is generated by a crystal c and oscillator valve 32 driving a power amplifier valve 33, the time base voltage being obtained from the secondary winding of a tuned transformer 34 in the plate circuit of the amplifier 33, The beat frequencies formed by mixing the normal and pulse frequencies are applied to a tuned circuit 27, 28, voltage tapped from the coil 27 being fed to a centre tap 36 in the secondary winding of the time base transformer 34 and thereby added to or subtracted from the secondary voltage so as to deflect the beam of the cathode-ray tube from its circular track for a short time and form a cross line or marker. The voltage pulse required for the frequency change of the oscillator valves 20 is obtained from a tap 37 on the bias resistance 38 of the power amplifier valve 33 through a diode 39, condenser 43 and radio-frequency choke 44. The amplifier 33 is biassed to near cut-off point and the voltage across its bias resistance 38 then drops to a low value and the diode 39 passes no current. When the oscillator valve 32 drives the amplifier 33, current rises and falls through the bias resistance 38 and the positive biassing potential rises and falls in step. The diode 39 passes current only when the bias voltage reaches-the most positive point, a voltage then being obtained across its load resistance 40 and being introduced into the grid circuit of the oscillator valves 20. For measuring distances over two or more ranges, a twin peak crystal of 100 kc. and 1000 kc., for example, may be used covering maximum paths of 5000 and 500 feet respectively and providing a switch to connect suitable coils in circuit. By adjusting the gain of the coils, circles of different diameter 'for each range may be provided, with a separate ,calibration on the cathode-ray tube.