Improvements in or relating to the detection of obstacles by electromagnetic waves

535,120. Locating objects by reflected waves. COMPAGNIE GENERALE DE TELEGRAPHIE SANS FIL. Nov. 30, 1939, No. 31194. Convention date, Dec. 1, 1938. [Class 37] Obstacles are detected and 'their distance measured by measuring the time of propagation of ultra short waves to the obstacle and back after reflection therefrom, the receiver being blocked during the transmission of each impulse by a blocking impulse generator synchronized by the transmitter. An ultra short wave transmitter and a receiver provided with highly directional aerials are arranged side by side. The period of the transmitter impulses is so chosen as to be equal to the time taken by the wave to pass outwards and return between the transmitter and an obstacle located at the limit of the range, the duration of the impulses being very short compared with this period. During the transmission the receiver is blocked, so that it can only receive the reflected wave. The receiver amplifies the received signal and applies it to the deflecting plates of a cathode-ray oscillograph, of which the sweep is synchronized by the same synchronizing voltage that controls the transmission impulses and the blocking of the receiver. The transmitter comprises a magnetron 4, Fig. 2, oscillating on a decimeter wave and coupled by a feeder 3 to an aerial 2 in the focus of a parabolic reflector 1. For facilitating amplification in the receiver the magnetron oscillator is modulated at a lower frequency by means of a pilot oscillator comprising a valve 6, oscillatory circuit 13, resistances 12, 14, and condensers 5, 11. The output of the pilot oscillator is applied to an amplifier 7 through a transformer 15 with tuned secondary. The output of the amplifier '7 is applied to the tuned primary of a transformer 16, the secondary of which is in the anode circuit of the magnetron 4 and ensures its modulation by anode control. The anode current of the modulation valves is provided by a stabilized rectifier 57. In order that the transmission shall be in synchronized impulses of short duration the magnetron voltage is maintained at a sufficiently high value for there to be no oscillations in the absence of a synchronizing voltage, the amplification of the valve 7 being at the same time cancelled by negative grid bias provided by the rectifier 57. Emission is caused by reduction of the anode voltage of the magnetron 4 and unblock-' ing of the modulation amplifier valve 7 by means of a positive voltage applied to its grid, the two voltages performing these operations being applied simultaneously for a very short time and being produced by an impulse generator. A valve 21 has applied to its grid through a transformer' 30 and resistance 24 a sinusoidal synchronizing voltage of the same period as the impulses. The plate current is passed through an inductance 32 shunted by a resistance 33 and the voltage across this is transmitted to the grid of a triode 22 through a resistance 25, condenser 34, and resistance 35, the value of the resistance 25 being large compared with that of the grid-cathode space so that the grid voltage is substantially equal to the cathode voltage. Impulses of positive voltage are produced across a resistance 36 in the plate circuit and are transmitted to the grid of a valve 23 and also to the grid of the modulation valve 7, which is unblocked and provides a modulation voltage on the econdary terminals of the transformer 16. The impulse applied to the grid of the valve 23 produces a corresponding variation of plate current, giving rise to a voltage of similar form in a resistance 17 in the anode circuit of the magnetron 4. The value of the resistance 17 and the slope of the valve 23 are so selected that the voltage drop produced by the impulse of plate current brings the point of operation of the magnetron 4 into the region in which it can oscillate. The synchronizing voltage is provided by a stabilized oscillator followed by an amplifier stage. The oscillator comprises a valve 42 and a double inductance 49, one part of which constitutes with the condenser 50 the oscillatory circuit connected to the grid, while the other part serves as reaction winding. The oscillations are transmitted to the grid of an amplifier valve 41 by means of a condenser 45 and potentiometer 44. In the plate circuit of this valve is a transformer 43 with two secondaries feeding the transmitter and receiver respectively. The receiver is shown in Fig. 3 and comprises a parabolic reflector 2 containing a receiving aerial 1 in the form of a half-wave doublet. A transmission feeder 3 transmits the very high frequency to a detector valve 5, the modulation frequency being amplified by an H.F. amplifier valve 24. The modulation frequency is transmitted along a high frequency. feeder 27 to a mixing valve 37 followed by amplifier valves 42, 53. A detector valve 64 follows, by which the impulse frequency is picked out. The impulse frequency is passed to a valve 100 where it is beaten with a neighbouring frequency so as to give a note frequency, which is amplified by a valve 107 and fed to headphones 114. The valve 107 also feeds a voltmeter relay 115 which closes the circuit of a luminous and audible signal indicator 118 at and above a certain strength of received si gnal. The detector valve 64 before detection feeds a mixing valve 76, which after the frequency change energizes the vertical deflecting plates of a cathode ray oscillograph 97 through an amplifier valve 84. A valve 170 generating narrow triangular impulses is connected to the oscillator 41, 42, Fig. 2, producing the impulse frequency at the transmitter. The impulses produced by the valve 170 produce at the plate of an amplifier valve 185 a saw-tooth voltage which is applied to the horizontal deflecting plates of the cathode ray tube 97. The impulses of the valve 170 are also applied to a blocking valve 179 which gives a rectangular impulse having a, duration and phase which can be regulated. The valve 179 acts on the valves 24, 42, 53 and blocks their plate current, thereby causing cessation of reception during the transmission periods. This blocking action may be suppressed when adjusting the receiver by means of a switch which cuts off the anode voltage from the valve 179. By varying the bias on the valve 179 by means of a potentiometer resistance the blocking duration of the receiver during the times of transmission may be regulated. The zone within which obstacles can be detected may in this manner be varied, reception due to nearer obstacles being suppressed.