Patent GB-594634-A (GB)
Title: Improvements in or relating to electrical apparatus for detecting the presence of moving objects
IPC: H01J31/02
Filed: 1941-01-20
Abstract: 594,634. Electric discharge apparatus; radiolocation. COLLARD, J. Jan. 20, 1941, No. 775. [Classes 39 (i) and 40 (v)] [Also in Group XXXVI] .Apparatus for detecting the presence of a moving object comprises means for transmitting oscillatory signalsat spaced time intervals and means for receiving these signals after reflection by the object. In order to distinguish signals reflected by the moving object from signals reflected by stationary objects, means are provided to convert into amplitude changes the phase changes which occur between successive signals reflected from the moving object by virtue of its movement and oscillations bearing a fixed relationship with the signals transmitted. As applied for detecting the presence of aircraft, a carrier wave having a frequency of 100 megacycles per second is modulated by short pulses of 2 microseconds duration occurring every 1300 microseconds. These signals are transmitted from a directive aerial and after reflection are received, amplified, and fed to a cathode-ray tube. A receiver 3, Fig. 1, produces output signals, the amplitudes of which are dependent on the phase difference between the transmitted and received signals. The output of the receiver is fed in succession to a series of condensers 4, one of the electrodes of each condenser being connected together and earthed through a common resistance 5. Signals appearing across this resistance are passed through an amplifier 6 to a cathode-ray oscillograph 7, the beam of which is traversed at a predetermined rate across the screen. As the beam commences to travel across the screen, a switch arm 8 commences to traverse the series of condenser contacts. At the end of their travel, the beam and the switch arm are simultaneously returned to their starting points to recommence the cycle. As the cycle is commenced, a signal is transmitted and the received reflected signal is applied to the condenser in contact with the switch arm at that instant, so that a charge accumulates on the condenser depending on the amplitude of the signal in the output of the receiver. If a reflected pulse is received from a stationary object the voltage produced at the output of the receiver will be the same as at the corresponding instant on the previous cycle and no additional charge is applied to the appropriate condenser. No current therefore flows through the resistance 5 and the cathode-ray is not deflected. If received signal is reflected by a moving object the voltages at corresponding instants in successive cycles will differ, so that current flows into or out of the appropriate condenser through the resistance 5 and causes a vertical deflection of the beam of the oscillograph. The switching is performed electronically as shown in Fig. 2. In an evacuated envelope is a cathode-ray gun 9 generating a beam of electrons which is deflected so that it impinges in turn on a series of conducting elements 10 mounted on a sheet of mica 11 backed by a conducting plate 12. The elements 10 are of material that emits secondary electrons under bombardment by a beam of primary electrons and form a series of condensers in association with the plates 11, 12. The elements 10 may be arranged in a circle, the cathode-ray beam being deflected by a rotating field. In front of the elements 10 is a grid 13 to which the output of the receiver 3 is connected. The plate 12 is earthed through a resistance 5 and connected to the amplifier 6. As the beam impinges on the elements 10 in turn, each element emits secondary electrons which are repelled or attracted by the grid 13, causing the individual condensers to charge or discharge and producing a voltage across the resistance 5 if the charge across one condenser differs from what it was in the previous cycle. An initial vertical deflection is given to the beam of the oscillograph simultaneously with the transmission of the signal, and the screen is provided with a scale, the position on which of the trace due to the reflection of the signal indicates the distance of the object. To reduce the effect of the capacity between the grid 13 and plate 12, the capacity between element 10 and the plate 12 is made large compared with that between the grid 13 and plate 10. Alternatively, or in addition, the plate 12 may be fed from the receiver 3 through a condenser with a voltage in opposite phase to the voltage applied to the grid 13.