Patent GB-581363-A (GB)
Title: Improvements in or relating to radio locating apparatus
IPC: G01S1/02
Filed: 1942-12-25
Abstract: 581,363. Radiolocation; cathode-ray tubes. BOWEN, E. G., TAYLOR, R. W., WILLIAMS, F. C., BROWN, R. H., and TOUCH, A. G. Dec. 25, 1942, No. 18363. [Classes 39 (i) and 40 (v)] An apparatus for detecting from an aircraft or other mobile vehicle the direction and distance of a distant body by reflected pulses of radiation comprises a transmitter radiating modulated pulses over a wide solid angle in a forward direction, a receiver for the reflected pulses, means for changing cyclically the direction - of maximum response of the receiver so that it occupies in turn four cardinal positions inclined above and below and left and right of the direction of movement of the vehicle, and means for presenting the received pulses on a cathoderay tube indicating apparatus against a time base which is synchronized with the repetition frequency of the radiated pulses in order to indicate both the range and the relative direction of the object reflecting the pulses. In the apparatus shown in Fig. 1, the modulated pulses are produced in a radio-frequency oscillator 1 excited by a master control relay 2 and are radiated by an aerial 3. The duration of each pulse is 2.8 microseconds, and the pulses are repeated at intervals of 1500 microseconds. The reflected pulses are received by four forwardly responsive aerials 4 comprising a pair having maximum responses slightly above and slightly below the forward d rect on respectively and another pair having maximum responses slightly to left and right respectively. These aerials are cyclically connected by a rotatable switch 5 to an amplifier 6 followed by a mixer 8, an intermediate frequency amplifier 9, a second detector 10, and an output circuit 11, the mixer 8 being connected to a local oscillator 7. While each modulated pulse is radiated out by aerial 3, the local oscillator 7 is triggered off by a gasfilled valve circuit 12 controlled by the master control relay 2, so that the receiving circuit is protected from the direct impact of the radiation. The output from the terminal circuit 11 is cyclically distributed by a switch 13 synchronized with the switch 5, so that the signals received by left and right inclined aerials are transmitted to deflection plates of an " azimuth " cathode-ray tube 14 and the signals received by the up and down inclined aerials are transmitted to an " elevation " cathoderay tube 15. The time base generator 16 for these cathode-ray tubes is controlled by the master control relay 2 ; and the brightness of the cathode-rays are maintained constant during the sweep by a correcting grid bias derived from a phase-reversing valve circuit 17 coupled by a condenser to the time base circuit 16. Figs. 2A and 2B represent typical indications given by the " azimuth '' and " elevation " cathode-ray tubes 14, 15. The mark 20 represents the residual " break-through " effect of the pulse at the moment of its emission ; this sets the lower limit of possible range observation. The mark 22 is the reflected pulse ; the preponderance of the mark 22 on the right of the vertical central line in Fig. 2A indicates that the reflecting body is on the right of the forward direction ; similarly the preponderence of the mark 22 above the central horizontal line in Fig. 2B indicates that the reflecting body is above the forward direction ; the distance between the marks 22 and 20 is the range of the reflecting body. The mark 21 is due to overlapping ground reflectors and sets the higher limit of possible range observation. In the modified apparatus shown in Fig. 3, the pulses are transmitted and received in the same manner, but the output of the second detector 10 of the receiving circuit is utilized in a different way. A portion of the output is fed through a circuit 30 to the Y-plates of a range-indicating cathode-ray tube 31. The circuit 30 comprises a pentode amplifier and a double diode serving as a voltage regulator ; one diode is shunted across a resistance in the grid circuit of the amplifier to maintain the grid at constant datum potential for signals of varying strength ; and the other diode is shunted across the Y-plates of the cathode-ray tube 31 to prevent any undesirable vertical shift. The X-plates of the cathode-ray tube 31 are fed with a time base from a generator 32, synchronized by the master control relay 2. The generator 32 comprises a screen grid valve rendered conductive by a priming impulse on its control grid causing the discharge of a condenser through the valve. The condenser then recharges through a resistance at an exponential rate, and this voltage rise is applied through a condenser to the X-plates of the cathode-ray tube. At the same time a correcting voltage is applied to the control grid of the cathode-ray tube to maintain the brightness of the trace constant. Another portion of the output of the second detector circuit 10 is fed to a valve circuit 33 through a filter serving to eliminate certain forms of interference. The valve circuit 33 gives a logarithmic output which is fed to the suppressor grid of a dual-control gate valve 34, the control grid of which is fed with " strobing " pulses generated in a circuit 38. This circuit comprises a pair of cross-coupled valves acting as a multivibrator. The grid of one of these valves is connected to the anode circuit of a pentode valve to the control grid of which delayed pulses from the master control relay 2 are applied. Each pulse causes a sharp drop of anode voltage, after which the voltage rises exponentially at a rate depending on the resistances in the anode and screen-grid circuits. When the rising voltage attains a given value, it triggers the multi-vibrator, and a sharp '' strobing " pulse is sent thereby to the gate valve 34. These " strobing " pulses are also fed to the control grid of the range-indicating cathode-ray tube 31. The timing of the " strobing " pulses is adjusted by a potentiometer 39. The setting of this potentiometer also controls the bias voltage of a pentode valve 48 which receives square-topped pulses generated by a valve 46 which is grid impulsed from a transformer via a terminal 47. The bias of the valve 48 determines its cut-off point and therefore the amplitude of the pulses it passes, which amplitude is therefore proportional to the range of the reflecting object. These pulses are fed through a transformer 49 and resistance 50 to the X-plates of a cathoderay tube 43. The gate valve 34, which receives pulses from the receiver of the reflected pulses and also " strobing " pulses, is operative only when these pulses occur at the same time. The output of the gate valve 34 is amplified at 35 and fed through a distributing switch 13 to the deflection plates of two cathode-ray tubes 36, 43. The switch 13 is synchronized with the switch 5, so that the pulses received by the azimuth aerials are fed to the X-plates, and the pulses received by the vertical aerials are fed to the Y-plates. The cathode-ray tubes 31,36 are intended to be watched by an observer, and the cathode-ray tube 43 by the pilot of the vehicle. Figs. 4A and 4C represent typical indications of the range-indicating cathode-ray tube 31. The vertical mark 32 indicates the time of the reflected pulses along the horizontal time base. The marks 21 indicate ground reflections. The points 42, where there is increase in brightness, indicates the time of the " strobing '' pulses. This cathode-ray tube is used by an observer to determine the range by the position of the mark 32, and also to enable-him to adjust the timing of the " strobing " pulses until these coincide with the reflected pulses as indicated by Fig. 4C, so that the gate valve 34 may be rendered active. When this occurs, a luminous spot 37, Fig. 4D, appears on the cathode-ray tube 36 and indicates the direction of the reflecting body in relation to the X and Y co-ordinates. When this spot appears, the observer switches the cathode-ray tube 43 in operation for the use of the pilot. This cathoderay tube 43 indicates the position of the reflecting body by a spot 44, F ig. 4F, and, in addition, this spot is oscillated by the pulses received from the circuit 47, 46, 48, 49, 50 described above, so that the spot 44 has wings 45, the span of which is inversely proportional to the range. This feature forms the subject matter of the divided Specification 581,375. To provide reference data for the rough estimation of this range by the pilot, the screen of the cathoderay tube 43 is formed with vertical side marks spaced apart to correspond to a range of 1000 feet, and with a central U-shaped mark the limbs of which are spaced apart a distance corresponding to a range of 2500 feet. Switch-over devices may be provided to connect the cathode-ray tubes to a longer time base and other controls to enable " homing " on to a distant beacon station. A radio-frequency oscillator suitable for the position 1 of Figs. 1 and 3 is shown in Fig. 5. It comprises a pair of push-pull valves V 9, V 10 with anodes connected to a Lecher circuit L7, L8 tuned by a sliding bridge. This circuit is connected through a resistance R48 and an inductance L11 in series to a terminal T1 fed with pulses from the master control relay 2, Figs. 1 and 3, and is coupled through sliding condensers C20, C21 to the transmitting aerial. The cathodes of the valves V9, V10 are connected to earth through tuned circuits variable by condensers C18, C19 and acting as phasing reactances. The grids of the valves are similarly tuned by the inductance of their leads L9, L10, and are coupled through a choke L12 to a terminal T2 fed with pulses from the master control relay 2. The oscillator is triggered by a positive pulse applied to the anodes through terminal T1 and is sharply cut off by the application of a negative pulse to the anodes through the terminal T1.