GB610583A - Improvements in and relating to radar systems - Google Patents

Abstract

610,583. Cathode-ray tubes. JOHNSON, C. A. April 10, 1946, No. 11061. [Class 39 (i)] [Also in Group XL (c)] In a pulse radar system including an electronic storage tube with a high velocity beam, means are provided for modulating the targetcathode potential difference of the tube in accordance with echo pulses and for causing the electron beam to scan repetitively over the same track on the tube target so that changes in the charge pattern on the target are only produced when the applied modulating signal derives from a moving object. As shown in the Figure, a transmitter 1 and receiver 2 are coupled through a T.R. switch 4 to a common aerial system 3 which is arranged to scan in azimuth. The receiver includes means for mixing received echo signals with a locally generated reference oscillation having a fixed phase relationship with the transmitted R.F. signals such as disclosed in Specifications 596,650 and 596,657 so that the video output due to moving objects comprises pulses which are amplitude modulated at the Doppler frequency. This output is applied through a cathode-follower 5 and coupling condensers 15 to the cathode 7, cathode heater, grid 8 and first and second anodes 9, 10 of an electrostatically-focused storage tube which comprises a conventional cathode-ray tube 6 without its phosphor screen, but with a foil or coating 20 on the outside of the end wall forming the signal plate, the end wall of the tube constituting the target; alternatively, a separate target is mounted in the tube comprising a mica sheet with a silver backing constituting the signal plate. The modulating signal is applied to all the electrodes 7-10 of the gun to avoid defocusing, but this is unnecessary if magnetic focusing is used. The third anode 11 is maintained at a constant negative potential to prevent capacitative coupling between the target and the applied signals and the electron beam is scanned spirally at the pulse recurrence frequency by sweep voltages from the generator 24. The wall anode 12 is earthed and the electron gun is maintained at a high negative potential so that a high intensity beam is produced and the cathode-earth potential difference is some 50 volts greater than the second cross-over value. Under such conditions the ratio of secondary electrons to primary electrons at the target is less than unity and the target charges negatively until the cathode-target potential difference is equal to the second cross-over value at which point the ratio is unity. The time variation of cathode potential results in a corresponding variation of potential along the scanning track on the target and when echoes from stationary objects are being received, the potential variation in successive scans is identical and an equilibrium charge pattern is produced on the target. When echoes from moving objects are being received, the Doppler modulation produces a variation in the charge distribution in successive scans and an output is obtained from the signal plate 20. The secondary electrons emitted by the target are collected by the wall anode 12 and hence the output may be taken from this electrode instead of from the signal plate. In a modification, the receiver output is applied to modulate the potential of the signal plate which is capacitatively coupled to the target and the output is then taken from the wall anode. In a further modification, the signals received during one rotation of the aerial system are recorded on the target, the electron beam being arranged to scan a rectangular raster.