Patent GB-900924-A (GB)
Title: New or improved method of radio scanning
Inventor: TUCKER DAVID GORDON
IPC: G01S13/42
Filed: 1958-03-17
Abstract: 900,924. Pulse radar; aerials; cathode-ray tube displays. NATIONAL RESEARCH DEVELOPMENT CORPORATION. June 16, 1959 [March 17, 1958], No. 8402/58. Class 40 (7). A scanning pulse radar comprises a transmitting aerial and a receiving aerial array whose equi-spaced elements are connected to a combining amplifier through respective signal channels, each associated with a respective phase shift element, means being provided for repeatedly varying the magnitude of the phase shifts during each pulse recurrence period so that the effective receiver directivity pattern repeatedly scans a sector during each pulse recurrence period, the period t of each scan being not greater than the duration of the transmitted pulse. The arrangement of the phase shift elements is such that in the static state the phase shift between signals derived from adjacent channels is greater than that which would be produced by transmission of the incoming signals over a distance equal to the spacing between adjacent aerial elements. The receiver aerial array may produce:- (1) a single fan beam which is scanned in one dimension; (2) two orthogonal fan beams which are scanned in two orthogonal directions, Fig. 24 (not shown); (3) a pencil beam which is scanned in a rectangular raster, Fig. 25 and Fig. 26 (not shown). The aerial elements may be coupled to respective equispaced tappings on an electrical delay line through respective frequency changers coupled to a common local oscillator whose frequency is varied at the sector scan frequency so that the relative phases of the signals derived from the different aerial elements vary cyclically and when combined at one end of the delay line give rise to a signal corresponding to that produced by a beam scanning the sector. Alternatively, the outputs from the frequency changers are combined directly and the local oscillator is connected to one end of a delay line, the local inputs to the frequency changers being derived from tappings on the delay line; a combination of the above two methods may also be employed. In both of the above arrangements the phase shift elements are connected in series but alternatively the aerial elements may be connected in parallel to the combining amplifier through separate phase-shifters which are varied electrically, e.g. ferrite or ferro-electric dielectric elements. The received signals may be displayed on a sector, P.P.I. or type B C.R.T. display, separate displays being provided for the two-dimensional scanning arrangements. The transmitting aerial may produce a beam of width # equal to that of the sector scanned by the receiving aerial array or it may produce a narrower beam of width #/s, e.g. s=6, which scans the sector # during a period st, the duration of the transmitted pulse being also st. The transmitting and receiving aerials may be on a common mounting which is rotated in azimuth. Receiver scanning in one dimension.-As shown in Fig. 1, the receiving aerial array comprises n elements 12 1 , 12 2  ... 12 n  coupled to respective tappings on a delay line 14 through respective channels 50 1 , 50 2  ... 50 n , each containing a frequency changer 22 (frequency changer 22 3  only shown) coupled to a common local oscillator 19 whose frequency is varied by a sawtooth or triangular waveform, Figs. 12 and 13 (not shown), of recurrence period t from a bearing timebase circuit 26. A magnetron 10 coupled to a transmitting aerial array 11 is triggered by a sawtooth waveform 30, Fig. 11, from a range timebase circuit 29, the timebase circuits 26 and 29 being synchronized by a master oscillator 31. The bearing timebase waveform is modulated at 27 with the range timebase waveform 30, Fig. 11, giving a resultant waveform 32, Fig. 10, and the waveforms 30 and 32 are applied to orthogonal deflector coils, Fig. 9 (not shown), of a C.R.T. 18 to give a zig-zag sectoral raster which is intensity-modulated by the rectified output from one end of the delay line 14. A full 360 degrees P.P.I. display is obtained by rotating the crossed deflector coils in synchronism with the aerial assembly; alternatively the range and bearing waveforms 30 and 32 may be applied to orthogonal rotor coils of a goniometer, Fig. 28 (not shown), whose orthogonal stator coils are coupled through amplifiers to fixed electrostatic deflection plates of the C.R.T. A type B display may be obtained by applying the range and bearing waveforms 30, 32 directly to orthogonal deflector coils. The aerial array 12 may comprise an array of waveguide horns, Figs. 2 and 5 (not shown), located on the focal line of a cylindrical parabolic mirror, Fig. 5 (not shown), and the delay line 14 may comprise a waveguide, transmission line or lumped circuit delay line, Fig. 8 (not shown), depending on the selected value of the I.F. The local oscillator 19 may be a klystron, Fig. 2 (not shown), or a backward wave oscillator, and each frequency changer 22 may be a crystal, the output circuit being adapted to select the difference frequency. In a modification, Figs. 3 and 4 (not shown), an output is taken from each end of the delay line 14, the two outputs corresponding to two aerial beams oscillating simultaneously in antiphase. In Fig. 3 (not shown), the two rectified outputs intensity-modulate the two electron beams of a double beam C.R.T. producing a type B display, the two electron beams being deflected in opposite directions by push-pull outputs from the bearing timebase circuit; this allows the scan period to be doubled. In Fig. 4 (not shown), the rectified output from the righthand end of the delay line is delayed by half a complete scanning period and combined with the rectified output from the left-hand end of the delay line to give a signal of improved signal/ noise corresponding to a single scanning beam. In the above arrangements co-phasal inputs to the frequency changers are derived from the local oscillator by an arrangement of branched waveguides, Fig. 6 (not shown), each local oscillator output being coupled to a corresponding waveguide signal channel 50 by means of a directional coupler. In a modification, Fig. 27, the local oscillator 19 feeds one end of a delay waveguide 53a which has slots or outlets feeding the signal channel waveguides 50 1  ... 50 n  through cross-type directional couplers. The difference signals in the channels 50 1  ... 50 n  as produced by crystal mixers 61 1  ... 61 n  are then combined directly. The delay waveguide 53a may be loaded with ferrite to increase the phase shift per unit length. Distortion of the output signal due to differential phase shifts and pulse envelope time delays in the different signal channels. Figs. 17 and 18 (not shown), may be reduced by including a pre-set compensating delay unit, e.g. 35 3 , Fig. 1, such as a length of waveguide or transmission line or a dielectric vane phase shifter 35, Fig. 27, in the signal channels 50. Signal distortion when the beam is at the limits of its scan is produced by differential delays, Fig. 14 (not shown), caused by the incident wave front being oblique to the array and such delays can be compensated by using a delay line with a parabolic phase-shift/frequency characteristic, Fig. 15 (not shown); in this case the local oscillator 19 must have a parabolic frequency characteristic. Such distortion may also be minimized by reducing the delay t 2  per section of the delay line to a value such that (n - 1) t 2  is not greater than t/n. This gives a frequency sweep #f not less than n(n-1)/t and provides an undistorted   scan of width (n - 1) times the beam width. Receiver scanning in two dimensions.-Two orthogonal fan beams scanning in orthogonal directions are produced by two orthogonal arrays, Fig. 24 (not shown), each array being coupled to a respective delay line 14, as described above, and the output from each delay line being applied to a respective range-angle C.R.T.; a common transmitting aerial may be employed. A pencil beam rectangular raster scan is produced by a plurality of verticallyspaced parallel arrays 12a 1 -12a n  ... 12m 1 - 12m n , Fig. 25, coupled to respective delay lines 14a ... 14m, as in Fig. 1, the magnitude of the frequency sweep of the common local oscillator being sufficiently great so that when a main beam due to a main diffraction peak reaches one limit of the horizontal scan sector a further beam due to the next main diffraction peak appears at the other limit of the scan sector so that the sector is scanned horizontally by a succession of beams during each frequency sweep of the common local oscillator. The outputs from the delay lines 14a ... 14m are applied to equi-spaced tappings on a further delay line 72 which is so designed that the frequency shift occurring between successive beams causes a vertical deflection of one beam width. The output from the delay line 72 may be applied to separate range-bearing and range-elevation displays. Transmitting aerial.-The transmitting array 11, Fig. 1, may produce a beam, Fig. 21 (not shown), of width # equal to that of the sector scanned by the receiving array 12. Alternatively it may produce a narrower beam of width #/s, e.g. s=6, which scans the sector during a period st, the duration of the transmitted pulse being also st; an advantage of such an arrangement is that any point in the sector is illuminated for a period less than the duration of the transmitted pulse. The scanning may be effected in s discrete steps, Fig. 22 (not shown), or continuously, Fig. 25 (not shown), and may be produced by an aerial array incorporating electrically-controlled ferrite or ferro-electric dielectric phase shifters of known type. Alternatively it may be produced by varying the radio frequency of each transmitted pulse during its duration and applying the pulses to a plurality of series-connected elements each producing a phase shift dependent on the applied frequency, the elements of the array being co