Patent GB-866972-A (GB)
Title: A new or improved apparatus for transmitting and receiving wave energy from a space sector with receiver beam scanning
Inventor: TUCKER DAVID GORDON
IPC: G01S15/42
Filed: 1956-05-16
Abstract: 866,972. Acoustic directional apparatus. NATIONAL RESEARCH DEVELOPMENT CORPORATION. May 15, 1957 [May 16, 1956], No. 15215/56. Class 40(7) A directional acoustic pulse-echo locating system comprises a directional or non-directional transmitting transducer and a separate receiving linear-array transducer having a vertical fan beam directivity pattern which is oscillated rapidly in azimuth about a reference axis 15 with an oscillation period not greater than the duration # of each transmitted pulse by coupling the elements of the receiving array to a combining amplifier through respective phase shift elements such that the phase shifts produced are varied electrically at the azimuthal scan frequency; the bearing and range of a reflecting object are indicated by the X and Y co-ordinates of an image spot on a type B cathode-ray tube display whose X and Y timebases are synchronized respectively with the pulse recurrence frequency and the azimuthal scan. As described the phaseshift elements comprise an electrical delay line with a linear phase-shift/frequency characteristic, the transducer elements being coupled to equispaced tappings on the line through respective frequency changers coupled to a common local oscillator whose frequency is varied at the azimuthal scan frequency such that the relative phases of the signals from the transducer elements when combined at one end of the delay line vary cyclically; alternatively the phase-shift elements may comprise thermionic valve reactance elements or inductances with magnetic cores whose permeability is varied by a D.C. polarising current. Also as described the azimuthal scan period is equal to the pulse duration # but it may be a submutiple of said duration. The invention is applicable to terrestrial, subterranian and subsqueous locating systems (e.g. fish location) and to locating flaws in solid objects. In one embodiment, Fig. 1, utilizing a double beam C.R.T. an output is taken from each end of the delay line, the two outputs corresponding respectively to two fan beams B1, B2, Fig. lb, oscillating symmetrically with respect to the axis 15 between the directions 15, 15a and 15, 15b respectively and in a second embodiment, Fig. 8, utilizing a single beam C.R.T. a single output is taken from one end of the delay line corresponding to a single beam oscillating symmetrically about the axis 15. A shown in Fig. 1 a 1 c/s. pulse generator 10 is coupled to the transmitting transducer 11 and to a range timebase generator 9 whose output is applied to a single set of Y deflection plates 24 of the double-beam C.R.T., a sawtooth bearing timebase of recurrence period equal to the pulse duration being applied from a generator 27 to the two sets of X deflection plates 23a, 23b thereby causing the two cathode-ray beams to scan symmetrically outwards from the centre line 28 of the screen. The receiving transducer array comprises five contiguous elements 12a ... 12e each having a directivity pattern (a), Fig. 15, the diffraction pattern due to the spacing of the elements being shown by the curve (b). The signal of frequency q=50 kc./s. received by each element of the array, e.g. element 12a, is applied in phase quadrature to a respective pair of balanced ring modulators 18a, 19a, Fig. 2 (not shown), together with phase quadrature components of a common local oscillation of frequency p varying synchronously with the bearing timebase between the limits 3 q/2=75 kc./s. and (1+k)q(=150 kc./s. when k=2), Fig. 5 (not shown), and derived, Fig. 3 (not shown), by heterodyning a 3À525 mc/s. fixed oscillation with phase quadrature outputs from an oscillator whose frequency is varied cyclically between 3À6 and 3À675 mc/s. by the bearing timebase voltage. The outputs from the modulators 18a, 19a are combined to give a resultant signal whose frequency (p-q) varies cyclically between the limits q/2 and kq and whose phase is determined by the phase of the signal received by the transducer element 12a. The amplitudes of the local inputs to the modulators 18a, 19a are controlled in units 21a, 21b, Fig. 2 (not shown), by an inverted sawtooth signal from the range timebase 9 to maintain a constant amplitude ratio between the received and local input signals to the modulators. According to the Provisional Specification spurious outputs from the frequency changers due to third order modulation may be avoided by using a local oscillation having a square waveform as shown in Fig. 8 (Prov.). The resultant output from each pair of modulators is applied to a respective tapping on a lowpass delay line 13, Fig. 4 (not shown), having a linear positive phase-shift/frequency characteristic, Fig. 7 (not shown) and it is shown that when the total phase shift of the line at the frequency q is 2# then the output from the right-hand end of the delay line corresponds to the signal received by a beam Bl, Fig. 16, scanning between the directions 15 and 15a and the output from the left-hand end of the line corresponds to the signal received by a beam B2 scanning between the directions 15 and 15b, the angular width of the scan sector bounded by the directions 15a and 15b being 2 k# where is the horizontal beam width of each beam B1 and B2. The left- and right-hand outputs are applied through respective detectors and amplifiers 14a, 25a and 14b, 25b to intensity modulate the cathode-ray beams scanning the right and left halves of the C.R.T. screen. A target on the axis 15 will produce a signal at each end of the delay line 13 at the beginning of each bearing sweep and in order to prevent an excessive response under these conditions the gain of the amplifiers 14a and 14b is reduced between the frequencies q and q/2, Fig. 6 (not shown). In order to minimise the effect of the second order maximum lobes of the array diffraction pattern (b), Fig. 15, the scan angle 2 k# should not exceed n# where n is the number of elements in the array. The effect of side lobes is also reduced by making the transmitting transducer 11 directional and such a transducer may comprise a strip array of elements with an energization (1+2 cos x#) where x is the distance from the centre of the array and is unity at the ends of the array. In the second embodiment, Fig. 8, comprising a single beam C.R.T., the bearing timebase is applied to a single pair of X deflection plates. Each frequency changer, Fig. 10 (not shown), connected between an element of the receiving transducer array 12 and a corresponding tapping on a delay line 30 comprises a first transformerless modulator 31a supplied with a fixed frequency S=1À05 mc/s. from a common oscillator 32, the resultant difference frequency S-q=1 mc/s. being selected by a filter 33a and applied to a similar modulator 34a together with the output from a common oscillator 35, Fig. 10 (not shown), whose frequency p is varied cyclically between 3À165 and 3À675 mc/s. by an output from the bearing timebase generator 27; the resultant difference frequency p+q-S which varies, Fig. 12 (not shown), between 2À165 and 2À675 mc/s. is selected by a filter 36a. In order to avoid distortion of the signal the frequency sweep should be greater than n/#. The delay line 30, Fig. 11 (not shown), is of the band-pass type with a phase-shift varying linearly from negative through zero to positive over the pass-band such that the output from one end of the line corresponds to the signal received by a beam oscillating symmetrically about the axis 15 and this output is applied to intensity modulate the cathode-ray beam. The relative amplitudes of the inputs to the delay line 30 are adjusted by variable gain amplifiers, Fig. 9 (not shown), interposed between the filters 33 and second modulators 34 to compensate for the attenuation in the line. Suitable component values are given for the delay lines 13, 30, the units 21a, 21b, Fig. 1, and the units 31-36, Fig. 8. The type of frequency changer used in the second embodiment may be employed in the first embodiment and vice-versa.