Patent GB-894345-A (GB)
Title: Improvements in or relating to radar apparatus
Inventor: CAWSEY DAVID
IPC: G01S13/36
Filed: 1959-11-02
Abstract: 894,345. Radar. DECCA RECORD CO. Ltd. Feb. 2, 1961 [Nov. 2, 1959], No. 37177/59. Class 40(7). In a C.W. Doppler radar system in which two different frequencies are transmitted one of the frequencies is derived, via a 3db directional coupler 11, directly from an oscillator 10 and the other is derived from the oscillator via the coupler 11 and a modulator 17 comprising two quarter wave plates in a waveguide with an annular ferrite member between them and provided with electromagnetic means 18, 19 operating in the range 2-25 kc./s. for producing a rotating transverse magnetic field across the ferrite member. The output from 17 then has a frequency which differs from the input frequency by twice the rotation frequency. The two outputs from coupler 11 are linearly polarised and the first quarter wave plate in modulator 17 transforms the input thereto into a circularly polarised signal which is subsequently reconverted to a linearly polarised signal by the second quarter wave plate. The two signals are then fed via rectangular waveguides (having sections as shown at A and C) to duplexers comprising ferrite circulators 12, 21, the output of the former being supplied via a phase shifter 13 and a 90 degrees twist waveguide section 22 to a Y junction 14 whilst the output from 21 is supplied directly to junction 14. The output from the latter is then fed via waveguide 15 (having a section as shown at D) to a directional aerial 16. Signals reflected by a (moving) target and received by aerial 16 are separated by virtue of their different planes of polarisation and pass to the ferrite circulators 12, 21 from which they are fed (together with a small amount of the transmitted signals due to the slight leakage of the circulators) to crystal detectors 23 and 24 respectively. The latter detect the Doppler beat frequencies which, after amplification in amplifiers 25 and 26 are fed to a phase comparator 27 comprising either a direct reading range indicating meter type or comprising a range calibrated phase shifter in one channel and a null signal audio detecting means. One of the Doppler beat frequencies is also fed directly to a frequency meter 28 for indicating target speed and may also be fed to means for providing an audible indication of the presence of targets. By noting the sign of the phase difference between the two beat frequencies it can be determined whether a target is approaching or receding. Where it is desired to mix each received signal with a signal of the other transmitted frequency it is only necessary to put a 45 degrees non-reciprocal ferrite polarisation rotator in the waveguide 15, the signals now of the modulation frequency being amplified if necessary before application to the detectors. In the case of complex targets errors caused by differential phase shift due to the use of two modes of polarisation may be overcome by transmitting both signals with the same polarisation. This may be achieved by putting an attenuating vane in waveguide 15 at 45 degrees to the plane of the walls so that the radiated signal comprises components of the two signals polarised in a plane normal to the plane of the vane. Alternatively, the same result may be achieved by replacing the Y junction 14 by a 3db directional coupler.