Patent GB-776259-A (GB)
Title: Improvements in or relating to radio navigation aiding devices
Inventor: WRIGHT PETER MAURICE
IPC: G01S13/75
Filed: 1954-09-14
Abstract: 776,259. Radar; phase modulation. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. June 30, 1955 [Sept. 14. 1954], No. 26556/54. Classes 40(5) and 40(7) A radar responder device comprises means for receiving frequency energy and returning it in the direction of its source, a portion, which is less than half the total, of the returned energy being phase modulated in a way characteristic of the particular device and the remainder unmodulated. Where the modulating part of the device is mounted on a buoy or other structure having a small reflecting area special means such as one or more corner reflectors may be necessary to return an adequate amount of unmodulated energy but in the case of mountings have a large surface area, such as a lightship adequate unmodulated reflection may be obtained from the mounting itself. Preferably the phase modulating means consists of a length of ferrite wave propagation material F on which the incoming wave is directed by a lens L. The distant end of the ferrite rod is provided with a shorting cap S and a magnetizing coil C is .wound round it and supplied with a bias voltage on which is superposed a characteristic audio frequency alternating current from motor generator set M, A. The incident wave I is reflected back phase modulated by the audio frequency. A number of modulating units of this kind can be grouped together, each pointing in a different direction or one or more may be rotated in the same way as the optical system of a lighthouse. If the lens is a wide angle one a number of ferrite rods may be grouped in focal region. Instead of using a lens with one or more rods a mirror can be used (Fig. 3 not shown). The mirror may be in the form of a "cheese" K (Fig. 4) directing incident energy into a horn H coupled to a guide leading to a modulating ferrite rod F of the type described. One, two, or four of such "cheese" units can be assembled and rotated by a motor MR. In another form of modulator (Fig. 6, not shown) incoming energy is directed into a shorted waveguide and a shaped metal vane rotated so as to project a varying extent through a slot in the guide. More complex modulating signals such as morse or speech may be applied. When'the responder is interrogated by a C.W. radar the phase modulation may be directly detected. If, however, it is interrogated by a pulse radar it is necessary to extract the modulation from the receiver vidio-signal. This may be effected by delaying the transmitter modulating pulses an adjustable amount and using the delayed pulses to gate the returned signals from the responder from the receiver vidio channel to the input of a narrow frequency band amplifier adjusted to the characteristic frequency of the responder. The output of the amplifier may be fed to an aural or visual indicator. To facilitate adjustment of the delay line and to identify the indication of the responder on the radar plan position indicator the delayed pulse may be differentiated and the two peaks of voltage produced rendered both of the same sign and applied to brighten the trace of the display tube to cause marks to occur on either side of any indication 'whose signal is passed by the gate.