Patent GB-720154-A (GB)
Title: Improvements in or relating to radio beacons
Inventor: RAMSAY JOHN FORREST
IPC: H01Q15/23
Filed: 1952-06-25
Abstract: 720,154. Radio navigation; keying carrier frequencies. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. April 28, 1953 [June 25, 1952], No. 16025/52. Class 40 (5) and 40 (7). A passive type of radio beacon which may be utilized in conjunction with moving craft radar equipment to give an indication of the craft's bearing from the beacon comprises a wide-angle aperture element in the focal region of which is situated at least one radio-wave modulatorreflector arrangement for modulating and reflecting back through said element energy reaching it therethrough, the frequency of the modulation imposed being dependent on the angle between the incoming signal direction and the axis of the element. Two different kinds of beacon are described, the first type intended for use with C.W. radar sets capable of detecting, and preferably measuring, Doppler frequency shifts in the reflected waves and the second for use with radar sets able to detect amplitude modulation in the reflected waves. A beacon of the first type may comprise a wideangle lens 1 having an axis 2 with a plane reflector 3 mounted for rotation about an axis 4 at right angles to the axis 2. For signals received along the axis 2 the signals reflected back will be of the same frequency as those received, but will have an amplitude modulation due to the rotation. For signals received other than along the axis, e.g. from a direction 11, the beam will strike the rotating reflector away from the axis 4 and will suffer a Doppler shift due to the rotation of an amount dependent on the distance from the axis 4 at which it strikes the reflector. The reflected signal will thus have a Doppler shift dependent on the angle between the direction 11 and the axis 2. Determination of the Doppler shift at the radar enables an estimate of the angle to be made, and by following a course to reduce the shift to zero and maintain it at zero the axis path 2 may be reached and followed. The reflector 3 need not extend on both sides of the axis 4 but may comprise a half-size element rotated about its edge. The axis 4 need not intersect the axis 2 but may lie out to one side of the reflector, Fig. 4 (not shown). In this case the axis 2 corresponds to a given value of Doppler shift which increases to one side and decreases to the other side. In a different form of beacon for this type of system the reflector 3<SP>1</SP> is axially vibrated as shown by the arrow X. Since in this arrangement the component of vibration decreases with increase of the angle between the arrival of the radar signals and the axis, the Doppler shift is a maximum on course. Instead of one large reflector 3<SP>1</SP> a plurality of small reflecting diaphragms arranged over the focal surface and each having a different frequency of vibration may be used. When the angle at which the radar signals are received is large, loss of effective echoing area occurs and may be compensated for at the radar receiver by giving the latter a rising characteristic for the shift frequencies involved. An aural, cathode-ray tube or frequency-meter indication may be obtained. Gases or solids subjected to and responsive to applied modulating fields may be used to obtain the bearing dependent shift instead of mechanical methods. An arrangement for amplitude modulating the reflected signals, Figs. 7 and 8, comprises a wide-angle lens 2 and a focal surface mirror 3 having a common axis 2. The mirror is rotated about the axis 2 and has series of rings (or spirals) of small holes 3h. The number of holes passing a small area off the axis 2 is arranged to increase with distance from the axis so that the reflected wave will be amplitude modulated at a frequency dependent on direction. Instead of holes raised areas on the mirror may be provided to give an amplitude modulation due to the defocusing of the beam. In a different arrangement, Fig. 9 (not shown), a large number of small mirrors covering the focal surface are used, each rotated at a different speed about an axis at right angles to the axis 2 to amplitude modulate any energy reflected by them at a characteristic frequency. In another arrangement, Fig. 10 (not shown), the focal region of the-lens is covered with an array of dielectric rods each coupled to a waveguide in which a suitable amplitude modulator operates to produce amplitude modulation at its own characteristic frequency. When it is desired to signal from the beacon to the radar a keyed shutter may be placed between the lens and any' of the types of modulator-reflector described, Fig. 11 (not shown). If the reflector is a spherical mirror MR it may be keyed by swinging it from its normal position shown in full line to the position shown in broken lines for morse signalling.