Patent GB-606505-A (GB)
Title: Improvements in or relating to radio direction finding apparatus
IPC: G01S1/02
Filed: 1945-05-04
Abstract: 606,505. Radio navigation. PATELHOLD PATENTVERWERTUNGS- & ELEKTROHOLDING AKT.-GES. May 4, 1945, No. 11408. Convention date, May 6, 1944. [Class 40 (vii)] In a radio beacon, a number of equi-angularly spaced radial beams produced by dipoles in reflectors are rotated or oscillated at the same angular velocity and some property of the beams, e.g. modulation frequency, pulserecurrence frequency or plane of polarization is varied in accordance with angular position such that the selected property of each beam has the same value for the same angular position. In the embodiment described, ten beams are produced and the modulation frequency of each is varied linearly in synchronism with the rotation between 60 and 115 kc/s., the last 5 kc/s. of a frequency sweep coinciding with the first 5 kc/s. of the next sweep. In order to produce the necessary phase displacement between the frequency variations for each beam, the output from a generator G, Fig. 2, varying in frequency between 120 and 170 kc/s. is heterodyned at the separate beam transmitters I, II ... by two local oscillators G 1<1> and G1<11>, G2<1> and G2<11> ... operating at frequencies 10 and 60 kc/s., 15 and 65 kc/s.... and the beat frequency outputs are passed through low- and high-pass filters F1<1> and F1<11>, F2<1> and F2<11> ... having cut-off frequencies 115 and 60 kc/s., the combined output from each pair of filters then having the necessary variation. The beams may be amplitude or frequency modulated and an amplitude or frequency-modulated speech signal may also be transmitted. To avoid rotating couplings, all the transmitting equipment is mounted with the aerials and the carrier for each beam modulator is derived from a single generator or from separate generators whose frequencies are automatically controlled by a single oscillator; only a single side-band need be transmitted directionally if an unmodulated carrier is radiated omni-directionally. In order to determine the modulation frequency transmitted on a particular bearing, a receiver located on that bearing heterodynes the demodulated signal with a variable-frequency local oscillator which is adjusted to give zero beat frequency. Since each beam has a finite width, a small range of frequencies on either side of the particular bearing frequency will be received and the beat frequency f-fa will decrease to zero and then increase again as shown in Fig. 4b, the local oscillator being adjusted so that the ranges of decreasing and increasing frequencies are symmetrical with respect to the zero beat frequency; this adjustment is facilitated by converting the beat frequency output into a D.C. voltage, the amplitude of which is proportional to frequency and applying it to the Y-plates of a cathode-ray tube. In a modification, Fig. 6 (not shown), the local oscillator is adjusted to produce a predetermined beat frequency which is applied to a discriminator giving a zero output at that frequency; this adjustment may be performed automatically by applying the discriminator output to control the local oscillator. To ensure secrecy, the correspondance between modulation frequency and bearing may be varied according to a pre-arranged plan, a warning pulse being emitted before making such a change.