Patent GB-590461-A (GB)
Title: Frequency modulated rotating radio beacon
IPC: G01S19/48
Filed: 1944-01-19
Abstract: 590,461. Directive radio systems. BENDIX AVIATION CORPORATION. Jan. 19, 1944, No. 1075. Convention date, July 16, 1942. [Class 40 (v)] A rotating beacon comprises a non-directional radiation pattern frequency modulated with one tone and a rotating directional radiation pattern of substantially the same carrier frequency, frequency modulated with a second tone ; the modulations being keyed to provide a reference signal at a predetermined point in the rotation.. The directional pattern 3, Fig. 1, which is modulated at 1200 c.p.s., is arranged to be of greater strength than the non-directional pattern 2, modulated with 500 c.p.s., except over a small arc corresponding with the null of the cardioid pattern. The directional pattern is rotated at a uniform speed of 1 r.p.m. and the modulation of both signals is keyed with the code character N as the null of the directional pattern is at true north. Due to the limiter, a frequency modulation receiver produces an output of 1200 c.p.s. tone only for the whole of the beacon rotation with the exception of the small arc 5 about the cardioid null when the non- directional signal is stronger and 500 c.p.s. modulation is heard. The bearing of the receiver relative to the beacon is thus determined by observing the time interval between the reception of the " North " signal and the change in the modulation note, allowance being made for the fact that the tone changes slightly before the passage of the cardioid null. Apparatus for producing the radiation patterns is shown in Fig. 2. Carrier from oscillator 23 is frequency modulated at 24 with 1200 cycle tone from source 26 and fed to amplifier 33, 34 which supplies power in opposite phase over lines 48, 49 and slip rings 14, 15 to aerials 6 and 8 which are spaced one half wavelength apart on the turntable 9 which is rotated by the motor 10. The output from modulator 24 is also supplied to amplifier 51 which supplies power over line 65 and slip ring 13 to the central aerial 7. The length of line 65 is so adjusted that the energy in aerial 7 is in phase quadrature with that in aerials 6 and 8, and the resultant radiation pattern modulated with 1200 cycle tone is of cardioid form. Carrier from 23 or from a separate oscillator of somewhat different frequency, is also modulated with 500 cycle tone from oscillator 27 in modulator 25 and fed to amplifier 68 which also supplies power over line 65 to the central aerial to produce the non- directional radiation pattern. Mounted on the shaft supporting the turntable 9 is a keying disc 16 provided with conducting sectors 17, 77 and the annular contact ring 18, Fig. 3 (not shown), which co-operate with brushes 19, 20 to control the keying relay 22, the contacts 30, 31 of which connect the tone sources 26, 27 to the frequency modulators. At a predetermined point in the rotation of the aerial system contact between brush 20 and sector 77 is interrupted and upon,contimuation of rotation the sectors 17 and the brush 20 co-operate to open and close contacts 30, 31 in a manner forming the code character N, the slot of the N just ending as the null of the radiation pattern or the edge of the tone change region lies at true north. As an alternative to the mechanically rotated aerial system, two pairs of fixed spaced aerials in space quadrature may be employed, the energy fed to the two pairs being amplitude modulated at the desired rotation frequency to produce the rotating radiation pattern.