Patent GB-550633-A (GB)
Title: Improvements relating to radio course-indicators
IPC: G01S1/02
Filed: 1942-02-27
Abstract: 550,633. Directive wireless signalling. STANDARD TELEPHONES & CABLES, Ltd. Feb. 27, 1942, No. 2590. Convention date, March 1, 1941. [Class 40 (v)] To avoid shifting of the indicated course due to partial failure of one of the modulators in a radio beacon employing electronically modulated overlapping beams, the beacon comprises means for separately modulating two portions of the .carrier wave energy in the same phase by the first modulating signal, and in opposite phase by the second modulating signal, means for combining the two modulation products in a first path so that the side bands due to the second modulating signal are substantially eliminated, and in a second path so that the carrier wave and the side-bands corresponding to the first modulating signal are substantially eliminated, and means for radiating in overlapping fields the combined products in the first and second paths. In the form shown in Fig. 1, carrier wave energy at a frequency f1 from a source 10 is supplied to two modulators 1À1, 12, to which are coupled a signal source 13 of frequency F1 which modulates the radio-frequency energy in opposite phase in the two modulators, and a signal source 14 of frequency F2 which modulates the energy in the same phase. The outputs from the two modulators are supplied in conjugate relation to a balanced bridge network formed by four equal transmission lines 21, 22, 23, 24 in one of which is a phase reversing means which is shown as a cross-over 29. The other diagonal points of the bridge are coupled by transmission lines 25, 26 to crossed loop or other antennae 27, 28. The carrier frequency energy f1 and the modulating energy F2 will arrive by the equal branches 21, 22 in phase at the line 25, whilst the modulating energy F1 in the two branches is in opposite phase, and is therefore neutralized. Consequently, the carrier frequency f1 with the side bands f1 Œ F2 will pass to the line 25. Owing to the crossover 29 the carrier f1 and the modulating frequency F2 arrive in phase opposition at the line 26, and only the side bands from the modulating source F1 will pass to this line. In order that antenna 28 may radiate the carrier frequency in addition to these side bands, the carrier frequency is supplied to the line 26 either directly, or, if feed-back to the outputs of the modulators is to be avoided, by way of a balanced bridge circuit. Should one of the modulators transmit less than the proper amount of energy there will be incomplete neutralization, and each antenna will radiate some of the side bands corresponding to the other, whilst one will radiate an increased and the other a decreased carrier wave. Owing to the balancing network 21 ... 24 these, changes will be equal and opposite, so that the total energy of the side bands f1 Œ F1 and f1 Œ F2 will remain the same at the course lie, and no shift of the indicated course will occur. An alternative to the network 21 ... 24 is illustrated comprising two transformers connected in "hybrid" fashion. Fig. 3 shows an alternative antenna system for a two-course beacon in which by means of a bridge network 50 fed from the lines 25, 26 a central antenna 53 radiates the carrier and the side bands due to both modulations, whilst the antennae 51, 52 radiate the side bands only.