Patent GB-552497-A (GB)
Title: Improvements in directional radio receiver systems
IPC: G01S1/02
Filed: 1940-02-23
Abstract: 552,497. Directive wireless systems. MARCONI'S WIRELESS TELEGRAPH CO., Ltd., COCKERELL, C. S., BRAILSFORD, J. D., and CUFFLIN, M. H. Feb. 23, 1940, No. 3462. [Class. 40 (v)] In a directional receiving system of the equisignalswitched-cardioid type, two push-pull oscillation-generating valves for switching the input have their plates and grids connected through the primary and secondary respectively of a tuned transformer and their cathodes connected through a resistance to a centre tapping on the secondary of the transformer and these valves may have additional electrodes for receiving the signals from the omnidirectional aerial and loop aerial respectively. Two valves for switching the output of the receiver and applying it, through condenser shunted resistances, to the indicating instruments (which may be of the one-needle or two-needle type), may have their grids and cathodes similarly connected to an additional secondary on the same transformer. The system coupling the omnidirectional and directional aerial systems includes a balanced condenser potentiometer whereby the apparatus may be adapted for either homing or taking of bearings, by adjustment of the relative strengths of the omnidirectional and directional inputs to the receiver, and consequently the precision of the readings. The frequency of the switching generator may be altered to suit either morse or telephonic modulation. Switching-circuits.-The push-pull oscillationgenerator for the switching frequency comprises the triode portions of two triode-hexode valves V<1>, V<2>, Fig. 1, whose plates are connected through the tuned primary L of a transformer, while their grids are connected through the secondary S<1>, S<2>, and their cathodes are connected together and, through a high resistance R, to the middle of the secondary. The switching frequency is 30 c/sec. (for telephonic modulation) or 80 c/sec. (for morse) according as an additional condenser is connected or disconnected by switch K6. Although the voltage applied to the grids is sinusoidal the effect (during the positive halfcycle in each valve) of providing a high centretap resistance R between the secondary and the cathodes, and (during the negative half-cycle) of the flow of grid current, is that the conductances of the hexode portions of the valves, which have their third grids connected to the triode grids, have a rectangular wave-form. Signals from the omnidirectional aerial OA are applied to the hexode control grids of both valves V<1>, V2, the hexode plates of which feed the coupling transformer KC in one direction or the other, according as one valve or the other is active. Signals from the directional aerial DA are fed directly to the coupling transformer. Switching of the cardioid is therefore effected by valves V<1>, V<2>. Switching and rectification of the output, as applied for instance to a two-needle (intersecting needle) indicator M, Fig. 4, is effected by valves V<3>, V<4>, whose grids are connected to an additional winding on the transformer TS<2> so that the operation of valves V<3>, V<4> is synchronized with that of valves V<1>, V2. The design is such that the conductances of the valves have a rectangular wave form. The I.F. or A.F. voltage from the receiver is applied through condensers C<13>, C<14> and chokes CH<3>, CH<4> to coils m<1>, m<2> of the two-needle indicator and is rectified by valves V<3> V4 in turn, so that the coils become effective alternately and the needles intersect on the middle line if the signal strengths from the two cardioids are equal. In a single-needle instrument, Figs. 3, 6 (not shown) the output of the receiver is fed through a single coil in alternately opposite directions. The valves V<3>, V<4> may be replaced by dry-plate rectifiers and mechanical switches, Figs. 6, 7 (not shown). The gain of the valves V<1>, V<2> is accurately balanced by connecting their screen grids to opposite ends of a potentiometer and their cathodes to an adjustable tapping at the middle of the potentiometer, Fig. 2 (not shown. The operational switch is so arranged that the operator selects the " balance " position in which the loop aerial is replaced by a dummy aerial and the balance of the valves can be adjusted, before he comes to the D.F. position. Variation of precision.-In order that the apparatus may be adapted for homing or for taking of bearings, the precision of the readings is varied by varying the effect of the omnidirectional aerial in relation to that of the directional aerial. This is done without readjustment of the parameters of the valves by the use of a balanced capacity potentiometer in the plate circuits of the hexodes. This potentiometer consists of condensers C<9>, C<11>, Fig. 1, and C<10>, C<12> to the junctions of which the hexode plates are respectively connected; but when the effects of the omnidirectional aerial, which is applied through these plates, is to be reduced, the earthed condensers C<11>, C<12> are shunted by additional condensers C<7>, C<8>. Sensitivity of indicators.-This is increased by the provision of condensers C<15>, C<16>, Fig. 4, shunted by resistances R<15>, R<16>. Under equisignal conditions the condensers are equally charged, but under off-course conditions current flows from the condenser which is receiving 'the stronger signal, through the indicator coils and into the other condenser, whose 'consequently increased charge prevents it from receiving further current from valve V<3> or V<4>. Such resistancecondenser combinations are applicable to a singleneedle instrument, Fig. 6 (not shown); but in the case of a double-needle instrument, Fig. 4 and Fig. 7 (not shown) an additional resistancecondenser combination R<17>, C<17> is provided in series with both coils ; its effect is to ensure that the needles shall move 'when signals are being received and that the on-course condition shall thus be distinguished from the no-signal condition ; it further ensures that under the offcourse condition one needle shall rise as the other falls so 'that a better intersection is obtained. The' cathodes of the valves V<3>, V<4> are biased by a potentiometer R<6> so that the indicators do not respond to noise in the absence of a signal. When wide variations of signal-strength occur, the A.V.C. of the receiver does not suffice to keep the intersection of the needles on the scale of the indicator. This defect is counteracted by connecting the lower ends of the coils m<1>, m<2>, through a choke L<3> and condenser C<20> to the receiver output, a limiter diode LD being shunted upon this connection ; the oscillations derived from the receiver are rectified by the diode and applied across the variable resistance R<17>, R<6> so as to regulate the height of the intersection of the needles. Specification 555,968 is referred to.