Patent GB-491222-A (GB)
Title: Improvements in variable inductances and transformers for radio transmitters and receivers
IPC: H01F21/04
Filed: 1937-02-26
Abstract: 491,222. Inductances; transformers ; wireless receiving circuits. MEYER, L. W. (Mallory & Co., Inc., P. R.) Feb. 26, 1937, No. 5882. [Class 38 (ii)] [Also in Group XL] A variable inductance or transformer comprises a coil of bare conductor 4a mounted on a rotatable insulating former 5 and coacting with a contactor unit mounted on a guide bar 7 adjacent the coil. The contactor moves along the bar 7 and comprises a carriage 6 in contact with the bar 7, a tiuck 21 carrying a grooved trolley wheel 18 which tracks on the conductor 4a, springs 24, between the truck 21 and the carriage 6, pressing the trolley wheel on to the conductor and means for ensuring conductive connection between the conductor 4a and the guide bar 7. The trolley weel 18 may be of conductive material such as silver alloy and the conductor 4a of copper heavily plated with silver or rhodium. The wheel 18 is bonded to the truck 21 by leaf springs 28 which make contact with its spindle and to the carriage 6 by a conductor 29, and the carriage is bonded to the guide bar by a leaf spring 30. Alternatively the wheel 18 may be of insulating-material and contact with the conductor is obtained by a silver leaf spring carried by the truck 21 and sliding over the conductor. The pitch of the coil may be non-uniform and the former may be cylindrical or conical. The guide bar 7 is grooved and has its centre line parallel to the coil side. The carriage 6 consists of a plate 19 carrying grooved strips 19a which face the grooves in the guide bar 7 and which carry balls 20 located between pins 27. A pair of studs 23 carry the truck 21 which is pressed away from the carriage 19 by the springs 24. In a ganged two unit variable inductance as shown in Fig. 1 the hollow coil formers 5 are secured at one end to a shaft 2 carried in bearings in a frame 14 and upright 15 which shaft is rotated by a wheel and knob 3. The guide bars 7 are also secured to the frame 14 and upright 15. The formers are turned down at 9 to form slip rings by which connection is made to the nearest ends of the windings. One of the inductances 4 has a fixed primary winding 16 mounted within it on a former secured to the upright 15. One or more switches may be mounted adjacent the guide bar 7 so as to be operated by a cam on the carriage 6 whereby particular wave bands may be switched out at predetermined points in the tuning range. The free end of the inductance may be earthed through a small inductance to prevent end turn short-circuiting. In Fig. 5 a ganged variable inductance and transformer, as in Fig. 1, is shown connected in a super-heterodyne receiver which consists of a pentagrid oscillator and mixer valve 35, an intermediate frequency amplifier stage 52 and a double diode triode detector 53a. The receiver has two wave ranges selected by the operation of a five pole double throw switch having positions I and II shown. In the lower frequency range position II the aerial 158 is connected through a wave trap tuned to the intermediate frequency of the receiver, and part of an inductance 32 to the coil 4 of the unit shown in Fig. 1. This coil forms a variable tuned circuit with a condenser 57 and is coupled to the input grid 36 of the valve 35 through a capacity 45. Fixed inductances 82 and 32 permit the use of the whole tuning range of the inductance 4. Automatic volume control is obtained from the detector 53 over a resistance 46. The oscillator anode grid 42 of the valve 35 is connected through coupling inductance 76, coil 16 which is coupled to the variable coil 4a, coil 60, and battery 75 to earth. In position II of the switch the coil 4a is connected across an inductance 34 and condenser 58 which latter is shunted by an inductance 5'9' coupled to the anode coil 60. The tuned circuit formed by the coils 4a, 34 and 59 and condenser 58 is coupled to the anode grid circuit at 16 and 60 and connected to the oscillator grid 41 through a condenser 47. The oscillator frequency is higher than the input circuit frequency by the fixed value of the intermediate frequency and this effect is obtained by the value of the condenser 58 and its padding inductance 59. In position I of the switch the aerial is connected to a coil 66 which is coupled to a coil 65 shunted by a condenser 64 and connected in series with the whole of coil 32 across the coil 4 whereby a higher frequency range is obtained. The oscillator coil 4a is now connected across a coil 33 in series with a condenser 63 and the coil 33 is connected to the oscillator grid. The oscillator anode grid 42 is coupled to the grid circuit by coils 76 and 33 and coils 16 and 4a. The coil 60 is short-circuited by the effective connection of a large condenser 67 across it. The oscillator frequency is now made lower than that of the aerial circuit by the amount of the intermediate frequency. The coil 32 is tapped as described to compensate for differences in the lead lengths of the various circuits. The inductances of the leads are considered in calculating the values of the lumped inductances and capacities. Values of the various inductances and capacities in the circuit and the dimensions of the coils 4 and 4a are given in the Specification.