Patent GB-579170-A (GB)
Title: Improvements in or relating to radio transmitters
IPC: G01S1/30
Filed: 1943-04-12
Abstract: 579,170. Wireless transmitters. O'BRIEN, W. J. April 12, 1943, No. 5873. Convention date, April 13, 1942. [Class 40 (v)] [Also in Groups XXXV and XXXVI (Appx.)] A transmitter is provided with means which, in response to changes in relationship between the antenna current and the exciting voltage brought about by changes in the antenna characteristics, adjusts circuit components which restore the predetermined phase or amplitude relation, or both, between the exciting voltage and antenna current. The transmitter may be used in radio beacon systems to prevent wandering of the indicated course line due to changes in antenna characteristics. In the circuit shown in Fig. 1, a rectifier 28 supplied from a current transformer 6 having its primary in series with the antenna feed line 3, and a rectifier 37 supplied from a potential transformer 40 having its primary in parallel with the feed line, produce opposed voltages across resistances 31, 45 which are respectively proportional to the antenna current and the exciting voltage and are normally balanced. Departure from this condition, due to change in antenna resistance, produces a resultant voltage which causes a polarized relay 22 to bring about operation of a reversible motor 12 to adjust the contact of a variable resistance 9 in the feed line until the normal voltage-current relation is restored. Changes in antenna inductance are compensated by similar means, including a variable inductance 10 in the feed line operated by a reversible motor 48 controlled by a polarized relay 50 which is energized in one or other direction when the opposed and normally equal voltages produced by rectifiers 60, 62 across resistances 63, 64 respectively become unbalanced. The rectifiers are fed from circuits each containing the secondary of the current transformer 8 and a corresponding half of the split secondary of the potential transformer 54. This results in the ratio of the voltages produced across the resistances 63, 64 being independent of the values of the current and voltage, but dependent on their phase relation, which latter will, therefore, be maintained constant by adjustment of the variable inductance 10 by the motor 48. In the circuit shown in Fig. 2, radio frequency energy is supplied from a line 107 to an antenna 100 through a variable gain valve 111 and amplifier 103, coupled by a transformer 113. The gain of the valve 111 is controlled by a grid voltage developed across a load 124 in the circuit of a rectifier 117 fed from a transformer 102 in the antenna circuit. Variations which might otherwise occur in the antenna current therefore produce corresponding variations in the gain of the valve 111, so that the current is maintained at its intended value. To maintain a fixed phase relation between the antenna current and exciting voltage, the phase relation between that current and the voltage applied to the valve 111 is kept constant. For this purpose a pair of rectifiers 144, 145 operate in circuits similar to those of the rectifiers 60, 62 in Fig. 1, and each comprising half of the split secondary winding of a transformer 140 in which, by way of an amplifier 139, is generated a voltage proportional to that applied to the grid of the valve 111. Each circuit also includes the output of a phase shifter 154 connected to a current transformer 106 in the output circuit of the amplifier 103. There is thus introduced into each rectifier circuit a voltage proportional to the antenna current, which voltage is brought into normal phase quadrature with that in the secondary of the transformer 140 by adjustment of the phase shifter 156. Change of this phase relation, due to change in the antenna constants, as in the circuit of Fig. 1, unbalances the potentials produced across the output resistances 147, 149. This determines the grid voltage controlling the impedance of a variable impedance valve 164 which acts as an inductive reactance in parallel with the tuned primary of the transformer 113. The circuit constants are so chosen that the effect of the impedance changes of the valve 164 on the tuning of the transformer primary are such as to compensate the phase shift due to the change of the antenna constants.