Patent GB-266068-A (GB)
Title: Improvements in or relating to thermionic transmitters & receivers
IPC: H03B5/10
Filed: 1925-11-27
Abstract: 266,068. Schotel, G. Nov. 27, 1925. Thermionic generators and amplifiers, particula.rly for short-wave signalling, are characterized by the use of two or more circuits tightly coupled by direct or indirect magnetic coupling and inserted in the plate or grid circuit of a valve, the shorter of the two or more resulting coupling- waves being selected for use. As shown in Fig. 1 applied to a transmitting-valve 1, the coil 6 provides a tight coupling between the aerial-earth circuit 6, 8, 12, and the plate circuit comprising the effective capacity between the plate 4 and filament 2, a block condenser 5, tapping 7, and coil 6. Assuming that the tappings 7, 8 coincide so that the coil 6 becomes a transformer of ratio unity, the two coupled circuits provide two natural frequencies. The lower frequency is roughly determined by the comparatively large capacity of the aerial and the high inductance of the coil 6. The higher frequency is approximately determined by the effective capacity between the plate and filament, in conjunction with the inductance of the aerial. In the latter may be included an aerial coil 12 and any inductance due to magnetic leakage in the choke or autotransformer 6. The oscillation of higher frequency is caused to build up by the use of a grid coil 9, which, in conjunction with the effective capacity of the grid, tunes the grid circuit to a frequency slightly higher than the desired frequency. The coil 9 practically short circuits the low-frequency oscillation, and prevents it from building up. The back-coupling in the valve is preferably provided by the inherent capacity between plate and grid, but may be provided by an additional coupling. If the coil 6 provides a step-up of ratio u, then the effective inductance in the plate circuit of the valve becomes u<2> times the aerial inductances, and consequently the frequency is lowered in the ratio u to 1. Adjustment of the tappings 7, 8 thus varies the operating frequency. The efficiency of the system is also varied by adjustment of the taps, owing to the alteration of the impedance of the oscillating circuit which for good efficiency should be larger than the resistance of the platefilament path of the valve. A separate tuning- arrangement may be provided such as means for varying the inductance of the coil 12. The coil 6 may be omitted, the aerial being connected through a variable coil 12 and block condenser 5 to the plate 4 of the valve. In other modifications the aerial is replaced by a condenser, or a variable coil, or a short-circuit, connected between the tap 8 and earth. When applied to a receiving-circuit as shown in Fig. 6, the coil 14 has one tap connected to the aerial 18, and the other tap to the grid 3 of the. valve 1, the effective capacity between grid and filament replacing the plate-filament capacity of the transmitter. The system is tuned by adjusting the taps or otherwise to the wave to be received, or approximately thereto for beat reception. When further valves are employed, the inter-valve circuits are arranged with a tight coupling, the higher frequency of the coupled circuits corresponding to the received wave. Directive signalling.-A pair of aerials spaced half a wave-length apart may be connected to equidistant taps on a coil which has its midpoint earthed, and has other equi-distant taps connected to the plates of a pair of push-pull valves, thus forming a double system corresponding to Fig. 1 and its image with the earth as a reflector. Alternatively, an indirect magnetic coupling may be provided between the coil connected to the aerials and the coil connected to the pair of valves.