Short-wave radio telephone and telegraph receivers

The receiver described in this paper is intended to cover a wave-length range of from 10 to 50 m. and to deliver speech or telegraph impulses to a commercial telephone or telegraph system. On account of the low level of atmospheric disturbance on these short waves it is possible to work with signal strengths of the order of one microvolt per meter, accordingly the receiver must be made very sensitive albeit of high reliability, and special care is needed to avoid valve noise and trouble from microphonic condensers and valves. The output required is +5 decibels to a commercial telephone line, reference volume being taken as 6 milliwatts. The receiver, which is a superheterodyne, consists of the following main parts: aerial-tuning panel, signal-frequency amplifier, first detector and beating oscillator, intermediate-frequency amplifier, low-frequency amplifier. In addition, there are certain auxiliary pieces of apparatus, such as manual gain control, automatic gain control, signal limiter, testing oscillators at signal frequency, intermediate frequency, and 1000-~ frequency. The whole equipment is mounted on plates carried on the standard telephone apparatus racks, and complete equipment is provided for checking all supply currents and voltages; alarm fuses and relays are included in all important circuits. The intermediate frequency amplifier operates at a frequency of 500 kc. and has an amplification of 100 decibels. The audio-frequency amplifier consists of a standard repeater panel and allows of a gain of 40 decibels. The filament supply requires about 10 A at 24 V, while the anode supply is at 130 V, the total current drain being 0.15 A. Certain of the units require an independent supply, which is obtained from insulated dry batteries.