Abstract :
The paper gives an account of the development and construction of the new transmitter now installed at the National Physical Laboratory for the emission of the special signals required in connection with modern research on the ionosphere. It also deals briefly with the methods adopted for the dissemination of standard-frequency signals for calibration purposes. The new installation has been designed to handle all the sending services required from the Laboratory over a continuous frequency range of 250 to 6000 kilocycles per sec.(wavelength 1200 to 50 metres). Owing to the rapid change in the emitted frequency required in many tests, a single oscillating circuit is employed and the oscillations are maintained by a push-pull system. A series modulator is provided for controlling the anode-circuit supply to the oscillator. The special emissions required for ionospheric research consist of frequency-change and pulse transmissions. In the former emission, the frequency is continuously varied over a small range by means of an entirely automatic mechanism which gives precision of both timing and frequency-change, while in the latter type of emission, pulses of waves may be emitted of short duration and at a repetition frequency synchronized with the a.c. supply mains. Two types of standard-frequency programmes are emitted. In one of these the output from the continuously-running 1000-cycle standard tuning-fork is used to modulate a suitable carrier wave; and in the other a standard radio frequency is emitted by using the transmitter as the final stage in an amplifier supplied by a quartz-crystal-controlled master oscillator. All the operations required in connection with these various emissions are effected by simple panel switchgear so as to render rapid changes in frequency and type of transmission possible, while a cathode-ray oscillograph equipment is incorporated for the observation of the modulation envelope. The whole installation was designed and constructed at th- e Laboratory for an input power of at least 4kW, although at present the input is limited by supply conditions to 2kW.
