Production and utilisation of micro-rays

The waves concerned are those of under 20 cm. wave-length; their behaviour places them in the region half-way between optics and radio-electricity. They are generated by a triode having an axial filament of pure tungsten, which is run at saturation emission current; surrounding this is the cylindrical "reflecting" electrode which is negatively biased and constitutes a virtual kathode. Between is a positively charged helical electrode (the "oscillating electrode") which experiences excitation at the h.f. potential, which is led out from its ends to a radiating doublet. In a practical communication system the doublet is placed at the focus of a paraboloidal reflector. In some cases it is convenient to replace the doublet by a half-wave aerial; the higher resistance this offers may be stepped down by a quarter-wavelength transmission line, the impedance of which is properly chosen. After describing the behaviour of the electrons within the tube, the author works out relations govening their motions and the currents collected by the electrodes, for the cases of plane and cylindrical electrodes. It is shown that in the presence of a h.f. potential the current collected by any point of the oscillating electrode varies with time, in such a way that the h.f. component of the whole leakage current to the electrode is out-of-phase with the h.f. potential. It is this dephasing which permits the oscillations to be sustained. For a given external circuit there is an optimum frequency, at which the output is a maximum. Further, the potentials of the two electrodes may be varied simultaneously and the frequency still be maintained at this value, the necessary relation being given by the "constant frequency curve." During such variation, the power output changes, its variation being linear over a considerable range, and by this phenomenon about 40% of linear amplitude-modulation is obtainable.