Abstract :
The possible methods of utilizing magnetrons to generate short-wave oscillations are indicated and the more important results of previous workers are described. The theoretical basis of ?electronic?and ?dynatron?oscillations is discussed, with particular reference to those features which can be investigated experimentally. It is shown that the wavelength of the electronic oscillations is determined mainly, if not entirely, by the electron time of transit. The general expression for the time of transit is given and hence expressions for the wavelength in terms of magnetic field strength are obtained for zero and saturated space-charge conditions. The wavelength is found to be about 36 per cent greater in the latter case. It is shown that initial electron velocity causes an appreciable reduction in wavelength in normal cases. The effect of magnetic field on space charge is considered and is found to lead to a small, possibly negligible, increase in wavelength. It is shown that the space charge is uniformly distributed when the magnetic field strength exceeds the critical value at which the electron orbits just touch the anode surface. This result has been previously stated by Hull, but Hull´s deduction that the electrons travel in circular orbits round the cathode is disputed. An attempt to provide a simplified theoretical explanation of the ?dynatron?characteristics of a ?split anode?magnetron leads to a false result from which it is concluded that no theory will provide an explanation which does not take into account the non-uniformity of the electric fields in the two halves of the valve. The general shape of the static characteristics is indicated by means of Habann´s theory, which is, however, not capable of giving proof of the existence of negative resistance in the case of a symmetrical oscillatory circuit, which is the case considered here. A qualitative explanation of the occurrence of negative resistance, i.e. of the greater fraction of the anode current r- eaching the lower-potential anode segment, is given. The object of the experimental investigation was to discover the nature of the fundamental relations in the electronic and dynatron types of oscillation, to compare these relations with the indications of the theory, and to apply the knowledge obtained to the production of a sufficient amount of power to be technically useful at the shortest possible wavelength. For electronic oscillations it is found that the experimental results are entirely in agreement with the theory in so far as it is applicable. In particular it is confirmed that the strength of the electronic oscillations is greatest at the ?critical? relation between anode voltage and magnetic field strength, and that the wavelength of the optimum oscillation is inversely proportional to the magnetic field strength. The actual value of the wavelength and the amount of the wavelength change due to space charge both agree with the theoretical values within experimental accuracy. It is concluded that the effect of magnetic field on space-charge distribution is not great enough to affect the wavelength appreciably. It is shown that apparently anomalous results can be explained by taking into account the stray capacitances, due in particular to the glasswork of the valve, across the oscillatory circuit. These lead to internal resonance effects which, although often a nuisance to the investigator, sometimes have the advantage of enabling a relatively large output to be obtained at a particular wavelength. The fact that the greatest output is, in general, obtained with the magnetic field not exactly in the direction of the electrode axis, which has been reported by Slutzkin and Steinberg and by Ranzi, was observed independently. The existence of an optimum field angle differing from zero is found to be due to the resultant spiral motion of the electrons balancing out the effect of cathode potential-drop for electrons arriving at part of the anode surface in su
