This paper describes an experimental investigation of a simple method of achieving a limited degree of electronic tuning in medium power magnetrons, originally proposed by Willshaw(l. By the insertion of an extra cathode in one (or more) of the block resonators, ancillary crossed-field diodes are formed which can be used to modulate the magnetron frequency by pulsing the current taken by the diodes. Initial experiments with a 1 00kW X-band magnetron showed small frequency shifts of a few MHz per tuning cathode. Experiments still in progress with a moveable cathode in a passive, driven, resonator, suggest that 1 OMHz tuning in an S-band magnetron should be feasible corresponding to 30MHz at X-band.
Experimental Magnetron TuningThe basic geometry of the tuning mechanism is shown in figure 1, where the tuning cathode lies in the same magnetic field as the magnetron cathode. By applying a negative voltage to the tuning cathode, the space-charge current in the vicinity of the resonator gap may be varied, with a consequent effect upon the resonator impedance and upon the circuit resonant frequency. This device differs from the conventional system of Smith and Shulman (2) in having no axial electron velocity, so that all electrons crossing the space are collected on the anode. Magnetron cathode Strip tuning cathode r.f. output Straps Magnetic field Fig. 1. Schematic layout of magnetron blockInitial experiments were performed in modified versions of the 1 00kW pulsed CV6035 magnetron.Measurements of the passive resonance at small-signal levels (ie a few m.w. power level) showed the same dependence of tuning sensitivity upon magnetic field as in Smith and Shulman -showing maximum values of opposite sign at about 1 0% above and below cyclotron resonance, as shown in figurd 2. The cavity loss maximised at resonance. Under the chosen conditions, the tuning diode is below the Hull cutoff -the current reaching the anode being the 'anomalous' current which always occurs in practice, allowing frequency shifts of typically 10-1 5MHz at about 200mA, 1 kV.
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