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Measurements of the loop impedance of an amplitude-modulated injection-locked uhf magnetron looking from the transmission line toward the magnetron are presented. The loop impedance is computed from the voltage standing-wave ratio and the position of the minimum-voltage plane of the injection current reflected from the magnetron output loop. While the plate voltage is varied, the changes of the loop impedance are observed under specific conditions which are important for the proper performance of the resonant injection system. Analyses of the phenomena occurring in the output transmission line of the synchronized magnetron are described. Matched and decoupled resistive loads as well as a reactive load are presented to the magnetron at the high-power end of the modulation cycle. It is demonstrated that the load impedance seen by the magnetron steadily and substantially changes during an amplitude-modulation cycle. However, at the low-power end of a 100-per cent modulation cycle the load is constant, and the value of it for a given type of magnetron with a predetermined setting of the magnetron tuner depends only upon the carrier frequency. The changes of load during the modulation cycle are discussed. These are graphically represented by a special kind of circular diagram.