The errors are associated with the selection of the quantities to be measured directly, which enter into the formulas to evaluate 2AH. In [4], for instance, these quantities are the passband of the first peak of the resonance curve of the coupled ferrite-cavity system and the passband of the loaded cavity. The error in calculating 2AH evidently cannot be less than the error in the quantities being measured in the indirect determination of ~heFMRlinewidth (i.e., finding 2AH by substituting the quantities measured directly into the appropriate formula). Moreover, the error in evaluating 2AH depends on the degree of approximation with which the formula selected relates 2AH to the quantities being measured. Since the methods of determining the FMR linewidth, which are based on perturbation theory, do not permit the determination of the 2AH of large-size specimens which are most used in practice, it is interesting to examine the possibility of decreasing the error in determining 2AH by starting from concepts of the theory of coupled systems and comparing the results obtained to those already known. One of the means of decreasing the errors in the quantity 2AH to be measured indirectly is to obtain a new formula and to measure the quantities therein with a lesser error than in known methods. A brief characteristic of the method and a description of the apparatus block diagram which permits execution of the necessary measurements with a 5% error in a broad range of values of ferrite-specimen diameters are presented below. The method proposed to determine the FMR linewidth is one of the cavity methods and has its foundation in the theory of coupled systems. To determine 2AH the strong coupling between the microwave cavity and the ferrite specimen is used. In this case the resonance curve of the cavity is split into two peaks, and the ferrite-cavity system can be considered as a system of coupled loops [5]. By making use of the theory of coupled systems under the condition that the cavity resonance frequency ~c equals mf, the FMRfrequency, the absolute value of the coefficient of reflection F r at the coupling frequencies can be found from the expression