The design, calibration and performance of resonance wavemeters for frequencies between 1,000 and 25,000 Mc/s

Abstract
The theory and practice of the design of wavemeters for use at frequencies between 1,000 and 25,000 Mc/s are discussed. The method of calibrating the wavemeters in terms of a standard of frequency is described, and the results of experimental investigations of a number of different types are given.The modes of resonance generally employed for these wavemeters are the principal coaxial mode, the E010-EH hybrid mode and the E010, H011 and H11n cylindrical wave-guide modes and the discussion and experimental work are restricted to instruments using these modes. The instruments can be designed to resonate at one fixed frequency or at frequencies within a certain range: in the latter case the adjustment is obtained by altering the length of the coaxial line or wave guide or by the insertion into the wave guide of a plunger of suitable dimensions. One of the main considerations of design is to ensure that the instrument does not resonate in modes other than the one desired, and that the frequency corresponding to a given setting is not ambiguous. Non-ambiguous ranges of frequency variation of approximately 6 and 12% are obtained with the H011 mode and H111 mode respectively; and with the E010 mode a range of the order of 10% is obtained with radial plungers, and a range of 3: 1 with an axial plunger, the mode of vibration in this case being the E010-EH hybrid.The frequency of resonance of a fixed-frequency wavemeter resonating in the E010 mode was measured with a precision of ±2 parts in 106; but for use with this accuracy the conditions of coupling to the exciting and detecting circuits, the temperature, and the humidity needed careful definition.Variable-frequency wavemeters, using the H011 and H111 modes, had a precision of the order of 1 part in 105, the limitation being in general the temperature coefficient of the instrument, which is comparable in magnitude with that of the material. The accuracy of the wide-range instruments, covering a frequency range of 3: 1 was of the order of 1 part in 104.The agreement between the measured and calculated values of resonant frequency for several wavemeters using different modes of vibration was about 1 part in 104, which was within the accuracy of the calculations owing to the physical imperfections of the wavemeters. It would appear that if resonators of the kind used for the wavemeters described were made and measured with the full accuracy now attainable, and the measured and theoretical values of resonant frequency were equated, it would be possible to derive a value for the velocity of propagation of electromagnetic waves accurate to about 1 part in 105.

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