The Realization of High-Frequency Impedance Standards Using Air-Spaced Coaxial Lines

Impedances of devices with coaxial inlets, having small reflection coefficients relative to a given characteristic resistance, can be determined accurately (± 0.1 per cent) by comparison with the nominally calculable characteristic impedance of a quarter-wavelength, air-spaced coaxial line in which the inner conductor is supported by the inner conductors of the apparatus connected at each end. In carrying out such comparisons, errors may be introduced by the presence of small series impedances at the connections between the rigid coaxial lines. Such connections cannot be eliminated because they must be made and broken during the process of making comparisons. Errors may also arise from the distributed resistance and the "internal" reactance of the line conductors, as well as from departure of the lines from uniformity in diameter. This paper describes experimental techniques for measuring these residuals and discusses their effects. The design requirements of suitable air-spaced coaxial lines as standards are stated, together with a brief mention of techniques for making comparisons with the standards.