The measurement of balanced and unbalanced impedances at frequencies near 500 Mc/s, and its application to the determination of the propagation constants of cables

Apparatus is described suitable for the measurement of balanced and unbalanced impedances at frequencies above 400 Mc/s. It consists of an air-spaced concentric line for unbalanced impedances and a screened twin line for balanced impedances. The length of line can in each case be varied by means of a movable bridge carrying a thermo-junction unit. The component to be measured is connected to the open end of the line, which is then adjusted to current resonance by the movement of the bridge. The impedance is evaluated from the readings of resonant length and the width of the resonant curve. Convenient working equations are developed, and results are given of a number of measurements made to check the validity of the assumptions involved. The results of measurements of a series of small ceramic condensers and carbon and metallized resistors are included in the paper. The main application of the apparatus is to the measurement of the propagation constants of radio-frequency cables, and a number of methods of making these measurements are described with a sample set of results. The methods are designed with a view to eliminating the experimental errors that arise. These errors are discussed in some detail in an Appendix. The accuracies achieved for a wide range of values of reactance and resistance are ± 2% and ± 5% respectively.