Capacitance between the coaxial chamber’s conductors

Abstract

Since the results of the conducting media electromagnetic field measurements are a function of the medium conductivity, an advance calibration of the instruments is necessary. The effective method of calibration is locating the instruments in the coaxial chambers being a coaxial line sections with extended dimensions where an electromagnetic wave is excited.

The coaxial chamber for the calibration of the conducting medium magnetic field strength measuring instruments is different by making the central conductor shaped as a cylindrical spiral and setting the instrument into the conducting liquid tank, which is located inside the volume bounded by cylindrical spiral. In order to eliminate the magnetic field parasitic component, which causes the measurements error, the central conductor of the chamber should be made of two wire spirals with opposite winding directions, and their currents should have the phase shift of π.

For the analysis of the wave impedance and the propagation constant of the transmission line equivalent to the coaxial chamber, the capacitance between two coaxial cylindrical spirals of the same radius and opposite winding directions, as well as the capacitance between each spiral and external conductor (screen) of the coaxial chamber, should be calculated. The pointed out problem has two parts. The first one is the calculation of the capacitance between the conductors placed in homogeneous medium (air). This problem has independent significance because the solution of it is necessary for the magnetic field strength instruments calibration in open-air conditions, and for the creation of the corresponding primary standards (the coaxial chambers) as well as for calculation of the balanced delay cables.

The second problem is the medium inhomogeneity influence consideration because a dielectric cylinder with the permittivity different from the air one is placed in fact inside the spirals.

The methods and the results of both problems solution are presented.