INTERFEROMETER FOR MEASURING DIELECTRIC PERMITTIVITY AND LOSSES IN THE CENTIMETER-WAVE RANGE *)

Recently, a distinctive interferometer has been constructed that makes it possible to measure dielectric permittivity and losses at a wavelength of 3.2 cm. In its design the instrument resembles a Michelson interferometer. Its schematic is shown in the figure.

A centimeter-wave generator \(G\) sends energy along two 80-ohm channels, into horn \(P_1\) and, in the other, through a directional coupler \(H\) and attenuator \(A\) to detector \(D\). From horn \(P_1\) emerges a beam of plane radio waves—a kind of “radio ray.” It falls on a semitransparent plate \(O\), where, as in an optical Michelson interferometer, it is split into two rays—a transmitted and a reflected one. Unlike the Michelson interferometer, only the transmitted ray is used here; the reflected ray is absorbed by screen \(C\). After passing through plate \(O\) and reflecting from the movable mirror \(M\), the radio ray returns to plate \(O\), is reflected from it, and enters receiving horn \(P_2\), and then passes into the T-shaped waveguide \(T\) with load \(H\). There it interferes with the “primary radio ray” (which has entered \(T\) from the generator through the directional coupler \(H\) and attenuator \(A\)). As a result of the interference, a standing wave is formed in waveguide \(T\), whose amplitude is measured by detector \(D\). The detector is rigidly fixed.

) W. A. Lengyel, PIRE 37*, 1242 (1949).

in the waveguide and does not shift when changes are made. The interference pattern is displaced in the waveguide, on the one hand, by the substance under investigation, and, on the other, by moving mirror \(M\). By compensating the phase difference introduced by the substance under investigation (it is placed in the path of the radio beam in \(D_1\) or \(D_2\)) by shifting mirror \(M\), this phase difference is determined and, from it, the electric permittivity.

The interferometer described makes it possible to carry out the following measurements.

1. Measurement of wavelength. Mirror \(M\) is set in such a position that, at the point where detector \(D\) is located, a minimum of the standing wave is observed. Then mirror \(M\) is shifted in some one direction; in the detector, maxima and minima of the current then alternate. The minimum distance through which mirror \(M\) must be shifted in order to obtain a current minimum again in the detector is, evidently, equal to half the wavelength. By this method a wave of \(3.2\) cm was measured with an accuracy of up to \(0.0003\) cm.

2. Determination of the dielectric permittivity of materials transparent to UHF. Before measurement, the system is adjusted with the attenuator to a standing wave, obtaining the smallest possible traveling-wave coefficient (this coefficient is the ratio of the minimum amplitude in the wave to the maximum). Then mirror \(M\) is set so that at the location of the detector there is a minimum of the standing wave. After this, a sheet of the material under investigation is placed in the path of the radio beam in \(D_1\) (the dotted line is shown in the figure). It introduces a certain phase difference, and the standing-wave minimum is displaced from the detector. By moving mirror \(M\), the system is again brought back to a signal minimum in the detector. Knowing the displacement of mirror \(M\) required for this and the thickness of the sheet, the refractive index of the substance under investigation and its dielectric permittivity are calculated.

3. Measurement of the complex dielectric permittivity of strongly reflecting materials. The sheet under investigation is placed not in \(D_1\), but in position \(D_2\). The displacement of the standing-wave minimum introduced by reflection from this sheet is measured and, in addition, the traveling-wave coefficient (since losses upon reflection from the sheet increase the traveling-wave coefficient), from which the permittivity is calculated.

M. Ginzburg