SU1569744A1 - Method of measuring complex reflection factor of materials in close zone - Google Patents

Abstract

This invention relates to a microwave measurement technique. The purpose of the invention is to improve the measurement accuracy. The essence of this method of measuring the complex reflection coefficient (QO) of materials in the near zone is explained by a device in which the signal from r-1 microwave reaches the directional coupler 2 and then is transmitted through the receiving-transmitting antenna 3 to free space. In the near zone of the antenna 3, a QO measure or sample 5 is located on the movement mechanism 4. The signal reflected from the QO measure or from sample 5 again enters the antenna 3, is allocated by the directional coupler 2 and is fed to the amplifier 6. Information about the amplitude and phase of the reflected wave Amplification meter 6 arrives at the computer 7. It also receives data on the position of the QO measure or sample 5. When processing the data, the computer 7 calculates the complex QO of the sample 5. To improve the measurement accuracy, move the measure of the QO or sample 5 For more than half the wavelength, the phase of the wave at the output of the antenna 3 is additionally recorded at distances between the aperture of the antenna 3 and the measure TO or sample 5 equal to not more than a quarter of the wavelength. 1 il.

Description

The invention relates to a technique for measuring at ultrahigh frequencies and can be used to create installations for measuring parameters of materials in free space.

The use of reflection coefficient (QO) measurement methods in free space allows non-destructive testing to be carried out in the development and production of materials, to investigate coarse-grained materials, etc.

The aim of the invention is to improve the measurement accuracy.

The drawing shows a device for carrying out a method for measuring the complex reflection coefficient of materials in the near zone.

The device contains a microwave generator 1, a directional coupler 2, a receiving-transmitting antenna 3, a movement mechanism 4, the sample under study 5, an amplifier 6, a computer 7.

The measurement of the complex reflection coefficient consists of the following sequence of operations: moving along the optical axis of the antenna for a distance greater than half the wavelength, QO measure and measuring at equal points, the distance between which does not exceed a quarter of the wavelength, complex samples N. amplitude and phase waves; moving along the optical axis of the antenna of the sample under study and measuring at the same points as the KO measures, complex samples of N-0 amplitude and phase of the wave.

By complex wave amplitude and phase readings is meant

N

ten

N -: w

| N, .e

 IN,

A1,

sh

where | N, -01, I N.; M | - modules of the reference wave amplitude in the measuring channel;

10 m Phase wave. The complex reflection coefficient of the sample under study is calculated by the formula

um z- n .-. e

 W (i)

(one)

Where

complex coefficient of reflection of the measure KO;

n is the number of counts; R. is the distance from the QO measure or the sample under study to the antenna aperture; - wavelength in free space; W (i) is a weight function.

According to the concept of partial waves, the field between the antenna and the sample can be represented as a sum of partial plane incident and reflected waves propagating at different angles to the optical axis of the antenna. The field structure in the near field of the antenna is not a flat TEM wave. The amplitudes of all partial waves incident on the antenna are summed with the weights. The reflection coefficient is a function of the distance from the antenna aperture to sample R. By performing, by analogy with the frequency method of analysis in radio engineering chains, the Fourier transform with integration by the parameter R. of the signal, one can find the spectral density, the value of which is proportional to the amplitude of the corresponding partial wave. The reflection coefficient at the normal incidence of an electromagnetic wave is associated with a partial wave propagating along the optical axis of the antenna. For this wave, the spatial frequency is

4I / s .. - - (. The sign - associated with the choice of

fishing-positive directions and its dimension). Since the partial waves incident on the antenna are proportional to the reflection coefficients of the QO measures and the sample under study,

45

.-

s; rf)

(2)

five

0

where S0, SM are the spectral densities when moving the KO measure and the sample under study. I

When measuring, a finite number of samples is determined at a selected interval of R, therefore, the spectral densities should be calculated based on a discrete Fourier transform (DFT) with a weighting function W (i), called a correlation window, which is introduced to reduce the limiting effect of the observation interval ,

five

The choice of a specific window does not play a significant role in measuring the energy of spectral harmonics using the DFT. The criterion for choosing the appropriate function W (i) can be the requirement to minimize the displacement of the spectral density estimate. This requirement is most satisfied with the Kaiser-Bessel window.

de

W (i)

In ()

b and

about / n.

 a modified Bessel function of the first kind of zero order; the parameter specifying the bandwidth of the correlation window; number of counts;

0Ј

. / N 1 (2

Blackman-Harris Window

W (i) a0-af-cos (- i) + ar cos (-) -, 6f

-a

ycos (- i),

a ... a - weighting factors determined depending on the level of side lobes from the table:, 1,2, ..., N-1.

When implementing the method of measuring the complex reflection coefficient of materials in the near zone, a Blackman-Hariss correlation window was used with side lobes level of -67 dB. When adding spectral densities at a frequency of - - in discrete form

of the Fourier transform of the DFT with the weight function W (i), expression (2) leads to expression (1). To more accurately determine the spectral densities, the observation interval should correspond to several periods of the frequency under study, and the frequency of

five

0

five

0

five

0

five

0

five

gasation exceed it no less than .-. twice. Therefore, the displacement of the QO measure and the sample under study is carried out over a distance greater than half the wavelength, and the amplitude and phase of the wave are recorded when the KO measure and the test sample are positioned at points whose distance does not exceed a quarter of the wavelength.

The data obtained allow us to calculate the spectral densities of plane partial waves propagating along the normal to the surface of the TO measure and the sample under study, and thereby eliminate the effect of field distortions in the near zone due to the difference in the field structure from the TEM wave on the results of measuring the complex coefficient reflections.

An apparatus for carrying out the method for measuring the complex reflection coefficient of materials in the near zone works as follows.

From the generator I, the microwave signal arrives at the directional coupler 2 and then is transmitted through the receiving-transmitting antenna 3 to free space. In the near zone of the antenna, on the movement mechanism 4, a QO measure or sample under test 5. A signal reflected from the KO measure or sample under study is fed back to the receiving-transmitting antenna 3, which is separated by a directional coupler

2 and is fed to the amplifier 6. The information about the amplitude and phase of the reflected wave from the amplifier 6 goes to the computer 7. It also receives data on the position of the CS measure or the sample under study. Performing data processing according to (1), the computer calculates the complex reflection coefficient of the sample under study.

Claims (1)

Invention Formula The method of measuring the complex reflection coefficient of materials in the near zone, which consists in alternately irradiating the antenna measure of the reflection coefficient and the sample under study and moving them along the optical axis of the antenna and registering the amplitude of the wave at the antenna output, characterized in that, in order to improve the accuracy, move the measure of the coefficient reflections and sample to be examined are performed on a distance greater than half the wavelength additionally records the phase of the wave at the antenna output when the distance between the aperture of the antenna and the measure of the reflection coefficient and the test sample is not more than a quarter of the wavelength, and the complex reflection coefficient is calculated by the formula "i Јc" ZN.0eJ (i) R 1 m jh. JTfl W (i) IN, „er1de i G - complex coefficient from ratios measure reflectivity; n is the number of counts; L wavelength in free space; R- is the distance from the aperture of the antenna to the measure of the reflection coefficient of the sample under study at which the reading is made; W (i) is a weight function; N., N. are the complex samples of the amplitude and phase of the wave when the sample under study is moved and the reflection coefficient measure, respectively.