The electrical performance index of the microwave darkroom is mainly characterized by the characteristics of the quiet zone. The characteristics of the quiet zone are described by the size of the quiet zone, the maximum reflection level in the static zone, the cross-polarization, the field uniformity, the path loss, the intrinsic radar cross section, and the operating frequency range.
The factors affecting the performance of the darkroom are diversified and complex. When using the light emission method and the energy physics method to simulate the darkroom performance, it is necessary to consider the transmission decoupling of the wave, the polarization decoupling, and the direction of the standard antenna. Figure factor, the vertical incident properties and oblique incidence properties of the absorbing material itself, multiple reflections and other effects. However, in the actual engineering design process, the performance of the absorbing material is often used as a key determinant of the performance of the darkroom.
1) Cross-polarization: Due to the non-strict symmetry of the darkroom structure, the inconsistency of absorption of various polarized waves by the absorbing material, and the darkroom test system and other factors, the polarization of the radio waves in the darkroom propagation process is impure. If the ratio of the measured field strength is less than -25 dB when the antenna to be tested is orthogonal and parallel to the plane of polarization of the transmitting antenna, the cross-polarization is considered to be satisfactory.
2) Multipath loss: The uneven path loss will rotate the polarization plane of the electromagnetic wave. If the wave to be tested is rotated in the direction of the wave, the fluctuation of the received signal does not exceed +-0.25 dB, and the multipath loss can be ignored.
3) Field uniformity: In the darkroom quiet zone, move the antenna to be tested along the axis, and the required undulation is not more than +-2dB; in the cross section of the quiet zone, move the antenna to be tested laterally and up and down, and the required signal fluctuation does not exceed +-0.25. dB.
2. Error in antenna measurement1) Error caused by limited test distance
It is assumed that the planar antenna is to be measured, and the received incoming wave is along the axial direction of its main beam. If the test distance is large, the fields accepted by different parts of the antenna to be tested cannot be the same, and therefore have a square root law phase difference. If the antenna to be tested is located at the boundary 2D2/λ of the far field region of the source antenna, the aperture edge and the phase center field have a phase difference of 22.5 degrees. If the test distance is doubled, the phase difference is halved.
For antennas measuring medium sidelobe levels, the distance 2D2/λ is usually sufficient, and the measured gain is about 0.06dB. The test distance is shortened, the measurement error is increased rapidly, and the side lobes merge with the main beam to form a shoulder. Even integrated. . Typically a 0.25 dB taper pin reduces the measured gain by approximately 0.1 dB and causes some slight error in the near side lobes.
2) Reflection
The direct wave is subject to interference from the surrounding object, forming a change in the field in the test area, which rapidly changes as the wave path difference as a function of position, causing the length of the undulation to be of the order of magnitude. . For example, a reflected wave 20 dB lower than a direct wave can cause a power error of -0.92 to +0.83 dB. , depending on the difference between the two; the phase measurement has an error range of ±5.7°. However, if the field of the reflected wave is 40 dB lower than the direct wave, the amplitude and phase of the side are only ±0.09 and ±0.6°, respectively.
Reflection is particularly detrimental in the measurement of low side lobes. A small reflection is coupled to the antenna to be tested through the main lobe, which completely covers the direct wave coupled to the side lobes. If the coupled direct and reflected wave intensities are equal, the measured sidelobe level will rise by about 6 dB or become zero in the measured lobe map.
3) Other errors
Factors that may also cause errors in antenna measurements are:
The coupling to the near field of the reactance at low frequencies may be significant. Measure the alignment error of the antenna; other interference signals; errors caused by the test cable.
Personal views are as follows:
1) Quiet area
The darkroom quiet zone is centered on the intersection of the rotary axis and the vertical axis of the darkroom.
2) Reflection level in the static zone:
The dark room like the antenna generally requires electrical performance to be measured according to the frequency and test accuracy of your test. The general specification of the darkroom of the antenna requires 30--50dB.
3) Field amplitude uniformity in the static zone: ±2dB in the longitudinal direction; ±0.25dB in the lateral direction;
4) Horizontal and vertical cross polarization characteristics in the static zone: -25dB;
5) Multipath loss: ≦ ± 0.20;
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