The improvement of anechoic chamber performance lies in reducing the measurement error

An anechoic chamber is a shielding chamber that has no reflective material on its inner surface. Simulating a free space signaling environment is mainly used for parameter measurement of microwave antenna systems. With the continuous development of electronic technology, anechoic chambers have been understood and applied by more people. So how to judge the performance of anechoic chambers? Let's take a look.

1、 Electrical performance index of anechoic chamber

Generally, it represents six indicators, such as static zone, reflectivity level, cross polarization, multipath loss, amplitude uniformity, and operating frequency.

1.Multipath loss: Uneven path loss can rotate the polarized surface of electromagnetic waves. If the test antenna is rotated in the wave direction, the fluctuation of the received signal should not exceed ± 0.25 dB, and the multipath loss can be ignored.

2. Field uniformity: In the quiet area of the anechoic chamber, move the antenna to be measured along the axis, and the fluctuation is required to not exceed ± 2dB; The antenna to be tested moves laterally up and down on the dead zone section, and the fluctuation of the received signal shall not exceed ± 0.25 dB.

3. Cross polarization: The structural symmetry of the anechoic chamber is not strict. The inconsistent absorption of various polarized waves by absorbing materials and the anechoic chamber testing system cause the polarization of electric waves to be impure during the propagation process in the anechoic chamber. If the polarization surface of the test antenna and the transmitting antenna are orthogonal and parallel, the intensity ratio of the test field is less than - 25dB, and the cross polarization is considered to meet the requirements.

2、 Performance index of anechoic chamber

When simulating the performance of an anechoic chamber using light emission and energy physics methods, the effects of radio wave transmission decoupling, polarization decoupling, standard antenna pattern factors, the vertical and oblique incidence properties of the absorbing material itself, and multiple reflections should be considered. However, in the actual engineering design process, the performance of absorbing materials is often a key determinant of the performance of anechoic chambers.

3、 Antenna measurement error

1. Errors caused by limited testing distance. Set the plane antenna to be measured, and the incoming wave is received along the axis of its main beam. If the test distance is large, the fields received by different parts of the antenna to be tested cannot be the same, so there is a square root law phase difference. If the antenna to be tested is located at the boundary 2D2 of the far field area of the source antenna/ λ， Its aperture edge and phase center have 22 fields with a phase difference of 5 degrees. If the test distance is doubled, the phase difference is halved. For antennas measuring medium sidelobe levels, the distance is 2D2/ λ Usually sufficient, the measured gain is approximately 0.06 dB. Shortening the test distance can quickly increase measurement error, and the side lobe can be merged with the main beam into a shoulder table, or even merged The 25 dB taper pin reduces the measured gain to about 0.1 dB and causes some errors in the near side lobe.

2. Reflection. Due to the rapid change of wave path difference as a function of position, direct waves are interfered by the reflection of surrounding objects, forming field changes in the test area, making the fluctuation length an order of magnitude of wavelength. For example, a reflected wave that is 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 error range for phase measurement is ± 5.7 °, but if the field of the reflected wave is 40 dB lower than that of the direct wave, the amplitude and phase of the side exit are only ± 0.09 and ± 0.6 ° errors. Reflection is particularly harmful in measurements of low sidelobes. A small reflection can completely cover the direct wave coupled to the side lobe by coupling the main lobe to the antenna under test. If the direct intensity of the coupling is equal to the reflected wave, the measured sidelobe level will increase by about 6 dB or become zero in the measured lobe map.

3. Other errors. Factors that may also cause antenna measurement errors include: significant near field coupling with reactance at low frequencies; Measure antenna alignment error; Other interference signals; Errors caused by testing cables.

The application of anechoic chambers is becoming increasingly widespread, and in order to meet people's needs, the parameters and various aspects of anechoic chambers will be best improved.