The electrical performance of a microwave darkroom is primarily defined by the characteristics of its quiet zone. This area is critical for accurate electromagnetic testing and is described in terms of several key parameters, including the size of the quiet zone, the maximum reflection level within it, cross-polarization levels, field uniformity, path loss, intrinsic radar cross-section, and the operating frequency range. These factors collectively determine the quality of the electromagnetic environment inside the darkroom.
The performance of a darkroom is influenced by multiple complex factors. When simulating darkroom performance using light emission or energy physics methods, it's essential to consider wave transmission decoupling, polarization decoupling, the orientation of the standard antenna, the vertical and oblique incidence properties of the absorbing material, and effects such as multiple reflections. However, in practical engineering design, the performance of the absorbing material often serves as the primary determinant of overall darkroom effectiveness.
1) **Cross-Polarization**: Due to structural asymmetry, inconsistent absorption of different polarizations by the materials, and other system-related factors, the polarization of radio waves within the darkroom may not be perfectly pure. If the measured field strength ratio is less than -25 dB when the test antenna is orthogonal or parallel to the transmitting antenna’s polarization plane, the cross-polarization is considered acceptable.
2) **Multipath Loss**: Uneven path loss can cause rotation of the electromagnetic wave's polarization plane. If the tested wave rotates in the same direction as the wave, and the received signal fluctuation remains within ±0.25 dB, then multipath loss can generally be neglected.
3) **Field Uniformity**: In the quiet zone, moving the test antenna along the axis should result in fluctuations no greater than ±2 dB. When moving the antenna laterally and vertically across the cross-section, the signal variation should not exceed ±0.25 dB.
**Antenna Measurement Errors**
1) **Error from Limited Test Distance**:
Assuming a planar antenna is being measured, and the incoming wave aligns with the main beam direction. If the test distance is too short, different parts of the antenna will receive slightly different fields, leading to a phase difference proportional to the square root of the distance. At the boundary of the far-field region (2D²/λ), the phase difference between the aperture edge and the phase center can be around 22.5 degrees. Doubling the test distance halves this phase difference. For antennas with medium sidelobe levels, the 2D²/λ distance is usually sufficient, resulting in a gain error of about 0.06 dB. Shorter distances increase errors rapidly, causing side lobes to merge with the main beam, forming a shoulder effect. A 0.25 dB taper pin might reduce the measured gain by approximately 0.1 dB, introducing minor errors in nearby side lobes.
2) **Reflections**:
Direct signals can be disturbed by surrounding objects, creating field variations that change rapidly with position. A reflected wave 20 dB lower than the direct wave can cause power errors ranging from -0.92 to +0.83 dB, depending on the phase difference. The phase measurement error can reach up to ±5.7°. However, if the reflected wave is 40 dB weaker, the amplitude and phase errors are reduced to ±0.09 and ±0.6°, respectively. Reflections are especially problematic when measuring low sidelobes. Even a small reflection can couple into the antenna through the main lobe, overwhelming the direct wave and causing the measured sidelobe level to rise by about 6 dB or even drop to zero in the lobe map.
3) **Other Errors**:
Additional sources of error include near-field coupling at low frequencies, alignment errors of the antenna, interference from other signals, and errors introduced by test cables.
**Personal Insights:**
1) **Quiet Zone**:
The quiet zone is centered at the intersection of the darkroom’s rotary axis and vertical axis.
2) **Reflection Level in Static Zone**:
Typically, the darkroom must meet a minimum reflection level of 30–50 dB, depending on the test frequency and required accuracy.
3) **Field Amplitude Uniformity in Static Zone**:
Longitudinal uniformity should be within ±2 dB, while lateral uniformity should not exceed ±0.25 dB.
4) **Cross-Polarization Characteristics**:
Both horizontal and vertical cross-polarization should be below -25 dB.
5) **Multipath Loss**:
Should be limited to ±0.20 dB.
Overall, ensuring high-quality performance in a darkroom requires careful attention to all these parameters, as even small deviations can significantly impact the accuracy of antenna measurements.
Limit Switch,Micro Limit Switch,High Limit Switch,Telemecanique Limit Switch
Shanghai Janetec Electric Co., Ltd. , https://www.janetecelectric.com