The electrical performance of a microwave darkroom is primarily defined by the characteristics of its quiet zone. This area is crucial for accurate electromagnetic measurements, and its quality is determined by several key factors: 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 parameters collectively ensure that the environment remains as free from external interference as possible.
Numerous factors influence the performance of a darkroom, making the design and simulation process complex. When using light emission or energy physics methods to model the darkroom’s behavior, it's essential to consider wave transmission decoupling, polarization decoupling, antenna orientation, absorption material properties, multiple reflections, and more. In practice, the performance of the absorbing material often becomes the primary determinant of the darkroom’s overall effectiveness.
1. **Cross-Polarization**
Due to structural asymmetry, uneven absorption of different polarizations, and system-related effects, the polarization of radio waves in the darkroom can become distorted. 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 direction of propagation, and the received signal fluctuation stays within ±0.25 dB, the multipath effect can be neglected.
3. **Field Uniformity**
In the quiet zone, moving the test antenna along the axis should result in a variation no greater than ±2 dB. When moving the antenna laterally and vertically across the cross-section, the signal fluctuation must remain within ±0.25 dB.
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**Antenna Measurement Errors**
1. **Error Due to Limited Test Distance**
When measuring a planar antenna, if the incoming wave aligns with the main beam, differences in field distribution across the antenna can create phase variations. At a test distance of 2D²/λ, the phase difference between the aperture edge and the phase center is about 22.5 degrees. Doubling the distance halves this difference. For antennas with medium side lobe levels, this distance is usually sufficient, resulting in a gain error of approximately 0.06 dB. Shorter distances increase measurement errors significantly, causing side lobes to merge with the main beam, potentially altering the pattern.
2. **Reflections**
Reflected waves can interfere with the direct signal, creating field fluctuations that vary with position. A reflected wave 20 dB lower than the direct wave can introduce amplitude errors between -0.92 dB and +0.83 dB, depending on the phase difference. Phase measurement errors may reach ±5.7°. However, if the reflected wave is 40 dB lower, the impact is minimal, with amplitude and phase errors of ±0.09 dB and ±0.6°, respectively.
Reflections are particularly problematic in low-side-lobe measurements. Even small reflections can couple into the antenna through the main lobe, overwhelming the side lobe signals. If the direct and reflected signals have equal intensity, the measured side lobe level could increase by 6 dB or even disappear entirely.
3. **Other Sources of Error**
Additional factors that can affect measurement accuracy include near-field coupling at low frequencies, alignment errors, other interfering signals, and cable-related inaccuracies.
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**Personal Insights**
1. **Quiet Zone**
The quiet zone is centered at the intersection of the darkroom’s rotational axis and vertical axis, ensuring optimal measurement conditions.
2. **Reflection Level in the Static Zone**
The darkroom is typically designed to meet specific frequency and accuracy requirements. Standard specifications often require a reflection level of 30–50 dB.
3. **Field Amplitude Uniformity**
Longitudinal uniformity is ±2 dB, while lateral uniformity is maintained within ±0.25 dB.
4. **Cross-Polarization Characteristics**
Both horizontal and vertical cross-polarization levels are generally required to be below -25 dB.
5. **Multipath Loss**
Multipath loss is kept within ±0.20 dB to ensure minimal signal distortion.
By carefully addressing these factors, the accuracy and reliability of measurements in a microwave darkroom can be significantly improved.
Shanghai Janetec Electric Co., Ltd. , https://www.janetecelectric.com