Field strength measurement is an important part of radio monitoring and plays an important role in radio management. Accurate field strength measurements can be used to understand the true state of the electromagnetic environment and provide effective technical data for frequency division and distribution.
Through long-term measurement and data analysis and comparison, it is possible to detect and reflect the abnormal changes of the electromagnetic environment in time, and change the passive situation that only the user can complain through radio interference, and the passive monitoring is active monitoring to meet the needs of ensuring safe production.
Four modes of field strength measurement(1) Use portable or mobile devices to make measurements and acquire relevant transient data or short-term data at one or more points. For example, a portable receiver is used to find a source of interference by using a relatively small and comparative field.
(2) Using the mobile device to perform measurements, and obtain statistical parameters of the mobile radio coverage area. Such as the operator's measurement of GSM network coverage.
(3) Short-term measurements at fixed points are generally used to support other monitoring services. If the fixed station is used to sense the signal, the field strength of the signal is read.
(4) Long-term measurements, including field strength records and curve record analysis, are stored and analyzed separately by computer. For example, the field strength of a certain frequency point or frequency band is measured for a long time at a fixed monitoring station.
Field strength measurement purposes are different, and the requirements for field strength accuracy are also different. When the field strength value is close to the standard value, it is required to determine whether the field strength value meets the standard requirements. Generally, the measurement uncertainty can be used to indicate the accuracy of the measurement results. Although the relationship between the measurement uncertainty and the accuracy is difficult to quantify, it is certain that the smaller the measurement uncertainty, the higher the accuracy. Therefore, in some cases, in addition to the field strength value, the measurement results should also give the uncertainty of its measurement.
To learn more about measurement uncertainty, refer to JJF1059, “Measurement Uncertainty Evaluation and Representationâ€. The basic concepts of “errorâ€, “accuracy†and “measurement uncertainty†are briefly introduced here. Each measurement yields a measurement that is often different from the true value being measured, and the error is equal to the difference between the measured value and the true value. Since the true value cannot be obtained, the error cannot be calculated, and the measurement uncertainty can only be used to indicate the reliability of the measured value. In JJF 1059, the measurement uncertainty is defined as "a parameter that characterizes the dispersion of the measured value and the measurement results."
Two ways to determine whether the field strength measurement Vmeas meets the standard requirements based on measurement uncertainty(1) Measurement uncertainty Ums is as small as possible
For communication users, if Vmeas+Ums is less than or equal to the standard value, it can be determined that the signal meets the standard requirements. If the user measures the signal level of a certain frequency point, it is seen whether the receiving station at the location will be interfered.
For management, if Vmeas-Ums is greater than the standard value, it can be determined that the signal does not meet the standard. If the management department measures the signal level of a certain frequency point, it is seen whether a certain transmitter signal will interfere with other receivers.
(2) Measurement uncertainty Ums is less than or equal to the recommended maximum uncertainty value URec
If all measured values ​​are less than or equal to the standard value, it can be determined that the signal meets the standard;
If any of the measured values ​​is greater than the standard value, the decision signal does not meet the criteria.
Unless limited by receiver bottom noise, atmospheric noise, or external interference, URec should meet the following requirements: URec = 2 dB when the measurement frequency is less than 30 MHz, and URec = 3 dB when the measurement frequency is greater than 30 MHz. To meet the above requirements, the field strength measurement system must be installed and used according to certain requirements. The specific requirements refer to Annex 1 of the ITU Recommendation ITU-R SM.378 "Field Strength Measurement in Monitoring Stations".
For communication users, when Ums is greater than URec: if all measured values ​​plus (Ums-URec) are less than or equal to the standard value, it can be determined that the signal meets the standard; if any measured value plus (Ums-URec) is greater than the standard value , then the decision signal does not meet the standard.
For the management department, when Ums is greater than URec: if all measured values ​​minus (Ums-URec) are less than or equal to the standard value, it can be determined that the signal meets the standard; if any measured value minus (Ums-URec) is greater than the standard value , then the decision signal does not meet the standard.
How to reasonably assess the uncertainty of field strength measurement
Field strength measurements require the use of calibrated antennas, coupling networks and/or transmission lines, test receivers or spectrum analyzers. To increase sensitivity, a low noise amplifier (LNA) can also be added. It can be seen that the biggest difference between the field strength measurement and the conduction measurement of the transmitting device is the use of a calibrated antenna. The characteristics of the antenna are particularly important factors in assessing the uncertainty of field strength measurement.
The components that affect the uncertainty in field strength measurements are as follows:·Receiver reading
·Attenuation between antenna and receiver
·Antenna factor
· Receiver sine wave voltage accuracy
Receiver selectivity relative to signal occupied bandwidth
· Receiver bottom noise
· Mismatch between antenna interface and receiver
·The frequency interpolation deviation of the antenna factor
• Changes in antenna factor due to changes in the height of the antenna from the ground and other mutual couplings
·Antenna directivity
· Antenna cross polarization
·Antenna balance
· Shadow effects (also called slow fading) and reflections caused by obstacles
An example of calculating the field strength measurement uncertainty is given below. This example is suitable for measuring the field strength at frequencies between 30 MHz and 3000 MHz at a fixed or mobile station.
Field strength E=Vr+Lc+AF+δVsw+δVsel+δVnf+δM+δAFf
+δAFh+δAdir+δAcp+δAbal+δSR
Specific to a particular monitoring station, the uncertainty of the input may be different from the one in the table. When assessing the extended uncertainty Ums of field strength measurements, all known factors in the measurement system must be considered, including equipment characteristics, quality and magnitude transfer of calibration data, known or closest distribution functions, and measurement procedures. Wait. When the main input in the synthetic standard uncertainty varies significantly with frequency, the uncertainty should be calculated for different frequency bands.
[Remarks] column description:(1) Receiver readings may vary due to measurement system instability and receiver noise. The measured value Vr is obtained from the average of multiple readings of the receiver, which is the experimental standard deviation of the mean.
(2), (3) The attenuation value Lc between the receiver and the antenna, and the uncertainty of the free space antenna factor AF, can be obtained by a calibration report giving the extended uncertainty and the inclusion factor.
(4) The uncertainty of the receiver sine wave voltage accuracy correction value δVsw can be obtained from a calibration report giving extended uncertainty and inclusion factors. [Note: If the calibration report gives the receiver sine wave voltage accuracy within a certain limit (2 dB), then the correction value δVsw should be 0, with a rectangular distribution and a half-width of 2 dB. ]
(5) When the resolution bandwidth of the receiver or spectrum analyzer is smaller than the signal occupied bandwidth and needs to be corrected, the bandwidth correction factor is required, which also introduces the uncertainty component δVsel. For example, when the resolution bandwidth of the RMS detector is only 10% of the bandwidth occupied by the CDMA signal under test, the correction factor is 10 dB. If the maximum allowable error of the resolution bandwidth is 10%, the uncertainty of the correction factor is 0.5 dB. .
(6) If the monitoring antenna is closer to the transmitting antenna and the bottom noise of the test receiver is much lower than the signal level, then the influence of the bottom noise can be ignored, otherwise the influence on the measurement result should be considered.
(7) Mismatch: Refer to ETSI TR 100028 for details.
(8) If the antenna calibration report only gives antenna factors at several reference frequency points, then the interpolation method is used to derive the antenna factors at the remaining frequency points. The thus calculated antenna factor uncertainty is related to the interval between the reference frequency points and the characteristics of the antenna factor as a function of frequency. If the calibration report gives an "antenna factor-frequency" plot, the antenna factor at each frequency point can be visually read.
The antenna factor frequency interpolation error has a correction value of zero and a rectangular distribution with a half-width of 0.3 dB. If the calibration report directly gives the antenna factor at the measurement frequency point, then the uncertainty of the antenna factor need not be considered.
(9) The antenna element height deviation of a single dipole antenna is different from that of a composite antenna. The antenna factor uncertainty of a single dipole antenna already includes a high degree of influence; the antenna element height deviation correction value δAFh of the composite antenna should be zero, have a rectangular distribution, and the half width can be characterized by a biconical antenna and a logarithmic period antenna. inferred.
(10) The calibration report generally only gives the antenna factor of the antenna in the main direction. For the composite antenna, the %E must also be considered.
Liquid Crystal Display,Lcd Screen Displays,Calculator Lcd Display,Lcd Display For Car Bluetooth
Dongguan Yijia Optoelectronics Co., Ltd. , https://www.everbestlcdlcms.com