Definition
Radio measurement refers to the measurement of the performance of the radio communication system and its equipment, components and components using electronic technology. The frequency range involved in the measurement can range from extremely low frequency 3Hz to high frequency 3THZ; the power level involved can range from 10-15W (-120dBm) to 108W. The measured parameters include frequency, power, attenuation, impedance, standing wave, field strength, phase, waveform and data. The instruments used include signal generators, frequency meters, power meters, impedance and standing wave measuring instruments, field strength meters, spectrum analyzers, test receivers, oscilloscopes, network analyzers, integrated circuit testers, etc., as well as measuring data signal parameters. Kind of instrument. Modern measuring instruments have multi-octave, multi-range, and multiple automation functions, and have high accuracy. For example, the atomic frequency standard can reach an accuracy of 10-14 orders of magnitude.
Characteristics
The difference between radio measurement and wired electricity measurement The object of radio measurement is a space-based system, while wired electricity is a space-enclosed system. Does it stimulate free space? The electromagnetic field is the biggest difference between wireless and wired systems. The part that highlights this feature is the antenna.
The difference between distributed parameters and lumped parameters
The antenna system of radio communication is a device that performs transmission mode conversion between the radio frequency signal of the transceiver and the electromagnetic wave in space. Spatial matching, so it must be distributed parameter devices (such as surface antennas and horn feeds), or lumped devices with obvious characteristics of distributed parameters (such as wire antennas, but the deformation of the distributed parameter device of the transmission line whose length can be compared with the wavelength) The characteristic lies in the existence of the wave to determine its electrical characteristics. The lumped parameter device uses electrostatic field, static magnetic field, electromagnetic, electrothermal conversion and on-off, and the existence of the control relationship determines its function, which is artificially strengthening its characteristics in a certain direction. The most obvious is the capacitor formed by overlapping multilayer electrostatic fields and the inductor formed by the static magnetic field coupling multiple loop currents in the same direction. When the operating frequency is increased to the size of the capacitor and the inductor, the size can be compared with the wavelength At the same time, the nature of the distributed parameters (ie the nature of the inductance and capacitance) will be revealed. This must be paid attention to in the radio measurement.
Strictly ensure the impedance matching of the high-frequency circuit to make the standing wave ratio Close to 1. Impedance mismatch will affect the measurement accuracy and communication efficiency, will interrupt the communication, and even burn the antenna and the transmitting system. The matching requirement is no exception for the low-frequency circuit.
The instrument interface is very Less use of double wires
In order to resist interference and realize the needs of wide-band measurement, radio measuring instruments mostly use coaxial interfaces. The upper frequency limit of the coaxial cables and connectors used by the instruments is up to 18GHZ, 2.4mm coaxial The system can reach 40GHz. If the frequency is higher than 18GHz, a waveguide connection is required. The characteristic impedance of the coaxial line is 75Ω below 300MHz, 50Ω below 18GHz, and 75Ω or 50Ω is optional below 1GHz. Between the communication system and the measuring instrument When the impedance of the interface is different, you can use the corresponding impedance converter.
Notes
Radiation interference, electric wave leakage, shielding grounding and coupling effects will affect the accuracy of the measurement. Sometimes It can’t even be measured. This is a problem that must be fully paid attention to in radio measurement.
(1) Radiation interference: Sky electricity, cosmic rays and various electrical equipment from space will produce different forms and different frequencies And radiation interference of different powers. Many parts of the communication equipment other than the measured point may produce radiation interference. The interference intensity may be so large that the measuring instrument is blocked. Appropriate measurement probes, effective measurement techniques, and good shielding and grounding can be used. Reduce the influence of interference and obtain usable measurement results.
(2) Shielding and grounding: Both the equipment under test and the measurement system should have good shielding. If necessary, the measurement should be carried out in a microwave dark room to make it open The system is close to a closed system. The transceiver level of wireless communication is very different. Use a coaxial measurement probe and ground at a suitable ground point to reduce the error caused by high magnification of the interference signal generated by the ground current in the ground circuit. .
(3) Coupling effect: the existence of the measurement system sometimes changes the working state of the communication system and Cause errors. The instrumentation equipment used to test the antenna sometimes destroys the distribution of the antenna's transmitting field. The measuring probe becomes a comparable load on the communication circuit. The probe may become a part of the oscillation circuit and affect the frequency and power. The higher the frequency, the more serious these problems will be. Sometimes the communication system cannot work normally because the measurement may also destroy the original shielding state. These problems must be properly handled during the measurement process.
Measurement of electromagnetic field
The surrounding area of the radiation source can be divided into three fields. Within one wavelength, the near field reactance is dominant, and the field strength is inversely proportional to the higher power of the distance. The electric field in the near field is measured with a dipole antenna and corresponding detection and receiving instruments, and the magnetic field is measured with a loop antenna. More than 10 wavelengths are the radiation far area of the small antenna, the reactance field has no effect, and the radiation field strength is inversely proportional to the first power of the field source distance. Between the reactance near field and the radiated far field is the radiated near field, or midfield. When three times the wavelength, the radiation field is about 25dB greater than the field strength of the reactance field. In addition to measuring the radiation field, usable data can be obtained.
Measurement automation and interface bus
The combination of measurement technology, microelectronics technology and computer technology makes automated measurement rapidly popular. The entire system under test can automatically determine various parameters by the computer within a few minutes. In order to cooperate with automated measurement, the interfaces between the measurement system and the system under test, between various instruments in the measurement system, and between them and the computer system are tending to be standardized. The International Electrotechnical Commission (IEC) developed the IEC-625 standard. my country has also set it as a national standard (GB249.1 ~ 249.2-85), which is the most widely used interface bus standard. The Vxi interface bus, which appeared in 1987, is small, flexible, and has a maximum data transmission rate of up to 40 times that of the above-mentioned system. It has been popularized and used by various countries and will be set as a standard.
Analog measurement
Analog measurement is mainly to simulate the change of environmental parameters such as temperature, humidity, pressure, etc., using convenient frequency and power to simulate practical frequency and power, using known Linear and non-linear transformation relations are commonly used methods to predict the performance of communication systems. For example, if the antenna size is reduced in the same proportion and the test signal frequency is increased correspondingly, the test data can be obtained more conveniently. The life of the equipment can also be simulated. It is common to simulate various measurements with computers, but any simulation is not real. In particular, the factors that affect the life span are complex. Although increasing the operating temperature, operating voltage, and increasing the load can accelerate the aging, the time effect is still difficult to fully simulate. Practical measurement is still necessary.
Remote sensing measurement
Remote sensing measurement is mainly used for unattended communication stations or dedicated long-distance measurement. Some stations also need to transmit measurement data to the central console, and can transmit information and monitoring data through the original channel or a dedicated measurement channel at regular intervals according to a prescribed procedure. Through radio transmission, various remote sensing and telemetry can be carried out on airplanes and satellites to obtain fairly accurate data or images.
Labor protection radio measurement
The human body is damaged in the long-term or short-term strong electromagnetic field. Corresponding safety standards are stipulated at home and abroad. The radiation safety standard implemented in my country is that the power density should be less than 0.038mW/cm2 (E≤12V/m). The electromagnetic radiation of some powerful transmitting stations, including extremely low frequency stations and satellite communication earth stations, in the near-field area obviously exceeds this standard. Therefore, it is necessary to isolate the dangerous area and take effective protective measures. Wear protective clothing and protective caps when necessary. To measure the field strength, a complex field meter or a simple handheld radiometer can be used.
Electromagnetic environment measurement
Electromagnetic environment measurement refers to the measurement of electromagnetic environment that needs to be carried out when building a new radio station, including the field strength of various radiated interference and the required main The strength of the signal and the fading of the radio waves, etc., so that the station is set in a good electromagnetic environment is a guarantee for good communication quality. In addition, the measurement of urban electromagnetic environment is also better than the scope of environmental protection measurement. A good electromagnetic environment can ensure the normal operation and reception of various communications, broadcast and television services. Supervise and manage the abnormal radiation of various electrical equipment.