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Standard of the Ministry of Electronics Industry of the People's Republic of China Antenna Test Method
Evaluation of Antenna Test Site
This standard applies to the evaluation of antenna test sites. 1 Overview
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After the antenna pattern test site is designed, constructed and installed, the state of the irradiated electromagnetic field in the test area, that is, the area where the antenna to be tested is installed, must be determined by experimental methods.
1.1 Purpose of evaluation
The test site should be evaluated according to the experimental evaluation outline to determine whether the error generated by the measurement using the test site is less than the specified value. The degree of experimental evaluation depends on the accuracy required for the measurement on the test site. 1.2 Factors affecting the irradiated field
Due to the reflections caused by various installation components, cables, obstacles on the surface or near the test site, and the uneven height of the test site itself, the irradiated field of the test area will deviate from the calculation results based on the ideal geometric shape of the test site. In addition, radio frequency interference is often the cause of deviation.
1.3 Classification of incident field in the measured area
For convenience, the incident field is usually divided into two parts for study. The field incident from the normal direction of the source antenna is called the near-axis incident field. The field arriving from the wide-angle direction is called the wide-angle incident field. The wide angle is relative to the line connecting the center of the measured area to the phase center of the source antenna. The near-axis incident field can be measured with a field detector on a plane perpendicular to the test field axis and coinciding with the expected antenna under test. This plane is called the measured aperture.
2 Measurement with a field detector on the measured aperture
2.1 Calibration of the locator
Before measuring with a field detector, the axes of the locator must be calibrated. For example, the vertical axis (the axis of the azimuth-pitch locator or the axis of the pitch-azimuth locator including the model tower) can be calibrated with an inclinometer or a precision level, and a plumb bob can also be used for the model tower. The horizontal axis can be calibrated optically, and for the model tower, an observation device can be installed on the top of the tower. In this method the optical line of sight of the observation device corresponds to the horizontal axis (axis) of the model tower, so that the degree of calibration can be verified. 2.2 Installation of the detector
After the axes have been calibrated, the installation of the field detector can be carried out. The field detector consists of an antenna, which is often a pyramidal horn or a log-periodic antenna. It is mounted on a bracket or slide rail that allows it to move along the I-beam support structure. The entire assembly is mounted on a suitable positioner so that the human field can be sampled as a function of the position on the measured aperture. A typical field detector sketch is shown in Figure 1.
Promulgated by the Ministry of Electronic Industry of the People's Republic of China on January 5, 1985 and implemented on July 1, 1986
2.3 Control of detector movement
Aluminum tube bracket
Drive motor
And synchronous housing
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Horn rotator
Detector antenna and mixer
Detector bracket
RF cable
RF cable pulley
Upper azimuth turntable
Constant tension cable
Constant tension cable tensioning wheel
Figure 1 Typical field detector mechanism
The movement of the detector antenna can be remotely controlled by a synchronization system or potentiometers. They indicate the instantaneous position of the detector. The received field amplitude can be plotted continuously and automatically as a function of position. These data are usually expressed in decibel curves. 2.4 Requirements for detector antennas
The detector antenna should have appropriate directivity in order to suppress wide-angle reflections and reflections from the detector mounting structure. As an additional precaution, an absorbing baffle should be placed behind the detector antenna. In order to include a major part of the test site surface within the half-power beamwidth of the detector antenna, its E-plane and H-plane beams should have appropriate widths. A common criterion for ensuring that this condition is met is: 4hr
,2arctg(
wherein: hr-
height of the antenna under test;
-length of the test field;
9-3dB beamwidth of the detector antenna. 2.5 Rotation of the detector antenna
(1)
It is necessary to have means to rotate the detector antenna about its axis so that two orthogonal polarizations, vertical and horizontal, can be measured. This is necessary because the test field may be used in either of these two polarization states, so the human field should be measured for both cases. It is practical to also measure the cross-polarization of the source antenna for both polarization orientations, because not only may the source antenna itself produce cross-polarization components, but also when the wave passes from an irregular object or Reflection on curved objects is often accompanied by a certain degree of depolarization. 2.6 The main task of detector measurement
The main task of detector measurement is to determine the source of reflection so that remedial measures can be taken. 2.6.1 Case of a single reflection source
If there is only one main reflection source affecting the incident field, then it is a relatively simple matter to determine its position based on the measurement data. 2.6.1.1 Spatial period of the interference pattern
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Assuming that the direct wave is incident on the aperture from a direction perpendicular to the aperture plane, and the reflected wave is incident on the aperture from a direction at an angle relative to this direction, it can be seen from Figure 2 that an interference pattern is obtained on the measured aperture. The spatial period of the composite wave is: P
ER wavefront
Ep wavefront
2 E ripple
Figure 2 Spatial cross-sectional diagram caused by reflected wave
（2）
If the detector antenna moves along a straight line within the measured aperture, that is, the intersection of the plane consisting of the propagation directions of the direct wave and the reflected wave and the plane containing the measured aperture, the field structure shown in Figure 2 is obtained. However, if the detector moves along any other direction within the measured aperture, the spatial period increases, and its value is:
Where: a-
singcosa
The angle between the detection path and the new direction obtained in the result of Figure 2. 2.6.1.2 Determination of the arrival direction of the reflected signal (3)
From the above results, it can be seen that by measuring the field strength along the radial line passing through the center of the measured antenna aperture, the direction of arrival of the reflected signal (i.e. angle) can be determined.
2.6 .1.3 Relative amplitude of reflected wave to direct wave The relative amplitude of reflected wave ER to direct wave Ep can be obtained from the value of the bee in the interference pattern. When expressed in decibels, the ratio is: ER
=201g[
-1+antilgo/20
1+antilgo/20
where is the difference between the maximum and minimum values of the measured interference pattern expressed in decibels. The relationship between Ep
and 0 is shown in Figure 3.
（P）
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Ep(dB)
Figure 3 Relationship curve between the amplitude of spatial interference pattern and the ratio of reflected signal to direct signal intensity 2.6.2 Multiple reflection source situation
Generally, there are several reflection waves, among which the ground reflection wave is the main one. In this case, the measured field is a composite spatial distribution. However, the position of the main reflection source can usually be determined based on the measured data. 3 Measurement of the incident field near the site axis of an elevated test site 3.1 Adjustment of the diffraction fence with the field detector
For an elevated test site, if the amplitude variation is too large, some measures should be taken to either absorb the reflected energy or to divert it away from the measured aperture. Surface diffraction fences are usually very effective for test site ground reflections. The position, size and direction of the fences can be adjusted with the field detector.
3.2 Aiming the source antenna with the field detector
3.2.1 Necessity of aiming the source antenna
Another important use of the field detector is the aiming of the source antenna. Since the source antenna is directional, any aiming error may produce an asymmetric amplitude taper of the field illuminated by the measured aperture. Aiming is performed in both the azimuth and elevation planes, which correspond to the horizontal and vertical movements of the field detector antenna, respectively. 3.2.2 Aiming steps
A convenient aiming method is described below. 3.2.2.1 Step 1
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First, move the detector antenna in the horizontal direction and determine the positions of two equal power points as references. Then adjust the source antenna until the geometric center of these two equal power points coincides with the center of the measured aperture. 3.2.2.2 Step 2
Rotate the source antenna 180° around its axis and check the aiming again. If the source antenna is still aimed, it means that the beam axis of the source antenna coincides with the rotation axis of the source antenna locator. Otherwise, corrective measures must be taken, that is, the source antenna must be redirected relative to the locator mounting surface. If the source antenna is a reflector antenna, the primary feed source must be repositioned. Repeat the above process until the beam of the source antenna is symmetrical relative to the rotation axis and is properly aimed relative to the measured aperture. 3.2.2.3 Step 3
Repeat the process of step 1 in the vertical plane. 3.2.2.4 Step 4
Repeat the process of step 2 in the vertical plane. 3.3 Measurement of relative phase at the measured aperture
After the influence of reflection is reduced to a satisfactory amplitude distribution at the measured aperture and the source antenna is well aimed, the relative phase at the measured aperture needs to be measured. If there is no reflection, the phase distribution at the measured aperture is mainly a function of the distance between the source antenna and the measured antenna. Therefore, for the elevated test field, this measurement is a precautionary measure rather than a necessity. 3.4 Measurement of polarization at the measured aperture
The remaining measurable characteristic of the human-radiated field is the polarization characteristic. If the field detector antenna is equipped with a rotator as shown in Figure 1, the polarization pattern at various locations on the measured aperture can be measured. Note that the source antenna should be aimed before making this measurement. 4 Measurement of incident field near the axis of ground reflection test site 4.1 Adjustment of source antenna height
Unlike the elevated test site, the design principle of the ground reflection test site is to use the reflection of the test site surface to produce a wide interference pattern in the elevation plane. The position of the source antenna should be adjusted so that the maximum value of the interference pattern is aligned with the center of the measured aperture. The method is to first place the source 4h. Then place the antenna at a certain position along the elevation plane with the measured aperture AR
(assuming that the reflection coefficient of the test site surface is -1), that is, the intersection line of h, = is measured with a field detector. Adjust the height of the source antenna until the field in the vertical direction is symmetrical relative to the center of the measured aperture. Since the optimal height may be different for each polarization, this process should be performed for both polarizations. For the ground reflection test site, it is usually not necessary to correct the orientation of the source antenna in the elevation plane, because a slight change in the direction of the source antenna beam in the elevation plane has little effect on the required interference pattern generated at the measured aperture. In the horizontal plane, it can be aimed in a similar way to the elevated test site. 4.2 Determine the location of additional reflections
As in the case of the elevated test field, the aperture to be measured should be probed to determine the location of the additional reflection source. If necessary, the reflecting object should be removed, or the reflected energy reaching the aperture to be measured should be minimized by using absorbing baffles and reflecting screens. 4.3 Polarization measurement at the aperture to be measured
For polarization measurements, the field detector must be oriented so that the linearly polarized detector antenna points to the phase center of the array formed by the source antenna and its mirror image. The phase center is not on the surface of the test field but at an approximate height h (see SJ2534.285 "Antenna Test Method Antenna Test Field Design"), h, is: (5)
Where, "—the amplitude of the reflection coefficient of the test field surface." "" can be simply taken as the ratio of the amplitude of the mirror-type reflected wave on the test field surface to the amplitude of the direct wave, and the ratio is shown in Figure 3. 5
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After determining this orientation, the polarization pattern at various locations within the measured aperture can be measured. 4.4 The case where the reflection test field is used for circular polarization If the ground reflection test field is used for circular polarization, it is hoped that the polarization of the source antenna is adjustable so that the polarization of the human radiation field can be precisely adjusted. By changing the relative amplitude and phase of the vertical component and the horizontal component of the source antenna, the The effect caused by the different reflection coefficients of the test field surface for the two field components. By emitting an appropriately elliptically polarized wave, the axial ratio of the incident field at any location at the measured aperture can be adjusted to better than 0.10 dB. Note that the characteristics of the test field surface vary with time and operating frequency. 5 Measurement of wide-angle incident field
5.1 Overview
The two most commonly used methods for wide-angle incident field identification are the antenna pattern comparison method and the longitudinal field detection method. Both methods are applicable to elevated test fields and ground reflection test fields.
5.2 Antenna pattern comparison method
5.2.1 The essence of the antenna pattern comparison method
This method is based on the following assumption: When there are no reflected signals or additional signals, small changes in the position of the antenna under test relative to the source antenna will result in a small change in the position of the antenna under test relative to the source antenna. The change will not change the measured azimuth radiation pattern of the antenna under test. On the other hand, if the radiation pattern of the antenna under test changes with the position when it is measured at several different positions of the antenna under test, it indicates the presence of reflected signals or additional signals. 5.2.2 Effect of reflected waves on sidelobe levels
To illustrate the effect that reflected waves may have on the measured radiation pattern of a directional antenna, consider the situation shown in Figure 4. The reflected wave is emitted from a direction that deviates from the beam axis of the antenna under test by 6 degrees. Usually, the wide-angle reflected wave level is at least 30 dB lower than the direct wave level. When the antenna lobe under test is pointed at the source antenna, the reflected wave is received on the sidelobe. The effect of the reflected wave on the measured mainlobe level can be ignored. However, if the antenna is rotated so that the main lobe points to the direction where the reflected wave arrives and the sidelobe points to the source antenna, the apparent sidelobe level is the same as that obtained when there is no reflected wave. The sidelobe levels measured are significantly different. The actual difference depends on the relative amplitude and phase of the direct wave and the reflected wave. Therefore, if the azimuth pattern of the antenna is measured at several positions on the axial direction of the test field, the measured patterns will be different due to the different relative path lengths of the two waves.
5.2.3 Specific implementation method
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a Test antenna pointing to the source
b Sidelobe pointing to the source
Figure 4 Antenna pattern comparison method How the sidelobe level of the antenna under test is affected during the measurement process
5.2.3.1 Method -
The antenna pattern comparison method is to record the azimuth pattern of the antenna under test at a sufficient number of different positions on the axial direction of the test field to obtain the maximum deviation of the sidelobe level. For convenience, all the patterns can be recorded on the same graph to determine the apparent direction of the incoming wave and its relative level. A measured example is shown in Figure 5. The level variation of these patterns over an azimuth of 120° is about 12 dB. Since the reflected wave is received in the main beam and the direct wave is received in the side lobe, 30 dB below the main beam peak, this level variation corresponds to a level of at least -34.5 dB for the reflected wave relative to the direct wave. The reflected signal level is determined from the measured data according to the curve shown in Figure 6. Many measurements must be made to ensure that the maximum variation is obtained, but the worst case may be missed. 5.2.3.2 Method 2bZxz.net
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?=Azimuth pattern angle 360
Figure 5 Comparison of azimuth patterns (360° cut), the longitudinal displacement of the rotation center is stepped by 100
(EP)
05-10-15-2025-30354045-50-55606570 Corresponding to the pattern level without reflection (dB) Figure 6 Space of given reflectivity level and antenna pattern level and amplitude of the involved pattern Note: When the ordinate value is lower than 5.7dB, the curve is treated as a straight line, which is actually slightly concave upward. When the ordinate value is above 5.7dB, all curves are corrected.
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This antenna pattern comparison method measures two azimuth patterns with the same center of rotation but with the antenna under test rotated 180° around the beam axis between the two cuts. For convenience, the direction of the horizontal axis of the chart recorder can also be reversed and the synchronizer adjusted appropriately so that the patterns recorded by the two cuts are the same when there is no reflected signal or additional signal. If reflections are present, the patterns will be different unless the reflections are symmetrical about the axis of the test field. These data can be used to infer the apparent direction of the reflected signal incident on the antenna under test. 5.2.3.3 Method 3
In some test fields, especially in the case of model towers, it is desirable to make equal conical cuts in azimuth directions symmetrical about the test field axis to compare the patterns. For example, rotate the azimuth locator by 6 degrees and rotate the model tower head to make the conical cuts. Then turn the azimuth locator to degrees and rotate the antenna under test 180° around the axis of the tower head to keep the starting point the same. After properly retuning the synchronizer, remeasure the directional pattern and compare them.
5.3 Longitudinal field detection method
Another method for determining the reflection level and direction is to use the longitudinal field detection method. 5.3.1 The essence of the longitudinal field detection method
Unlike the field detectors mentioned above, the longitudinal field detector is oriented so that its beam axis is perpendicular to the direction of movement. The moving path of the detector antenna must be long enough to detect the maximum and minimum points of the interference pattern formed by the direct wave and the reflected wave. 5.3.2 Distance between constructive interference points
The approximate distance P between the constructive interference points between the direct wave and the reflected wave is: Pi=
2sin2(0/2)
Where: - The angle between the direction pointing to the source antenna and the direction of arrival of the reflected wave. 5.3.2.1 Determination of the direction of arrival of the reflected wave
From the above formula, it can be seen that by measuring the amplitude change period of the received signal, the approximate direction of the source of the reflected signal can be determined9.5.3.3 Specific implementation method
(6)
The level of the reflected signal relative to the direct signal can be approximately determined based on the peak-to-peak variation of the measured data. The radiation pattern of the detector antenna should be taken into account when analyzing the data. In order to perfectly determine the reflected signal or the additional signal, it is usually necessary to carry out longitudinal measurements with the detector in several different azimuth directions.
6 Identification of anechoic chambers
6.1 Quiet zone
The design goal of an anechoic chamber is to make the uniformity of illumination in a given area of the room meet the specified indicators. This area is usually called the quiet zone, and the antenna under test is placed in this area. 6.2 Quietness
The actual size of the quiet zone can be specified according to the direct wave emitted by the source antenna, that is, it is determined based on the technical indicators of the change in the amplitude and phase of the direct wave from the center of the quiet zone to the edge of the zone. This is because the "quietness" of this area depends on the reflection size of the indoor walls, floor and ceiling. At present, the quality factors of anechoic chambers have not been standardized. It is necessary to determine the ratio of the "equivalent" reflected wave to the direct wave. The so-called equivalent reflected wave is the combined effect of all reflected waves incident on the detector antenna used to measure the echo-free chamber. This means that the radiation pattern of the detector antenna will undoubtedly affect the measurement results.
6.3 Method for identifying the free space standing wave ratio in an echo-free chamber 6.3.1 Transverse detection method
When the antenna radiation pattern is measured in an echo-free chamber, it is usually identified by the free space standing wave ratio method. 6.3.1.1 Orientation of the detector antenna
This method is similar in principle to the field detector method mentioned above, except that the orientation of the detector antenna is not always perpendicular to the direction of movement, but is made adjustable as shown in Figure 7, which prepares the conditions for selecting a more directional antenna as the detector. Standard gain antennas are often used for this purpose.
6.3.1.2 Determination of the direction of the equivalent reflected wave
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For a given orientation, the detector antenna is continuously moved along a transverse path. Other directions of movement may also be selected. 1 Transverse movement of the detector antenna
|Figure 7 Geometric relationship of the free space standing wave ratio method The linear movement of the antenna is coupled to the recorder to record the interference pattern. As long as the received direct wave component is greater than the reflected wave component, the ratio of the reflected wave to the direct wave can be determined using the curve in Figure 3. This process is repeated for various orientations of the detector antenna. The direction with the largest ratio is the direction of arrival of the equivalent reflected wave. Measurements should be made in several different transverse directions, including horizontal and vertical directions. All measurements should also be made with horizontal and vertical polarizations.
6.3.2 Longitudinal detection method
Wide-angle human radiation, including signals reflected from the back of the echo-free chamber, can be determined using a longitudinal detector (Clause 5.3). This measurement is made with the detector bracket parallel to the anechoic chamber and the detector moved in the longitudinal direction. The free space standing wave ratio is measured as a function of the orientation of the detector antenna relative to the direction of movement. The entire rear portion of the anechoic chamber can be characterized in this way, including critical points such as corners. 6.3.3 Selection of detector antenna
6.3.3.1 Omnidirectional antenna
As pointed out earlier, the measurement results are related to the detector antenna radiation pattern. To avoid this effect, an isotropic detector antenna can be used, such as a tri-pole antenna consisting of three mutually orthogonal dipoles. This antenna is designed in such a way that the antenna pattern is essentially isotropic. Antennas have been designed that respond equally well (within ±1 dB) to signals coming from any direction and of any polarization. The bracket used to hold the detector should be designed very carefully to avoid causing additional reflections in the chamber. The advantage of this detector is that only three orthogonal scans are required for each required frequency to obtain complete reflectivity data. In general, since the equivalent reflected wave is the sum of all reflected waves from all surfaces, the reflectivity level produced by this system is higher than that obtained with a directional antenna. This method is particularly useful when the antenna to be tested in the room is quasi-isotropic. 6.3.3.2 Directional Antennas
If the directivity of the antenna under test is moderate or relatively strong, the reflections at the rear of the room should be suppressed. In this case, it may be more appropriate to use a directional detector to measure the free space standing wave ratio. 6.4 Pattern Comparison Method for Identifying Anechoic Chambers Antenna pattern comparison has been used to identify anechoic chambers. This method is not recommended as the primary method for identifying anechoic chambers because it is difficult to determine the maximum reflectivity level.
7 Identification of Reduced Range Test Sites
A reduced range test site can be measured using the method of Chapter 2, i.e., using a field detector to make amplitude, phase and polarization measurements of the irradiated field in the area under test.
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Additional Notes:
This standard was proposed by the Standardization Research Institute of the Ministry of Electronics Industry. This standard was drafted by the 39th Institute of the Ministry of Electronics Industry. The main drafters of this standard are Ke Shuren, Dong Dunchang and Wang Shuhui. 114 Pattern comparison method for identifying anechoic chambers Antenna pattern comparison method has been used to identify anechoic chambers. This method is not recommended as the main method for identifying anechoic chambers because it is difficult to determine the maximum reflectivity level.
7 Identification of reduced-range test field
The reduced-range test field can be measured using the method in Chapter 2, that is, using a field detector to measure the amplitude, phase and polarization of the irradiated field in the measured area.
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Additional notes:
This standard was proposed by the Standardization Research Institute of the Ministry of Electronics Industry. This standard was drafted by the 39th Institute of the Ministry of Electronics Industry. The main drafters of this standard are Ke Shuren, Dong Tunchang and Wang Shuhui. 114 Pattern comparison method for identifying anechoic chambers Antenna pattern comparison method has been used to identify anechoic chambers. This method is not recommended as the main method for identifying anechoic chambers because it is difficult to determine the maximum reflectivity level.
7 Identification of reduced-range test field
The reduced-range test field can be measured using the method in Chapter 2, that is, using a field detector to measure the amplitude, phase and polarization of the irradiated field in the measured area.
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Additional notes:
This standard was proposed by the Standardization Research Institute of the Ministry of Electronics Industry. This standard was drafted by the 39th Institute of the Ministry of Electronics Industry. The main drafters of this standard are Ke Shuren, Dong Tunchang and Wang Shuhui. 11
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