Antenna test method introduction

About the near field and the far field
Radio waves should be called electromagnetic waves or EM waves for short, because radio waves contain electric and magnetic fields. The signal from the transmitter and via the antenna will generate an electromagnetic field. The antenna is the converter and interface for the signal to free space.
Therefore, the characteristics of the electromagnetic field vary depending on the distance from the antenna. The variable electromagnetic field is often divided into two parts-the near field and the far field. To clearly understand the difference between the two, it is necessary to understand the propagation of radio waves.
electromagnetic wave


Figure 1 shows how a typical half-wave dipole antenna generates electric and magnetic fields. The retransmitted signal is modulated into a sine wave, and the voltage changes in polarity. Therefore, an electric field is generated between the antenna elements, and the polarity changes every half cycle. The current of the antenna element generates a magnetic field, and the direction changes every half cycle. The electromagnetic fields are orthogonal to each other at right angles.
1. The electromagnetic field surrounding a half-wave dipole includes an electric field (a) and a magnetic field (b). The electromagnetic fields are spherical and at right angles to each other.
The magnetic field next to the antenna is spherical or arc-shaped, especially the magnetic field close to the antenna. These electromagnetic fields are emitted from the antenna, and the more they are outward, the less obvious, and the characteristics gradually tend to be flat. The receiving antenna usually receives plane waves.
Although the electromagnetic field exists around the antenna, they will expand outward (Figure 2). After going beyond the antenna, the electromagnetic field will automatically break away as an energy packet and propagate independently. In fact, electric and magnetic fields are generated by each other, and such "independent" waves are radio waves.


2. Within a certain range from the antenna, the electric and magnetic fields are basically flat and intersect at right angles. Note that the propagation direction and the electromagnetic field are at right angles. In Figure (a), the propagation direction and the electromagnetic field line direction are orthogonal, that is, the vertical paper faces inward or outward. In the figure (b), the magnetic field lines are perpendicular to the paper surface outwards, as shown by the circles in the figure.
near field
There seems to be no formal definition of the near field, it depends on the application itself and the antenna. Generally, the near field refers to the distance from the antenna to 1 wavelength (λ). The unit of wavelength is meter, and the formula is as follows:
Λ= 300/fMHz
Therefore, the calculation method of the distance from the antenna to the near field is as follows:
Λ/2π = 0.159λ
Figure 3 shows the radiated sine wave, the near field and the far field. The near field is usually divided into two regions, the reaction zone and the radiation zone. In the reaction zone, the electric and magnetic fields are strong and can be measured separately. Depending on the type of antenna, a certain field will become dominant. For example, a loop antenna is mainly a magnetic field. The loop antenna is like the primary of a transformer because it generates a large magnetic field.

3. The boundary between the near field and the far field, and the wavelength of the operating frequency band are shown in the figure. The antenna should be located where the sine wave starts on the left.
In the radiation zone, the electromagnetic field begins to radiate, marking the beginning of the far field. The strength of the field is inversely proportional to the distance of the antenna (1/r3).
The transition zone shown in Figure 3 refers to the part between the near field and the far field (some models do not define the transition zone). In the figure, the far field starts at a distance of 2λ.
far field
is similar to the near field, and there is no uniform definition of the beginning of the far field. Some think it is 2 λ, some insist that it is 3 λ or 10 λ away from the antenna. There is another saying that is 5λ/2π, and others think that the distance should be 50D2/λ based on the size D of the antenna.
Some people think that the boundary between the near field and the far field starts at 2D2/λ. It is also said that the far field starts at the place where the near field disappears, which is λ/2π mentioned above.
The far field is a real radio wave. It travels in the atmosphere at a speed of 300 million meters per second, which is close to 186,400 miles per second, which is equivalent to the speed of light. The electric and magnetic fields support and generate each other, and the signal strength is inversely proportional to the square of the distance (1/r2). Maxwell described this phenomenon in his formula.
Maxwell's equations
In the late 1870s, before the invention of radio waves, Scottish physicist James Clark Maxwell predicted the existence of electromagnetic waves. He synthesized the laws of Ampere, Faraday, and Ohm, and formulated a set of equations to express how electromagnetic fields generate and propagate, and concluded that electric and magnetic fields are interdependent and support each other. In the late 1880s, German physicist Heinrich Hertz proved Maxwell's theory of electromagnetic fields.
Maxwell created four basic equations to express the relationship between electric field, magnetic field and time. The electric field generates mobile charges, that is, electric current, over time, thereby generating a magnetic field. Another set of methods is to say that a changing magnetic field can generate an electric field. The electromagnetic waves emitted by the antenna propagate by themselves in space. This article does not list these equations, but you should remember to include some different equations.
application
The change in the intensity of the far field propagating in space is determined by the Friis formula:
Pr = PtGrGtλ2/16π2r2
In the formula, Pr = received power; Pt = transmit power; Gr = receive antenna gain (power ratio); Gt = transmit antenna gain (power ratio); r = distance to the antenna. The formula is applicable in unobstructed open spaces within sight.
There are two issues to discuss here. The received power is inversely proportional to the square of the distance r, and proportional to the square of the wavelength, that is, electromagnetic waves with longer wavelengths and lower frequencies travel farther. For example, with the same power and antenna gain, a 900MHz signal will travel farther than a 2.4GHz signal. This formula is also often used to analyze the signal strength of modern wireless applications.
In order to accurately measure the propagation of the signal, it is also necessary to understand the radiation pattern of the antenna in the far field. In the near-field reaction zone, the receiving antenna and the transmitting antenna may interfere with each other due to the coupling of capacitance and inductance, resulting in erroneous results. On the other hand, if there is a specific measuring instrument, the radiation pattern in the near field can be accurately measured.
Near field is also very useful in the field of communication. The near field mode can be used for radio frequency identification (RFID) and near field communication (NFC).
RFID is the electronic version of the barcode. It is a thin label with a chip inside. The chip integrates storage and a specific electronic code, which can be used for identification, general or other purposes. The tag also contains a passive transceiver. When approaching the "reader", the strong RF signal sent by the reader will be recognized by the tag. The antennas of the reader and the tag are both loop antennas, which are equivalent to the primary and secondary of the transformer.
The signal identified by the tag is rectified and filtered into direct current, which supplies energy for tag storage and forwarding. The transmitter sends the code to the reader for identification and processing. Active tags sometimes use batteries to extend the sensing distance beyond the near field. RIFD tags have different frequency ranges, 125kHz, 13.56MHz and 900MHz.
At 900MHz, the wavelength is:
Λ= 300/fMHz
Λ= 300/fMHz
Λ = 300/900 = 0.333 meter or 33.33 cm
Λ = 300/900 = 0.333 m or 33.33 cm
Therefore, according to the near-field distance calculation formula:
Λ/2π = 0.159λ = 0.159(0.333) = 0.053 meter (about 2 inches)
Λ/2π = 0.159λ = 0.159(0.333) = 0.053 m (about 2 inches)
The sensing distance usually exceeds this number, so the distance actually extends to the far field at this frequency.
NFC also uses storage and specific codes similar to credit cards. The battery-driven internal transponder can transmit the code to the reader. NFC also uses the near field, generally in the range of a few inches. The frequency of NFC is 13.56MHz, so the wavelength is:
Λ= 300/fMHz
300/13.56 = 22.1 meters or 72.6 feet
The near field distance shall not exceed:
Λ/2π = 0.159λ = 0.148(72.6) = 11.5 feet
Because of low power consumption, the actual sensing distance rarely exceeds 1 foot.
The principle and method of measuring reflection level in near-field working area
Reflection level test principle of near-field working area
The free space voltage standing wave ratio method is used to measure the reflection level of the near-field working area. The measurement principle is based on the presence of direct and reflected signals in the microwave anechoic chamber. The field strength at any point in the microwave anechoic chamber is the vector combination of the direct signal and the reflected signal. , A standing wave is formed in the space, and the magnitude of the standing wave value reflects the magnitude of the reflection level in the microwave anechoic chamber.

VSWR method measurement principle diagram
When the main lobe of the receiving antenna is aligned with the transmitting antenna, the received signal is ED. Move the receiving antenna, the relative phase of the direct signal ED and the reflected signal ER of the receiving antenna will change. At this time, the amplitude of the signal received by the receiving antenna will fluctuate. As shown in the figure, this fluctuation reflects the inherent standing wave in space. In this way, the reflection level can be obtained.
darkroom space standing wave diagram
When the receiving antenna is turned to an azimuth q lower than the level a (dB), the received direct signal Eq=ED10a/20. When the reflected signal and the direct signal are in phase, the combined field is represented by b

When the reflected signal and the direct signal are reversed, the composite field is represented by c:

is the reflection level:

Therefore, by measuring the space standing wave curve and the receiving antenna pattern, the reflection level of the microwave anechoic chamber can be calculated.
  testing method
In the near-field working area, test the absorption characteristics of specific frequency bands for the absorbing materials of the main reflecting wall.
Selection of test location
When testing the reflection level of the near-field working area, the transmitting antenna is first placed on the central axis of the darkroom, the receiving antenna is placed in a reasonable position facing the tested wall, and the reflection level is tested by moving a distance along the two antenna axes. The test location is shown in the figure.

Schematic diagram of near-field quiet zone test position (top view)

Schematic diagram of near-field quiet zone test position (side view)

Schematic diagram of test equipment connection
test steps
A) Connect the test system, and place the transmitting antenna and receiving antenna in the test position I according to Figure 2-5;
b) Set the signal source frequency to 1GHz, adjust the output power to an appropriate level to make the transmitting antenna radiate signals, and the receiving antenna is in the direction of the transmitting antenna, moving along the line to be tested, and record the received signal curve. The test curve is used as the line of travel The reference level line;
C) Turn the receiving antenna toward the direction of the wave-absorbing material of the tested wall, move the receiving antenna along this line of measurement, and record the spatial standing wave curve;
D) Change the antenna polarization mode and repeat the measurement of the above steps a) ~ c);
E) Repeat steps b) ~ d) at 2GHz, 5GHz, 10GHz, 18GHz, 40GHz frequency points, until all frequency points are measured;
F) Change the position of the transmitting antenna and the receiving antenna, as shown in Figure 2-3, respectively to position Ⅱ, position Ⅲ, repeat the above steps b) ~ e) measurement;
G) Change the height of the transmitting antenna and the receiving antenna, as shown in Figure 2-4, to H2 and H3 respectively, and repeat the measurement of the above steps b)~f).
  data processing

Far-field quiet zone amplitude uniformity test method
The uniformity of the amplitude of the far-field quiet zone refers to the change in the amplitude of the received signal when the transmitting antenna remains stationary and the receiving antenna moves along the specified travel line in the quiet zone.
During the amplitude uniformity test, the receiving antenna moves laterally along the stroke lines of different heights in the area shown in the figure, and the amplitude data of each position in the area is collected. After data filtering and processing, the amplitude of a circular plane in the quiet zone is obtained. Uniformity measurement results. By measuring multiple planes in the quiet zone, the amplitude uniformity test result of the entire quiet zone is obtained.
Schematic diagram of the test stroke line (the cross section of the quiet zone)
Test interval setting
The test interval distance of far-field amplitude uniformity is shown in the table:
table Selection of stroke line
interval category
test interval
test surface
0.5m
Test in-plane stroke line
0.2m
Schematic diagram of test equipment connection
test steps
A) Connect the test system, and place the transmitting antenna and receiving antenna in the test position I according to Figure 3-2;
B) Set the signal source frequency to 1GHz, adjust the output power to an appropriate level to make the transmitting antenna radiate signals, and move the receiving antenna in the direction facing the transmitting antenna along the line to be measured, and record the received signal curve;
C) Change the test stroke line to measure at different heights within a test surface;
D) Change the antenna polarization mode and repeat the measurement of the above steps a) ~ c);
E) Repeat steps b)~d) at 3GHz, 5GHz, 10GHz, 18GHz frequency points, until all frequency points are measured;
F) Change the test surface, repeat the above steps b) ~ e) measurement;
  data processing
Large antenna test
The antenna measurement required to match the antenna to a certain application. The antenna engineer needs to judge how the antenna will work in order to determine whether the antenna is suitable for a particular application. This means using antenna pattern measurement (APM) and hardware in-loop simulation (HiL) measurement techniques.
Although there are many different methods to carry out these measurements, there is no ideal method that can adapt to various situations. For example, low-frequency antennas below 500MHz usually use an anechoic chamber, which is a technology that appeared in the 1960s. Unfortunately, most modern antenna test engineers are not familiar with this very economical technology, and do not fully understand the limitations of the technology (especially when it is higher than 1GHz). Therefore, they cannot play to the utility of this technology.
If measuring antennas with frequencies as low as 100MHz, it is more important for antenna test engineers to understand the advantages and limitations of various antenna test methods (such as cone-shaped microwave anechoic chambers). When testing antennas, antenna test engineers usually need to measure many parameters, such as radiation pattern, gain, impedance, or polarization characteristics. One of the techniques used to test the antenna pattern is far-field testing. When using this technique, the antenna under test (AUT: Antenna Under Test) is installed in the far-field range of the transmitting antenna. Other techniques include near-field and reflective surface testing. Which antenna test field you choose depends on the antenna to be tested.
To better understand the selection process, consider this situation: a typical antenna measurement system can be divided into two independent parts, namely the transmitting station and the receiving station. The transmitting station is composed of a microwave transmitting source, an optional amplifier, a transmitting antenna and a communication link connecting the receiving station. The receiving station is composed of AUT, reference antenna, receiver, local oscillator (LO) signal source, radio frequency down converter, locator, system software and computer.
In the traditional far-field antenna test field, the transmitting and receiving antennas are located in each other’s far field, and the two are usually separated far enough to simulate the desired working environment. The AUT is illuminated by a source antenna far enough away so that the electrical aperture of the AUTA wavefront close to the plane is produced on the upper surface. Far-field measurements can be made in indoor or outdoor test fields. Indoor measurements are usually carried out in a microwave anechoic chamber. The darkrooms are rectangular or tapered, and are specifically designed to reduce reflections from walls, floors, and ceilings (Figure 1). In the rectangular microwave anechoic chamber, a wall absorbing material is used to reduce reflection. In the conical microwave darkroom, the cone shape is used to generate the illumination.
Figure 1: These are typical indoor direct-beam measurement systems. The figures are cone (left) and rectangular (right) test fields.
Near-field and reflection measurements can also be carried out in indoor test fields, and are usually near-field or compact test fields. In a compact test field, a plane wave is generated on the reflecting surface to simulate far-field behavior. This makes it possible to measure the antenna in a test field that is shorter than the far field distance. In the near field test field, the AUT is placed in the near field, and the field on the surface close to the antenna is measured. Then the measurement data is mathematically converted to obtain the far-field behavior (Figure 2). Figure 3 shows the plane wave generated by the reflective surface in the quiet zone in the compact test field.
Figure 2: In the compact test field, the flat waveform is generated by reflection measurement.
Generally speaking, antennas with less than 10 wavelengths (small and medium-sized antennas) are easy to measure in the far-field test field, because the far-field conditions can often be easily met within a manageable distance. For electrically large antennas, reflective surfaces and arrays (more than 10 wavelengths), the far field is usually many wavelengths away. Therefore, near-field or compact test fields can provide more feasible measurement options, regardless of whether the cost of the reflective surface and the measurement system rises.
Suppose the antenna test engineer wants to make measurements at low frequencies. The defense sector is particularly interested in this, because they need to study matters such as the use of antennas at low frequencies in order to better penetrate the structure of the ground penetrating radar (GPR) system (for radio frequency identification (RFID) operating in the 400MHz range) Tags), and support more efficient radio equipment (such as software-defined radio (SDR)) and digital remote sensing radio equipment. In this case, the microwave anechoic chamber can provide a good enough environment for indoor far-field measurement.
Rectangular and conical are two common types of microwave anechoic chambers, the so-called direct irradiation method. Each type of darkroom has different physical dimensions and therefore different electromagnetic behaviors. The rectangular microwave anechoic chamber is in a real automatic space state, while the conical anechoic chamber uses reflection to form a behavior similar to free space. Due to the use of reflected rays, quasi-free space is formed instead of truly free space.
As we all know, the rectangular darkroom is easier to manufacture, and its physical size is very large under low frequency conditions, and the working performance will be better as the frequency increases. On the contrary, the conical darkroom is more complicated to manufacture and longer, but the width and height are smaller than the matrix darkroom. As the frequency increases (such as above 2GHz), the operation of the cone-shaped darkroom must be very careful to ensure a sufficiently high performance.
By studying the wave absorbing measures used in each type of darkroom, the difference between rectangular and conical darkrooms can be understood more clearly. In a rectangular darkroom, the key is to reduce the reflected energy in the darkroom area called the quiet zone (QZ). The quiet zone level is the difference between the reflected rays entering the quiet zone and the direct rays from the source antenna to the quiet zone, in dB. For a given quiet zone level, this means that the normal reflectivity required by the rear wall must be equal to or greater than the quiet zone level to be achieved.
Because the reflection in the rectangular darkroom is a kind of oblique incidence, which will compromise the efficiency of the absorbing material, the side wall is very critical. However, due to the gain of the source antenna, only less energy is irradiated to the side walls (floor and ceiling), so the gain difference plus the oblique incidence reflectivity must be greater than or equal to the quiet zone reflectivity level.
Usually only the side wall area where there is specular reflection between the source and the quiet zone requires expensive side wall absorbing materials. In other cases (such as at the launch end wall behind the source), shorter absorber materials can be used. A wedge-shaped absorbing material is generally used around the quiet zone, which helps to reduce any backscatter and prevents negative effects on the measurement.
What wave-absorbing measures are used in the cone-shaped darkroom? The original purpose of developing this darkroom is to avoid the limitations of the rectangular darkroom when the frequency is lower than 500MHz. In these low-frequency bands, the rectangular darkroom has to use low-efficiency antennas, and the thickness of the side wall absorbing materials must be increased to reduce reflections and improve performance. Similarly, the size of the darkroom must be increased to accommodate larger absorbing materials. Using a smaller antenna is not the solution, because lower gain means that the side wall absorbing material must still increase in size.
The cone-shaped darkroom does not eliminate the specular reflection. The cone shape brings the mirror area closer to the feed (the aperture of the source antenna), so the specular reflection becomes part of the illumination. The mirror area can be used to create a set of parallel rays entering the quiet zone to produce illumination. As shown in Figure 3, the final quiet zone amplitude and phase taper are close to the expected values in free space.
Figure 3: Plane waves generated by the reflective surface on the quiet zone in the compact test field.
Using the array theory can explain the illumination mechanism of the cone-shaped darkroom more clearly. Consider that the feed is composed of a real source antenna and a set of images. If the image is far away from the source (electrically), then the array factor is irregular (for example, there are many ripples). If the image is closer to the source, then the array factor is an isotropic pattern. To the observer at the AUT (in the far field), the source he sees is the pattern of the source antenna plus the array factor. In other words, the array will look like independent antennas in free space.
In a cone-shaped darkroom, the source antenna is very critical, especially at higher frequencies (such as 2GHz and above), when the darkroom behavior is more sensitive to small changes (Figure 4). The angle and handling of the entire cone is also important. The angle must be kept constant, because any change in the angle of the cone will cause illumination errors. Therefore, maintaining a continuous angle during measurement is the key to achieving good taper performance.
Figure 4: In a typical cone-shaped darkroom, the layout of the absorbing material looks simple, but the area closer to the source antenna (the cone-shaped dark area) is very important.
Like the rectangular darkroom, the reflectivity of the wave-absorbing material on the receiving end of the conical darkroom must be greater than or equal to the required quiet zone level. The side wall absorbing material is not so important, because any rays reflected from the side wall of the darkroom cube part will be further absorbed by the back wall (there is a high-performance absorbing material at the back wall). As a general "experience", the reflectivity of the absorbing material on the cube is half of that of the absorbing material on the back wall. In order to reduce potential scattering, the absorbing material can be placed at a 45-degree angle or a diamond shape, of course, a wedge-shaped material can also be used.
The 　　 table provides the characteristics of a typical conical microwave anechoic chamber, which can be used to compare with a typical rectangular anechoic chamber. A smaller amount of cone-shaped absorbing material means a smaller darkroom and therefore lower cost. These two darkrooms provide basically the same performance. However, it should be noted that if the rectangular darkroom wants to achieve the same performance as the conical darkroom, it must be made larger, using longer absorbing materials and a larger number of absorbing materials.
Figure 5: A small cone-shaped darkroom from 200MHz to 40GHz for antenna testing.
Although it is clear from the previous discussion that the cone-shaped darkroom can provide more advantages than the rectangular darkroom at low frequencies, the measurement data shows that the cone-shaped darkroom has real usability. Figure 5 is a small cone-shaped darkroom from 200MHz to 40GHz, with an overall size of 12×12×36 feet and a quiet zone size of 1.2 meters. Here a double-ridge broadband horn antenna is used to illuminate the quiet zone of the lower frequency. Then use Agilent's N9030A PXA spectrum analyzer to measure the quiet zone with a log-period antenna. The reflectivity measured at the 200MHz point is greater than 30Db (as shown in Figure 6). Figures 7 and 8 show the source antenna on the top of the feed and the scanning antenna in the quiet zone, respectively.
Figure 6: As can be seen from the figure, the reflectivity measured at the 200MHz point is greater than 30dB.
Figure 7: The test in the figure uses a double-ridged speaker as the source.
There are many different methods like APM and HiL for antenna measurement. The measurement technique is to choose the correct antenna test field, depending on the antenna to be tested. For medium-sized antennas (10 wavelengths), it is recommended to use a far-field test field. On the other hand, the cone-shaped darkroom can provide a better solution for frequencies below 500MHz. They can also be used at frequencies above 2GHz, but you need to be careful when operating to ensure adequate performance. By understanding the correct use of the cone-shaped microwave anechoic chamber, today’s antenna test engineers can use very useful tools to carry out antenna measurements in the 100MHz to 300MHz and UHF range.
Figure 8: The test in the figure uses a log-period antenna to scan QZ to measure reflectivity.