Matching the antenna to an application requires accurate antenna measurements. 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 the use of antenna pattern measurement (APM) and hardware in-loop simulation (HiL) measurement techniques. In the past five years, the defense sector has become increasingly interested in these techniques.
Although there are many different methods to carry out these measurements, there is no ideal method that can be adapted 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 above 1 GHz). Therefore, they are unable to maximize the effectiveness of this technology.
With the increasing interest in antenna measurements with frequencies as low as 100MHz, it is becoming increasingly 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 antenna patterns is far-field testing. When using this technique, the antenna under test (AUT) 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 separate parts, namely the transmitting station and the receiving station.
The transmitting station consists 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 a 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 to produce a near-planar wavefront at the electrical aperture of the AUT. Far-field measurements can be made in indoor or outdoor test fields. Indoor measurements are usually carried out in a microwave darkroom.
This kind of Anechoic Chamber is rectangular or tapered and is specially designed to reduce reflections from walls, floors, and ceilings. 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.
Near-field and reflection measurements can also be made 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 undergo mathematical conversion to obtain the far-field behavior.
Generally speaking, antennas with less than 10 wavelengths (small and medium-sized antennas) are the easiest 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 an 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.
Rectangle and cone are two common types of microwave anechoic chambers, the so-called direct irradiation method. Each kind of darkroom has different physical dimensions and therefore different electromagnetic behavior.
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, what is ultimately formed is quasi-free rather than truly free space.
As we all know, the rectangular darkroom is easier to manufacture, and its physical size is very large at low frequencies, 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 sufficiently high performance.
The difference between rectangular and conical darkrooms can be understood more clearly by studying the wave-absorbing measures used in each darkroom.
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.
Since the reflection in the rectangular darkroom is an oblique incidence, which will compromise the efficiency of the absorbing material, the sidewall 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 sidewall area where there is a specular reflection between the source and the quiet zone requires expensive side wall absorbing materials. In other examples (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 absorbing measures are used in the conical darkroom? The original purpose of developing this darkroom was 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 sidewall 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 sidewall absorbing material must still increase in size.
The cone-shaped darkroom does not eliminate specular reflections. 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.
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 anisotropic 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 above 2GHz), when the darkroom behavior is more sensitive to small changes.
The angle and handling of the entire cone are 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 the measurement is the key to achieving good cone performance.
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 sidewall absorbing material is not so important, because any rays reflected from the sidewall of the darkroom cube part will be further absorbed by the back wall (the best performance absorbing material is on the back wall).
The reflectivity of the absorber on the cube is half that of the absorber on the back wall. To reduce potential scattering, the absorbing material can be placed at a 45-degree angle or diamond shape, of course, wedge-shaped materials 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 in order to achieve the same performance as the cone-shaped darkroom, the rectangular darkroom must be made larger, using longer absorbing materials and a larger number of absorbing materials.
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.
There are many different methods like APM and HiL for antenna measurements. The measurement technique is to choose the correct antenna test field, depending on the antenna to be tested.
For medium-sized antennas (10 wavelengths in size), a far-field test field is recommended.
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.
C&T RF Antennas Inc has more than ten years of antenna research and development and production experience and is committed to the development of the wireless antenna industry.
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