The rectangular microwave anechoic chamber and the conical microwave anechoic chamber 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 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 (for example, above 2GHz), the operation of the cone-shaped darkroom must be very careful to ensure a 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 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 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 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 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 that enter 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.
The array theory can be used to 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 above 2GHz), 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 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 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 the best absorbing material on the back wall). As a general "experience", 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 characteristics of the cone-shaped microwave anechoic chamber can be compared with the 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 more absorbing materials.