Anechoic chamber generally, for radiation test, the test site is divided into three types: full anechoic chamber, semi anechoic chamber and open field. The radiation tests conducted in these three test sites can generally be considered to conform to the propagation law of electromagnetic waves in free space.
The full anechoic chamber reduces the interference of external electromagnetic wave signals to the test signals. At the same time, the electromagnetic wave absorbing material can reduce the multipath effect caused by the reflection of walls and ceilings on the test results. It is suitable for emission, sensitivity and immunity experiments. In practical use, if the shielding effectiveness of the shield can reach 80db~140db, the interference to the external environment can be ignored, and the situation of free space can be simulated in the full anechoic chamber. Compared with the other two test sites, the whole anechoic chamber has the least ground, ceiling and wall reflection, the least interference from the external environment, and is not affected by the external weather. Its disadvantage lies in the limited test space due to cost constraints.
Similar to the full anechoic chamber, the semi anechoic chamber is also a hexahedral box with shielding design, which is covered with electromagnetic wave absorbing materials. The difference is that the semi anechoic chamber uses a conductive floor and is not covered with absorbing materials. The semi anechoic chamber simulates the ideal open field, that is, the field has an infinite good conductive ground plane. In the semi anechoic chamber, since the ground is not covered with absorbing material, a reflection path will be generated, so the signal received by the receiving antenna will be the sum of the direct path and the reflection path signal.
Open area test (OAT) is a flat, open, oval or circular test site with uniform and good conductivity without any reflector. The ideal open area surface has good conductivity and infinite area. The signal received by the receiving antenna between 30MHz and 1000MHz will be the sum of direct path and reflection path signals. However, in practical applications, although good ground conductivity can be obtained, the open field area is limited, which may cause the phase difference between the transmitting antenna and the receiving antenna. In the emission test, the open field is the same as the semi anechoic chamber.