GTEM cell plays the role of antenna in RF radiation test

The anechoic chamber, as a test environment that replaces the outdoor wide field, has been widely used because of its sound characteristics. However, due to the high cost and expensive equipment that must be used, it is restricted to develop into small and medium-sized enterprises. The GTEM cell recommended here is also known as the gigahertz (GHz) transverse electromagnetic wave chamber, which is a new type of electromagnetic compatibility test equipment developed in the past decade. Its output power range can be from DC to quantity. Above GHz, the internal available area is relatively large. It is particularly commendable that the total price of the cell itself and its supporting equipment is not too expensive and can be accepted by most enterprises. GTEM cell has made steady progress in China and has become the first choice for the company to conduct RF radiated magnetic field immunity test for equipment with small appearance specifications.

1. GTEM cell introduction

GTEM cell is designed according to the principle of coaxial and asymmetric square transmission lines. In order to prevent the reflection and resonance of the internal electromagnetic wave, the GTEM cell is designed as a sharp cone, its input end is selected as a N-type coaxial connector, and then the core conductor is expanded into a fan-shaped plate, called a plate. The square and symmetrical plant area is produced between the plate and the bottom plate of the small room. In order to make the spherical wave (strictly speaking, the spherical wave is transmitted from the N-type connector to the GTEM cell, but due to the small customized opening angle, the spherical wave is similar to the plane wave.) have good transmission characteristics from the input end to the load end, the terminal of the board becomes a non-reflective terminal because of the use of distributed resistors to match the network. The absorbing material is also pasted on the port of the small room to further absorb the electromagnetic wave of high frequency. Therefore, a symmetrical test area is formed between the plate and the bottom plate of the small room. In the experiment, the test product is placed in the test area. In order to ensure that the uniformity of the test product will not unduly harm the uniformity of the field due to the placement of the test product, the height of the test product should not exceed one third of the distance between the plate and the base plate.

2. Principle

GTEM small indoor field strength is positively correlated with the input signal voltage V from the N-type connector, and inversely proportional to the safety distance h between the plate and the bottom plate: E=V/h In the matching system, the correlation between the voltage between the plate and the bottom plate and the signal input power of the N-type connector V=(RP) 1/2=(50P) 1/2, so the field strength E=(50P) 1/2/h If the difference between the measured value and the theoretical value is considered, the above formula should also be multiplied by the index. k. So the specific field strength is

E=k（50P）1/2/h

It can be seen from the above formula that if the same power is introduced in the GTEM cell, the closer the board position is to the bottom plate. (the lower the value of h), greater field strength can be obtained; If the in-phase field strength is caused, the larger the space, the greater the field strength. (the more h value) the greater the input work required.

The above results show that the test sample can also be put into the test sample for the smaller test sample. The front part of the GTEM cell can use smaller signal input power to obtain strong enough field strength. Please note that the height of the test product should not exceed 1/3 between the selected part of the plate and the bottom plate.

3. Application of GTEM electromagnetic compatibility test in small and medium-sized laboratory

3.1 Advantages of GTEM small indoor RF radiated magnetic field immunity test:

The field strength generated by the GTEM cell is much higher than that generated by the antenna. Therefore, using a small RF power amplifier can generate extremely strong electric field, which greatly reduces the price of all test systems. This is a very good RF radiated magnetic field immunity test scheme for small machines.

GTEM does not use antenna in the small room to conduct immunity test of radio-frequency radiated magnetic field, which can be easily used for automatic test, greatly reducing the test time and the technical standards of the test personnel.

After amplification, the signal source is introduced into one end of the GTEM small room (according to the N-type coaxial connector), which can generate a strong uniform magnetic field between the plate and the base plate. The field intensity is detected in the electric field monitoring probe placed near the part to be measured, and then the input power value is obtained through the computer, and the signal source is immediately adjusted to achieve the required field intensity value. The measurement control software controls the signal source to scan the number of radiated fields in a certain step. In addition, there is a video monitor (the camera is installed in the small room of the GTEM, which is not drawn in Figure 4. The experimenter observes the working situation of the test product with the small outdoor monitor under the influence of the radio frequency electromagnetic field of the GTEM.

Operation steps:

① Place the test object and field probe in the GTEM cell;

② External signal source, according to the power amplifier, GTEM creates a small indoor uniform electric field;

③ Define the detection frequency band, deployment method and deployment depth;

④ Adjust the output level of the signal source (note that do not exceed the large input level allowed by the power amplifier);

⑤ Detect the field strength of GTEM small room according to the field strength monitor to make it reach the required strength;

⑥ Repeat the process ③~⑤ to observe the electromagnetic radiation sensitivity of the tested product.

3.2 Electromagnetic disturbance radiation transmission test system of test products

In essence, the electromagnetic disturbance radiation transmission value of the test product can also be detected in the GTEM cell. At this time, the plate and plate in the cell will replace the antenna in the darkroom test to accept the radiation disturbance caused by the test product. The N-type connector in the GTEM cell affects the receiver, and the radiation transmission of electromagnetic interference during the operation of the test product can be detected according to the impact on the receiver. Then, through the computer and processing software, determine the test results of the radiation emission of the test product. Notice that there is one. Compare the test results in the GTEM cell with the test results in the wide field or radio anechoic chamber, find out the rules (establish the mathematical model) and make corresponding adjustments, which is the problem that the GTEM cell test tool needs to solve. In addition, the placement of the test in different small rooms of GTEM will lead to the difference in the relative distance between the plate and the bottom plate, which is also an important factor for the different test results. The experimenter should try his best to give full attention.

The operation steps for conducting electromagnetic interference radiation transmission test in GTEM cell are as follows:

① Place the test object in the GTEM cell;

② The external influence on the receiver, the radiation disturbance level of the test object;

③ The range of scanning frequency, detection mode and resolution bandwidth are set according to the detection standard;

④ Influence the radiation disturbance level value of the tested object detected by the receiver;

⑤ The final test result is obtained by data processing with computer and software.

Note that there is a polarization problem in the cell (by changing the direction of the antenna, testing in a wide area and in a radio dark room) whether using GTEM to do the immunity test of the radio-frequency radiated magnetic field or the emission test of the radiated electromagnetic disturbance formed in the work of the test product itself. The plates and bottom plates in the GTEM cell act as antennas, and their positions cannot be changed. Therefore, in order to change the polarization direction of the electric field, we can only rely on manually changing the placement direction relative to the plate and bottom plate to complete the test.