Common mobile phone antenna testing methods

With the rapid development and application of mobile communications, China's mobile phone industry has also continued to grow and develop, and of course, China's mobile phone users are also growing rapidly. Among the RF devices of mobile phones, mobile phone antennas are passive devices. As the only "tailor-made" device on mobile phones, the particularity and importance of mobile phone antennas inevitably require very strict testing requirements for antenna performance during its research and development process, so as to ensure the normal use of mobile phones. Now let's briefly introduce several common mobile phone antenna testing methods in the research and development process of mobile phone antennas:

1. Anechonic chamber

A microwave darkroom is also called a non-reflection room or an absorbing darkroom. A microwave darkroom consists of an electromagnetic shielding room, filtering and isolation, grounding device, ventilation waveguide, indoor power distribution system, monitoring system, absorbing materials and other parts. It is a shielded room lined with absorbing materials. It can absorb most of the electromagnetic energy projected on the six walls and better simulate the free conditions of space. A darkroom is a test equipment that antenna design companies need to build, because the test of mobile phone antennas is more accurate and systematic, and its test indicators can be used to measure the performance of a mobile phone antenna. It is mainly used by antenna companies, but it is expensive.

2. TEM CELL test

Use TEM CELL to test the active index of antenna, because microwave darkroom and antenna test system are expensive, usually more than one million, general mobile phone design and development companies do not have such equipment, and use TEM CELL (also more triangular cone) instead of testing. Like the test purpose of microwave darkroom, TEM CELL is also an antenna test environment that simulates an ideal space. The metal box can provide sufficient shielding function to eliminate the influence of external interference on the antenna, and the internal absorbing material can also absorb the incident wave and reduce the reflected wave. TEM CELL cannot perform passive test on the antenna, but can only test active index. Due to space limitations, the absorbing material of TEM CELL is relatively thin, and for the split-shaped absorbing material, the absorption of the incident wave is increased by multiple reflections between the split tips. Therefore, the absorbing materials in the microwave darkroom are relatively thick, while the absorbing materials of TEM CELL are not thick, so the absorption of the incident wave is not very sufficient, which will lead to inaccurate test results.

In addition, the height of TEM CELL is not high enough, which is also one of the reasons why TEM CELL cannot be used for quantitative testing. According to the far-field test analysis of antenna radiation, for mobile phone antennas in the EGSM / DCS frequency band, the distance between the tested mobile phone and the antenna is at least greater than 1 meter; therefore, we can see that almost all 2D darkrooms are much greater than this distance. TEM CELL is smaller than this distance, so this is also one of the reasons why TEM CELL cannot measure accurately compared to microwave darkrooms.

Therefore, TEM CELL can only do qualitative analysis of antennas but not quantitative analysis. In the laboratory, the differences between several prototypes can be qualitatively analyzed to compare their performance, but it cannot be used as an accurate standard value to measure the performance of the antenna. The performance of the mobile phone antenna can only be roughly judged by comparing it with other "golden samples". TEM CELL generally only finds the best square value, which makes the test results sensitive to the position of the mobile phone.

In addition, there is another test tool called a shielded box, which some design companies use to perform active tests on mobile phone antennas. This method is not feasible. On the one hand, the test distance is too close, and on the other hand, there is not enough absorbing material, so external interference has a greater impact on the antenna test. This makes the test results sensitive to the position. A slight change in the position will result in a significant change in the test results. Therefore, this test method has little reference value for the performance of mobile phone antennas.

3. Test antenna performance using a coupling test board

In order to ensure the quality of the product during the production process, antenna coupling tests are often performed. The test device to be used is: the coupling test fixture is connected to the comprehensive tester, and the mobile phone is fixed on the fixture. In the early stage of production, a reasonable coupling compensation value is given based on the test results of several prototypes, a power standard is determined, and then the maximum power of the mobile phone is tested. If it is higher than this power standard, it means that the product meets the production requirements. If it is lower than this requirement, it means that there is a problem with the antenna and related components. The following problems can be found through antenna coupling testing:

(1) The antenna matching circuit has poor soldering and missing parts, etc.

(2) There is a problem with the electronic/structural parts around the antenna.

(3) The antenna is not assembled properly.

(4) There is a problem with the quality of the antenna itself.

It should be pointed out that the antenna coupling test is a consistency test of the product, not a test of product performance. The antenna indicators mentioned above are all tests for the far field, and the antenna coupling test is a test for the near field. The antenna of the tested mobile phone is very close to the coupling fixture antenna. The near field is an objective existence of the antenna itself. Once the structure and antenna of the entire mobile phone are determined, the near field can also be determined. Therefore, it is possible to judge whether there is a problem with the antenna part based on whether the test results are within a certain range. The antenna coupling test only tests the maximum power of the antenna, and does not test other items. Even if it is tested, it is meaningless.

It is very wrong to use antenna coupling test to verify antenna performance and to judge performance based on the test results of different mobile phones. At present, I still encounter many domestic mobile phone companies that judge antenna performance through coupling test, which forces antenna companies to adjust the resonant frequency to pass the coupling test standard.

When it comes to the design of mobile phone antennas, antenna testing is a very important step. Without an accurate design, there is no way to measure the quality of the antenna design. Therefore, choosing a good testing system is also very important for the development of mobile phone antennas.