Detailed Mobile Phone SAR and OTA Test Technology and Standard Requirements

Due to the widespread popularity of radio equipment (such as mobile phones) worldwide, more and more government departments, telecommunications regulatory agencies, etc. require electromagnetic wave radiation to be reduced to an appropriate level.

1. What is SAR?

1.1 What does SAR mean?

The English name of SAR is called Specific Absorption Rate. Chinese is generally called electromagnetic wave absorption ratio or specific absorption rate. It is the electromagnetic wave energy absorption ratio of a mobile phone or a wireless product, which is defined as an induced electromagnetic field is generated in the human body under the action of an external electromagnetic field. Since various organs of the human body are lossy media, electromagnetic fields in the body will generate electric current, resulting in absorption and dissipation of electromagnetic energy. SAR is commonly used in bio doping to characterize this physical process. The meaning of SAR is the electromagnetic power absorbed or consumed by human tissue per unit mass, in W/kg, American standard (1.6mw/g, 1g average).

1.2, how is SAR tested?

The human body model, measuring instrument, probe pair and mechanical arm form a SAR measuring system, and the system is placed in the shielding room. The inside of the human body model is a liquid substance. The electromagnetic properties of the liquid are consistent with the electromagnetic properties of the human tissue. The probe can move freely within the test for testing. Finally, the SAR value is calculated by a formula.

Mobile phone OTA test

1. Introduction to OTA Testing

2.1 Passive test and active test of mobile phones

At present, in the RF performance test of mobile phones, more and more attention is paid to the test of the radiation performance of the whole machine. This radiation performance reflects the final transmission and reception performance of the mobile phone. At present, there are two main methods to investigate the radiation performance of mobile phones: one is to determine the radiation performance of the antenna, which is a relatively traditional antenna test method, called passive test; the other is in a specific microwave dark room. Testing the radiant power and receiving sensitivity of a mobile phone is called active testing. The OTA (Over The Air) test is an active test.

The passive test focuses on the radiation performance of the mobile phone from the radiation parameters of the antenna, such as gain, efficiency, and direction of the antenna. The passive test considers the influence of the whole machine environment (such as the device around the antenna, the cover and the cover) on the performance of the antenna, but the final radiated transmit power and receiving sensitivity after the antenna is matched with the whole machine, the passive test data cannot be obtained. It is directly known that the test data is not very intuitive.

The active test focuses on the radiation performance of the mobile phone from the transmission power and receiving sensitivity of the mobile phone. The active test is to test the transmission power and receiving sensitivity of the whole machine in all directions in three dimensions in a specific microwave darkroom, and more directly reflect the radiation performance of the whole mobile phone.

The CTIA (Cellular Telecommunication and Internet Association) has established standards for OTA (Over The Air). The OTA test focuses on testing the radiation performance of the whole machine and gradually becomes a test item that is recognized and recognized by mobile phone manufacturers.

OTA test full wave darkroom

2.2 Purpose of the OTA test

At present, only mobile phone models that have passed the FTA (Full Type Approval) certification test can be put on the market. In the FTA test, the RF performance test mainly tests the RF performance of the mobile phone in the cable connection mode; as for the radiation emission and reception performance of the mobile phone, There are no clear rules in the FTA test, and the OTA test just makes up for the shortcomings of the FTA test in this area. At the same time, the terminal manufacturer must have a clear understanding of the radiation performance of the mobile phone produced, and through various measures to improve the emission and reception indicators of mobile phone radiation. If the radiation performance of the mobile phone is not good, it will cause problems such as poor cell phone signal, poor voice call quality, and easy dropout. This is also a problem with more customer complaints.

When the mobile phone is talking, since the human brain is close to the antenna of the mobile phone, the transmission and reception performance of the mobile phone will be lowered, and the transmission and reception performance of the mobile phone will be reduced. In the mobile phone development process, the impact of the human brain on the transmission and reception performance of the mobile phone should be quantitatively measured, and the optimized design should be performed so that the transmission and reception performance cannot be reduced too much, that is, the electromagnetic coupling effect of the human body and the antenna is reduced.

In order to investigate the radiation performance of mobile phones, in addition to examining the passive performance of mobile phone antennas, the active performance of the whole machine is also an important aspect. At present, the active performance of the whole machine is more and more valued by the terminal manufacturers. Therefore, in the investigation of the radiation performance of the mobile phone, the two kinds of radiation performance should be considered together. At present, terminal antenna manufacturers generally require antenna suppliers to provide passive and active test reports in research and development.

3. OTA test and other major parameters of the mobile phone

3.1 Main test parameters and related calculations in the OTA test In the OTA test, the radiation performance parameters are mainly divided into two categories: receiving parameters and transmitting parameters.

The transmission parameters are TRP and NHPRP; the receiving parameters are TIS and NHPIS.

TRP (Total Radiated Power): obtained by dividing and averaging the emission power of the entire radiation sphere. It reflects the transmission power of the whole mobile phone, which is related to the transmission power and antenna radiation performance of the mobile phone in the case of conduction.

NHPRP (Near Horizon Partial Radiated Power): A parameter that reflects the transmission power of an antenna near the H-plane of a mobile phone.

TIS (Total Isotropic Sensitivity): Reflected in the radiation sensitivity of the entire spherical surface. It reflects the receiving sensitivity of the whole mobile phone, which is related to the conduction sensitivity of the mobile phone and the radiation performance of the antenna.

NHPIS (Near Horizon Partial Isotropic Sensitivity): A parameter that reflects the receiving sensitivity of the antenna near the H-plane.

For the handheld terminal, the OTA test will also examine the above parameters of the terminal in the case of a simulated human head, and compare the changes of the relevant parameters in the presence or absence of the simulated human head.

3.2 Other related antenna parameters

When looking at antenna performance, there are other parameters that need to be understood: APIP, Gain, Directivity, EIRP, ERP.

Gain(dBi): the ratio of the radiated power of the antenna at a certain point in space to the power of the ideal non-directional point source antenna at the same input power. The gain unit is dBi, and the antenna test report provided by the mobile phone antenna manufacturer The gain in is generally in dBi.

Gain (dBd): The ratio of the radiated power of an antenna at a certain point in space to the power in the maximum radiating direction of an ideal half-wave dipole antenna at the same input power. The unit of the gain is dBd.

Directivity: The ratio of the power produced by an antenna at a point in space to the power produced by an ideal non-directional point source antenna at the same radiated power.

Efficiency: The ratio of the antenna radiated power to the antenna input power.

APIP (Antenna Port Input Power): The power added to the antenna port is the power output from the PA to the antenna port. The power level is mainly related to the conducted transmission power of the mobile phone.

EIRP (Effective Isotropic Radiated Power): The equivalent isotropic radiated power is the product of the power obtained by the antenna and the gain expressed by the antenna in dBi, reflecting the amount of power radiated by the antenna in all directions.

PEIRP (Peak Effective Isotropic Radiated Power): Peak equivalent isotropic radiation power.

The concept of ERP (Effective Radiated Power) is the same as that of EIRP, but ERP is the product of the power obtained by the antenna and the gain expressed in dBd.

4. Constraints on TRP and SAR indicators in OTA testing

The TRP reflects the radiation performance of the far field of the antenna, while the SAR reflection is the near-field radiation performance of the antenna. For the TRP indicator in the OTA, it is generally expected that the TRP is relatively large so that the power from the PA to enter the antenna is effectively radiated, and the connectivity of the wireless interface is better. In the SAR test, it is hoped that the TRP value is relatively small, so that the power absorbed by the human brain is relatively small, and it is guaranteed to pass the SAR test standard. Therefore, the TRP index and the SAR index are a pair of contradictory indicators. How to ensure that both indicators meet the relevant standards in the antenna design, meet the design needs, and must be considered at the beginning of the antenna design.

Here are some solutions:

(1) It is most important to choose the appropriate antenna form. For example, the Monopole in the built-in antenna is characterized by high efficiency and high SAR. Therefore, it should be known before use, that is, the coupling effect between Monopole and the human brain is strong. The PIFA antenna has better overall performance. Because its side close to the human brain is blocked by the PCB, its high-frequency band is 5-6dB less than the maximum radiation direction in the human brain. Therefore, the SAR value of the PIFA antenna is relatively low. An ideal antenna from among the built-in antennas.

(2) Considering the SAR problem at the beginning of the design of the antenna, mainly designing on the structural problem, combining the antenna of the appropriate form with the structure of the mobile phone to ensure the performance of the antenna while satisfying the SAR index, such as taking the antenna on the PCB. The bottom of the measure. For external spiral antennas, attention should be paid to the distance between the antenna and the human brain to ensure that the SAR test is met.

(3) In the later stage of design, it is found that the SAR test exceeds the standard, which can be solved by lowering the performance of the antenna. For example, using a material with a slightly lower loss, this needs to be carried out in cooperation with the antenna manufacturer.

(4) Change the antenna routing mode, adjust the direction and other measures.

(5) Reduce the output power of the PA as the standard allows.

The above method is to achieve a compromise between the two in the case of meeting the needs of SAR and TRP testing.

5. Summary

CTIA’s OTA test indicators directly reflect the radiation performance of mobile phones and are therefore increasingly valued by test organizations and related vendors. In the determination of the antenna of the mobile phone, it is necessary to comprehensively consider the passive and active performance indicators, and comprehensively evaluate the performance of the whole antenna.

MORAB’s OTA antenna test darkroom meets CTIA’s OTA test standard. The near-field conversion far-field measurement scheme of the SG24 used in the test system is the most accurate antenna test method. It has the advantages of high repeatability, high accuracy, and high resolution. The SG24 is one of the few test systems in the world that is fully compliant with CTIA requirements. Provides the TRP/TIS test required by CTIA. Test coverage: CTIA OTA (GSM900, DCS1800, Cellular 850, PCS1900); 3D antenna field measurement, gain, antenna efficiency, pattern, polarization performance.

From：http://www.hj-antenna.com/