IEC 61000-4-39:2017 Radiated fields in close proximity - Immunity test

Particular considerations for IEC 61000-4-39 

This part of IEC 61000 is an international standard which gives immunity requirements and test procedures related to radiated disturbances caused by radio-frequency fields from devices used in close proximity. 

It is impossible to ignore that the everyday electromagnetic environment has greatly changed. Not long ago, handheld, frequency-modulated (FM) transceivers for business, public safety, and amateur radio communications represented the predominant RF applications. Distribution was limited (for example, by licenses) and in most cases the radiating antennas were outside buildings to get a high efficiency. The situation changed once technology allowed the manufacturing of compact wireless phones with low weight and a reasonable price. Wireless services (DECT, mobile phones, UMTS/WiFi/WiMAX/ Bluetooth®1, baby monitors, etc.) have come into widespread use and acceptance. Recognizing the fact that equipment for these new technologies could have the antenna inside the building and even inside the device housing and be omnipresent in nearly any setting including at work, in the home and in public transportation creates new situations for exposure of equipment to RF energy. 

With the new digital technologies, the traditional modulation methods of AM and FM has given way to digital modulations with a variety of different amplitude and bandwidth characteristics. While overall time-averaged transmit power levels might have generally decreased over time due to improved network density and migration of services, the maximum possible (peak pulse) power levels in other bands have increased significantly. Moreover, the incorporation of multiple transmitting antennas (to support for example WiFi and Bluetooth links), evolving form factors, higher bit rates to facilitate data transfer and Internet access and the use of wireless headsets have resulted in a more complex and diverse pattern of use and exposure. Increased portability of transmitting devices has also drastically reduced the separation distance between sources of radiated RF energy and equipment likely to be disturbed by that energy. 

It should be expected that the wireless technology revolution will continue to evolve with new applications using increasingly higher microwave frequencies.

 Immunity testing according to existing standards, such as IEC 61000-4-3, 61000-4-20, 61000- 4-21 and 61000-4-22, may not be suitable to assess compatibility with the complex electric and magnetic fields generated by RF emitters located in close proximity (for example, within a few centimetres) of the surface of electronic equipment. The power levels required for the higher disturbance intensities associated with such very small separation distances may make application of some of the existing test standards quite challenging or cost prohibitive. 

New technologies use also magnetic fields. The fields are inhomogeneous and vary appreciably in both magnitude and direction over a region of space. Typically they can be generated by motors, power transformers, switching power supplies, higher-powered electronic article surveillance (EAS) gates or transmitters of radio-frequency identification (RFID) systems, inductive charging systems and near field communication (NFC) devices. The fields from such sources decrease rapidly as the distance from the source increases. 

Because these new technologies use a very large range of the frequency spectrum it is necessary to use different test methods which consider the physical behavior of magnetic coupling in the lower frequency range and the more electrical based characteristic in the higher frequency range. Additionally, the widely diverging physical and electrical characteristics of equipment types that may be affected by portable transmitters in close proximity, as well as the applications for which such equipment is used, indicate a need for multiple test methods. 

At present this document covers magnetic field disturbance sources in the frequency range 9 kHz to 26 MHz.In the frequency range 26 MHz to 380 MHz no testing is yet defined. In the frequency range 380 MHz to 6 GHz testing using a TEM horn antenna is defined. It has been argued that especially in the frequency range above 380 MHz the specified test methods do not take into consideration the possible variations in field impedance from real life close proximity transmitters, which may represent sources having field impedances far below the far field impedance of 377 Ω (predominantly magnetic field sources) and far above 377 Ω (predominantly electrical field sources). In the frequency range above 380 MHz the signal wavelength is such that the reactive nearfield from the source begins at only a few centimeters from the source (around approximately 0,1 λ). At this distance the field impedance approximates more and more to the far field impedance of 377 Ω. The TEM horn antenna represents a field source which is not far from 377 Ω.

Activities are ongoing to identify antenna types that can be characterised by field impedance and radiation pattern over a specified illumination window size, which for the ease of testing should be as large as possible and should preferably cover a large frequency range. Antenna types that are not covered by manufacturer’s intellectual property rights, and which can be unambiguously characterised by for instance near field scanning or numerical model characterisation, are preferred for the present basic standard.