This article is the second in a two-part series that reviews the basics of electromagnetic compatibility tests. We’ll now turn our attention to radiated EMC tests.
In the first article of this two-part series, we introduced the need for electromagnetic compatibility (EMC) tests and standards. Since it is impossible to test all the equipment in the world at the same time, standardized ways of testing all the systems going to the market are defined.
The first article talked about tests for conducted EMC, which have two goals: first, verify that the power emitted through the wiring harness lines is below the limits, and second, verify that equipment can support some amount of injected current without being disturbed.
Part Two of this series introduces tests for radiated EMC, which have the same goals as conducted EMC tests. But instead of testing the wired medium, tests for radiated EMC deal with the electromagnetic spectrum.
The Goals of Radiated EMC Tests
Radiated EMC tests have two goals. One is to measure the radiated energy coming out of a device and the wiring harness connected to it. The other goal is to check that the device under test (DUT) can keep working in a normal operation mode, even when an electromagnetic field is applied directly to it.
Energy can be transmitted in the form of electromagnetic waves. According to Maxwell’s equations, any current circulating through a conductor generates a magnetic field. During radiated emissions tests, this generated field amplitude has to be below a specified limit.
Conversely, any radiated field falling upon a conductor will provoke a circulating current. When performing radiated susceptibility tests, the DUT is monitored at all times, allowing engineers to analyze the moment at which a failure occurs. As in conducted tests, radiated tests use a ground plane to give a return path to common mode currents.
Antennas for Radiated Tests
For both emission and susceptibility tests, different types of antennas are used. The selected model will depend on the frequency range and the desired radiation pattern.
The antenna resonant frequency (the frequency at which the antenna will radiate the maximum energy), is related to the antenna length. Hence, to cover all the frequencies of EMC tests, antennas are changed when shifting from one frequency range to another.
What are Radiated Emissions Tests?
In radiated emissions tests, the electromagnetic waves coming out from the DUT are measured. Emissions can be produced intentionally—for example, in a device using Wi-Fi—but most of them are unintentionally caused by currents circulating through the internal components and the wiring harnesses.
The methods reviewed here are based on CISPR 25 and the ISO 11452-2, which define the limits and procedures for the measurement of radio disturbances starting from 150 kHz.
All the equipment must be placed inside an anechoic chamber, which attenuates all the emissions in all the desired bandwidth. Absorber materials are added to the walls for the removal of reflected energy.
A list of antennas used in EMC tests is shown in the table below. When possible, tests are performed with both vertical and horizontal polarizations.