Among the various EMC tests, the radiated emission test is one of the most important as it demonstrates that the unit does not interfere with other equipment nearby through electromagnetic radiation. The radiated emissions of a product can be challenging to assess without an anechoic chamber due to two main factors.

•The first factor is ambient noise, which consists of nearby radio and TV broadcast transmitters, handheld devices like walkie-talkies, equipment, and machinery used during the assessment, and ESD events.

•The second factor is reflections caused by metal structures, including racks, cabinets, junction boxes, conduits, and pipes. If the testing is not designed and performed correctly, there can be a significant difference between chamber testing and benchtop testing, sometimes up to a 20dB difference. Therefore, it is essential to carefully consider and address these challenges during benchtop testing to ensure an accurate assessment of a unit's radiated emissions.

This section provides a “hybrid” approach to facilitate a thorough understanding and effective implementation of the method for benchtop radiated emission assessment. Figure 1 lists some of the equipment that is often involved in performing both near and far field measurements.

Step 1 – Near field Assessment

A near-field measurement of the subsystems is essential to ensure that all switching frequencies (including, but not limited to clock frequencies of microcontrollers, switching frequencies of switched mode power suppy, motors, etc) and their harmonics are recorded, as these spectrums may appear in the far-field measurement. In cases where the product can be fitted in a TEM cell, I prefer to test and record the module using the TEM cell quickly. Most of the time, a product may not fit in a TEM cell; therefore, we use near-field probes (both magnetic and electric field loops) to “sniff” the subsystem and record spurious levels that could potentially radiate in the far field.

It should be noted that the purpose of these measurements is not to correlate the results in the far field. Instead, the information obtained from the near-field measurements is used to determine the frequencies of critical spurious emissions in the far-field results.

It is not recommended to use near-field measurement results to directly predict far-field emissions. This is because near-field readings are highly dependent on the geometry of the source and its properties, making it difficult to provide correlations between measurements performed in the near field and those done in the far field. While it is generally true that the stronger the field near the source, the stronger it will register in the far field, this correlation is not precise enough to provide reliable predictions.

Step 2 – Measuring Cable Radiation

After conducting a near-field assessment, the next step is to use an RF current probe to measure a sampling of cables. When using an RF current probe to measure common mode current on cables, it is recommended to make several measurements along the cable as standing waves on the cable can cause readings to differ between different parts of the cable. Harmonics between 30 and 500 MHz should be noted down.

Henry Ott and Clayton Paul outlined a method to convert RF current probe measurements into electric field strength radiated from cables carrying RF currents. It is a derivation/simplification of the full treatment outlined in “Antenna Theory – Analysis and Design” (C. Balanis). Engineers may find this application note from Tekbox useful if they want to explore the details. Min has also demonstrated this method in his popular Youtube video.

Step 3 – Far-field Measurement

As mentioned before, a typical indoor lab environment, whilst dry and warm, is electromagnetically cluttered so that both ambient sources and metallic objects abound to confuse the measurement. For which reason it may be worth considering taking the far-field test outdoors on a nice day, when the ambient should be reduced to only the local broadcast stations and mobile phones, and the metallic objects can be mostly eliminated. In the final step of the radiated emission assessment, the radiated emissions from the product are measured using antennas. Both full-size and reduced-size antennas are available in the market for this purpose. While reduced-size antennas can be advantageous for far-field measurements above 200 MHz, as they can be moved around easily and placed in locations where a full-size antenna may not fit, they may not be suitable for measuring radiation fields between 30 MHz and 200 MHz. This is because reduced-size antennas often have lower sensitivity and a higher antenna factor (AF) compared to full-size antennas, resulting in higher system noise floors that can exceed the test limits being used for comparison. Therefore, it is always recommended to use a full-size antenna for measuring radiated emissions between 30 and 200 MHz.

Previously recorded near-field measurement results can be useful in determining whether the far-field radiation comes from the product or ambient noise. Software that can load multiple results can be helpful in comparing and analysing both the near and far field measurements.

An example of this is shown in Figure 2. In this case, the red trace shows the near-field measurement results, while the green trace shows the far-field measurement. As can be seen, the ambient noise can be distinguished so that we can focus on the noise generated by the product (the blue pointers shown in Figure 2). Quasi-peak measurements can then be performed at selected spot frequencies to determine whether the noise might exceed the limit line. Strictly speaking, only a fully compliant measurement can tell if the noise actually does exceed the limit.

A video demonstration can be found here.