At the most recent IEEE Symposium on EMC + SIPI, there was a lot of very successful standards development activity. One thing we’re particularly proud of is getting the effort to renew IEEE 299 up and running. That’s the most-used standard sponsored by the IEEE EMC Society, titled “IEEE Standard Method for Measuring the Effectiveness of Electromagnetic Shielding Enclosures”. It covers the frequency range 9 kHz – 40 GHz, extendable to 50 Hz – 100 GHz, for enclosures that have all linear dimensions greater than 2 m.
The dimension specification is particularly important, and two other standards we are working on address the additional complication of measuring smaller enclosures. IEEE 299.1 is titled “Standard Method for Measuring the Shielding Effectiveness of Enclosures and Boxes Having All Dimensions between 0.1 m and 2 m”. Then there’s a new standard being developed, IEEE P2710 “Recommended Practice for Techniques to Evaluate the Performance of Enclosures and Other Methods for Electromagnetically Shielding Portable Electronic Devices”. That last is meant to create standardized methods for evaluating the shielding performance mostly of conductive fabric pouches, the kind advertised to shield cell phones, tablets, and laptops from unwanted RF signals.
299.1 and 2710 are facing a much more challenging task in standardizing methods for evaluating shielding in extremely space-constrained environments. There are two challenges: the physical size and the “electrical length” of the enclosed space. Electrical length is the size scaled to the wavelength of the RF signals you’re working with. We often say that something is “electrically short” if it is less than λ/10 or λ/20. At that point, you can treat the structure like a lumped element in a circuit diagram. Items that are larger will have additional interactions with the fields involved that have to be taken into account. That’s obviously very dependent on the frequencies involved. A cubic electronics enclosure that is 1 m on a side will probably be “electrically” short at 30 MHz, but not at 1 GHz.
Then there’s the practical effect of the physical size. One of the most common shielding effectiveness measurement techniques for enclosures is to have a transmitter outside the enclosure, then a receiving antenna on the inside, along with a data acquisition system (DAQ), see Figure 1. With a roomy enclosure, you can minimize the interactions between the Rx antenna and the enclosure walls, and with the DAQ.
In Figure 2 you can see that it is much harder to simply fit the Rx antenna and DAQ equipment into such a small space (such as a pouch meant to contain a cell phone), and that’s without thinking too hard about the interactions between the fields generated by the DAQ and reflecting from the pouch walls, or other reverb effects from the setup. You can use smaller and smaller antennas, but then you need more and more amplification to make up for the poor gain. Where will you put the amplifier, and how will you power it?
IEEE 299.1 and IEEE P2710 are struggling a bit in their renewal and development efforts, respectively. With advancements in electronics miniaturization and computational electromagnetics over the last decade, there is room for substantial improvement and valuable development here. If either of these efforts interests you, please get in touch with me at standards@emcunited.com to see about getting involved.
