EMC Pre-compliance and Tables - Page 1
Forum section: electronics / projects

author: Greece2000
author_rank: Contributor
timestamp: October 07, 2023, 07:09:06 am
content: Look at the following image.You have a DUT (black box) and it's sitting on a reference ground plane (red line) that is on top of a wooden table (brown). The table has metal legs (grey).The metal legs may cause reflections from any field that goes through.My question is since we have a reference ground plane on top of the table will a majority of the field (blue) be reflected off the reference ground plane and not interact with the metal legs of the table?Or will the metal legs be a problem?

author: corey
author_rank: Contributor
timestamp: October 07, 2023, 09:48:07 am
content: I don’t think the legs will have any appreciable effect on the radiated emissions/susceptibility measurements. Maybe only marginal but if you have any issues with your device, that will show up more than any interaction with the legs. But I don’t know how good your measurement equipment is or the performance of the device. The E-field will not reflect off the ground plane but I guess you’re talking about high frequency/RF. Where’s your measurement antennas?

author: Greece2000
author_rank: Contributor
timestamp: October 07, 2023, 11:03:19 pm
content: It will be 800mm in the air, inline with the reference ground plane, and either 1m or 3m from the table.

author: Greece2000
author_rank: Contributor
timestamp: October 08, 2023, 11:13:26 pm
content: To be clear you mean the legs won't have an effect because the E field is absorbed by the reference ground plane and for high frequency components they will be in their far field at distances where they are close to the table surface and thus will be reflected? That does mean there is potential for the low frequency components of the magnetic field to be affected by the metal structure under the reference ground plane and cause different far field results for low frequencies.

author: Greece2000
author_rank: Contributor
timestamp: October 09, 2023, 12:00:26 am
content: From Testing for EMC Compliance by Montrose:"EUT Turntable. A continuously rotatable, remotely controlled turntable is required. The turntable must be able to support the weight of the EUT and accessory equipment. Most turntables can handle a large amount of weight. Do not forget to add the weight of multiple test personnel to the support weight of both table and EUT. A turntable facilitates determination of direction of maximum radiation at each EUT emission frequency. For floor-standing units, the turntable shall be metal in construction and installed to be flush with the metal ground plane. Bonding of the turntable to the ground plane must occur. This connection is generally provided by flexible gaskets or shims.For table-top systems, the turntable should be nonmetallic, located on top of the reference ground plane. The height of the support table for table-top equipment must be nonmetallic and located at a height of 0.8 m above the ground plane."

author: SiliconWizard
author_rank: Super Contributor
timestamp: October 09, 2023, 12:05:10 am
content: I've always seen wooden EMC tables (including their feet, with no nails, screws or conductive elements) in all labs I've been to and I seem to remember that it's a requirement in the corresponding EN/IEC standards, although that would have to be checked back. Whether you need to comply with any standard, I don't know, and whether this would make any objective difference in your particular case with the particular tests you will do, it would be bold to make a claim here IMHO.

author: Greece2000
author_rank: Contributor
timestamp: October 09, 2023, 12:08:42 am
content: Yeah it's necessary to have it be non-conductive. It's just annoying that it's so hard to find a portable table that is not conductive.I would have thought something like that would be widely available but all of the mechanical folding contraptions are made from metal.

author: SiliconWizard
author_rank: Super Contributor
timestamp: October 09, 2023, 12:13:30 am
content: Quote from: Greece2000 on October 09, 2023, 12:08:42 amYes, unfortunately the most common to find are those specialty wooden tables made for EMC tests that are relatively expensive. One option if you're tight on budget would be to build it yourself, using for instance only wood and glue, if you have the skills.