Resonance is something I had not considered, but could be an issue if it occurs below 30 MHz.
Not sure what exactly is going to resonate here, though. The capacity of the test sample enclosure relative to the floor ground has to tank with an inductance. The only inductance is if there is a wire or cable shield between the test sample enclosure and the floor ground. That could certainly be a green wire if such were part of the power harness, or it could be a shield of a test sample-attached cable. But in this model, the table legs have no place. Now if there were sufficient capacity between test sample enclosure and the table leg(s), that could look like a short at resonance. The earlier model I described clearly goes open-circuit. -- Ken Javor (256) 650-5261 From: John Woodgate <[email protected]> Date: Thursday, December 28, 2023 at 5:03 PM To: Ken Javor <[email protected]>, <[email protected]> Subject: Re: [PSES] Metallic table for conducted emissions? I agree with your interpretation of the enquiry. I think it is necessary to watch out for the legs. if they are frames (or some more complex construction) rather than single pillars, becoming resonant loops at some frequencies. I suppose single pillars could also resonate. On 2023-12-28 22:00, Ken Javor wrote: Ken et al, I think the query was just the opposite. The tabletop is wooden, but do the legs need to be wooden as wel? The FCC paradigm (AFAIK) places the test sample 80 cm above ground, thus limiting parasitic capacity. My gut response is they should be fine with metal legs, as long as the tabletop is thick enough to limit stray capacity. If one had OCD tendencies, one could measure the capacity between a piece of metal laid on the tabletop and the ground plane, and as long as the capacity were suitably low (single-digit picofarads?) that would suffice. -- Ken Javor (256) 650-5261 From: Ken Wyatt <[email protected]> Reply-To: Ken Wyatt <[email protected]> Date: Thursday, December 28, 2023 at 3:53 PM To: <[email protected]> Subject: Re: [PSES] Metallic table for conducted emissions? Hi Brian, The answer is yes and no. Yes, you can get a general idea, but no, you really need a ground plane to conduct the CM currents back to the LISN, which needs to be bonded to the plane. Just find a metal supply store and buy enough aluminum to cover one of your benches. In a pinch, and for quick troubleshooting, I just tape down heavy duty aluminum foil and copper tape the LISN to the foil. You’ll see examples of the technique in my Volume 2, Chapter 3 of the trilogy. Cheers, Ken _______________________ I'm here to help you succeed! Feel free to call or email with any questions related to EMC or EMI troubleshooting - at no obligation. I'm always happy to help! Kenneth Wyatt Wyatt Technical Services LLC 56 Aspen Dr. Woodland Park, CO 80863 Contact Me! New Books! Web Site | Blog The EMC Blog (EDN) Subscribe to Newsletter Connect with me on LinkedIn On Dec 28, 2023, at 2:13 PM, Brian Gregory <[email protected]> wrote: Hello fellow experts, we're looking to build a conducted emission pre-compliance test station to FCC Part 15 Subpart B requirements (residential applications). Is a non-metallic table a necessity for reasonable accuracy? We have a number of lab benches with wood tops and metal legs that would fit far better than jamming an all wooden, non-folding table into our modest space. thanks all and Happy New Year, Colorado Brian This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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