Shielding of open TEM cell

Am 24.08.21 um 16:58 schrieb Klaus Vestergaard Kragelund:

Someone I knew from univ in a previous life but lost out of sight over the decades had also built such a cell and IIRC he ended up with lots of ferrite tiles (AVX ??).

Lessons learned resulted in

(more on the site)

Running such a lab is a cut-throat business. Customers tend not to come back when they do not get the results they want, no matter what the DUT looks like. And the "good" customers have a lab of their own and need only someone accredited to sign and carry the risks. No one can make a living out of this and even trying to provide a solution is perceived as messing around with a perfectly finished product.

There once was an experiment by the authorities in .de where they sent around a carefully designed piece of sh*t and even some accredited labs were off by 20 dB. That created quite an uproar.

cheers, Gerhard

Yes, running a lab is not my thing. I just need it for my own measurements

So the labs was not calibrated regulary? I thought that is a requirement

I have done many emissions measurements free field in the garden at frequencies up to 1GHz and then later had them verified by a test lab (usually Intertek). It is unusual to get a difference of more than 2 or 3 dB.

John

It'll still resonate, you just won't know where in advance. At least it's easy to calculate the eigenmodes of a rectangular box.

Cheers

Phil Hobbs

Yes, but the closed paths will be very long and complicated, and likely lossy (especially if there is RF absorber on the floor).

I'd guess that there will be a resonance band somewhere, but it won't be as prominent as the first few eigenmodes of a rectangular box, and may be avoidable in practice.

By the way, I got the idea of crooked rooms breaking resonances up from architectural acoustics. Helmholtz resonances are far lower than singing-in-the-shower resonances.

Joe Gwinn

My previous employer used a commercial lab. Their HF wizards were worth every Swiss Franc: the knew exactly where to put a piece of ferrite or copper foil to get the DUT within specs. Wim

I doubt that very much. The resonances won't be harmonically spaced in general, but they'll still be there.

But the physics is different. Helmholtz resonances are of the mass-and-spring variety, unlike organ pipes.

Cheers

Phil Hobbs

But you won't have standing waves covering the entire area of each pair of two parallel walls. You'll get lots of narrow strips, each at a different frequency. And lots of zig-zag paths. Sort of like billiard-balls in a crooked pool table. This should shape the amplitudes of the eigenmodes.

Yeah. I don't think that there is a Helmholtz resonance for EM waves in cavities. I was pointing out a limitation in my analogy.

Joe Gwinn