You then try a number of fixes, and take readings using this setup. It is easy to determine if you're close to 6dB drop that you need.
Since there’s no expectation of that uncertainty being systematic, taking a relative change pushes your error bars +3dB.
That 6dB drop is under the error bars, so it’s pretty meaningless, and that’s with a professional setup.
In other words, there’s no expectation that the numbers you’ll get from these will translate into anything useful, at most you have a ‘hotspot’ detector.
This kind of debugging is very routine. You do it manually with E/H field probes and a spectrum analyzer in between lab visits. This setup makes it more repeatable by automating it.
To comply with CISPR (as an example), a lab only has to validate <4.5dB (in the 30 MHz to 300 MHz range), and up to 5.2 dB (from 30 MHz to 1 GHz). See for instance [1].
By the way, if you are doing this kind of testing routinely, you need to check your PCB layout process. This fine grained tweeting should be a last resort option.
[1]http://ing.univaq.it/emc-chap-it/download/MUinEMC_Revision%2...
Finding the problem and fixing it is voodoo (including magical copper tape and metal charms for the wires..), any help is extremely welcome and can save so much money.
(Even better if it improves upon the usual EMV divining rod.)
(It's a university, which just means we have more expensive things. There is no dedicated person for this, most of the work is done by students with limited time and no domain knowledge in anything hardware at the beginning. And we're still chronically underfunded, of course.)
(So yes, we do model, but usually the science side of it. Most of the instruments stay in the lab, go in cars or just outside worldwide (also ships, stratospheric balloons etc, nobody cares about EM radiation too much as long as you don't overdo it), so the hacky homegrown versions are totally fine. Building scientific instruments in a research group has some advantages because they can be highly custumized, but that means you need to have a person with it who knows which parts to kick for which problems, or where to add or remove some parts. The first iterations are eg expensive optic components, some basic micro-controller or embedded computer system, cardboard and tape.)
(For the more sophisticated things like satellite instrument design you do get dedicated more specialized staff on your team (probably working at ESA or NASA etc), so they will do all the electronics design and probably modelling, so you're free to do what is actually your job, optimizing the thing for science.)