The tighter the shield is, the higher the high pass frequency on the waveguide will be. Each part is like a little launcher section I think.
I thought about it this way:
How much of an effect does putting a ground plane on the bottom and the top of the circuit have? I usually at least have a ground plane.
If you have two ground planes, does a extra box do anything really, between the planes? The capacitance of an enclosure even if you don't consider waveguide stuff is small considering the distance comparison between even a SMD part compared to a normal PCB thickness, even if you don't have a ground plane sandwich.
The dielectric constant of a PCB is much higher then of air, so I imagine even things like stripline would not be effected too much.
I figure the worst case of extra ground capacitance (im not sure you can even think of it this way too well) would be if you use tiny SMD parts and literary sandwich it down. But, the effect is uniform on the entire PCB, I think.
So, the effect would be that there might be a bit more coupling to ground. Everything should be in the near field (if you can think about it this way and not wave guide, I don't really know whats correct), so I don't think you would really have issues with things like reflections. I think the most imporant thing would be to keep shield impedance low/well matched to ground plane.
I do wonder if there is a case to analyze where the ground plane is really wimpy and the shield has a lower impedance, so you get some kind of significant internal shield current.. what comes to mind is datasheets that talk about RF isolators, where they recommend a floating capacitance between both sides to I guess reroute/concentrate emissions. But on a properly designed board the current should want to go into the ground plane more then anything I think. If the shield has crappy impedance then something might use it to capacitvely jump to another area of the PCB like in the recommended application of floating capacitance layers for use with RF isolators.
It gets more complicated with magnetics I think, since you start to introduce mutual inductance. In this case I have read on the forum *such as in my LISN threads* that you should really have good spacing between the magnetic elements and the enclosure, I think teslacoil said some kind of rule of thumb of distance related to magnetic shielding. It was basically some kind of factor related to the dimension of the inductor to reduce the mutual inductance to low levels.. I believe that following this rule basically doubled my enclosure size.
I bet there are other considerations but I think this is a somewhat meaningful basic analysis.
If you look at a RF can ground, you will see a higher end ones or ones for higher frequency use basically make a VIA sandwich. I suspect that unless you have some kind of shitty ground then basically your performance will get better the lower the impedance is between the shield and your PCB reference. I would suspect that if you have to do something funny and nonintuitve then something is wrong with the circuit or its really really low noise.
I suspect that the biggest problems might be complicated IC's that have fast internal clocks or multiple frequencies for other reasons to very high levels, but I don't know much about fast IC's. A older PIC will have emissions into the GHz range for instance, so if its a really big enclosure maybe you can have unexpected behavior from reflections /radiation. I would imagine a good solution to be to put a ferrite pad ontop of such devices that pollute in such ways, if they have a problem with the shield, or to generally dampen the whole thing.
I think you want to keep the shield as tight as possible and then use ferrite to minimize waveguide behavior?
Someone put this into better language please.
For instance Capacitors also tend to be have a ground referenced shield (so they are directional). It makes me think that its fairly OK to just put something in a shield box and assume it will generally work better. There are also shielded transistors:
https://www.radiomuseum.org/tubes/tube_af114.htmlhttp://pdf.datasheetcatalog.com/datasheets/90/74138_DS.pdfIn spectrum analyzers I have seen shielded transistors on a double ground plane backed PCB inside of another can (for a logarithmic amplifier)
I want to know why that guy got worse performance from his PLUTOSDR when he shielded it though. I want to know how that shield was grounded.