Author Topic: Microwave shielding  (Read 927 times)

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Offline prutser

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Microwave shielding
« on: January 27, 2022, 12:33:44 pm »
In most electronics components shops you can get standard aluminium die-cast boxes for RF.  If we look at many professional products (e.g. measuring equipment) we see customized machined enclosures, those usually have quite thick 'walls'.
Is there any noticable difference when e.g. putting a 10GHz mixer or amplifier in a nice machined shieleding enclosure vs the standard die-cast boxes ?

Offline T3sl4co1l

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Re: Microwave shielding
« Reply #1 on: January 27, 2022, 01:18:28 pm »
Note that the material thickness doesn't matter; 10GHz doesn't go more than a few micrometers into most any metal.  The bulk is mostly incidental (but does help when power dissipation is needed, as for PAs).  Machined enclosures are used more to get the shield walls closer to the circuit.  Since the circuit is composed of varying sizes of traces, filter structures, amplifiers and other components, the envelope is always varying, so, easy enough to mill everything out of a solid block and call it good.

Why the proximity?  Keeps cavity modes suppressed.  The cutoff frequency of a cavity is determined by the max width of its minimum cross section (or something like that?).  Keeping it tight to the circuit, keeps those to higher frequencies.

Whereas the diecast box, you're probably just stuffing a board in there, with a few bulkhead connectors.  The connectors might not even have great mode transitions.  Which... probably, the kind like the 5-prong vertical SMAs and whatnot, are about the best?  Anything open-structure or wired is NFG of course.  Edge-launch, well if you can get the board into the damn thing, not too bad, but you'll be limited on board area that way, have to leave clearance to get the connector into the hole.  And the board is then left free-hanging on one edge, not great mechanically or electrically.

All hope is not lost, of course; it just means you may need to invest in some RF absorbent materials for the diecast box -- particularly when placed within 1/4 to 1/2 wave of the surface (enclosure, PCB or both), this absorbs the incident plus reflected wave interacting with the wall or whatever.  Note that if those fields couple noticeably with, say, resonators, filter structures, whatever on board, they'll kill the Q -- that's leakage that would otherwise be reflected in-phase with nearby shields, but here can only be absorbed, or allowed to ping around inside the enclosure, likely also coupling into other undesirable / unintended paths.

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Online TimFox

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Re: Microwave shielding
« Reply #2 on: January 27, 2022, 02:49:19 pm »
A major problem with non-precision boxes at high frequencies is that there may be a thin air gap between mating surfaces, between the screw locations, which functions as a slot antenna at short wavelengths.
WWII-vintage electronics made use of springy "finger stock" to ensure a continuous conducting path, and more modern boxes can use "gasket" material with wire mesh around an elastomer core for the same function.

Online coppercone2

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Re: Microwave shielding
« Reply #3 on: January 27, 2022, 04:24:38 pm »
the walls are thick but the lid is not, its so you can ratchet down on the top plate I think and slightly deform the top plate across the bottom plate (its like a anvil)


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Re: Microwave shielding
« Reply #4 on: January 28, 2022, 03:39:13 am »
A major problem with non-precision boxes at high frequencies is that there may be a thin air gap between mating surfaces, between the screw locations, which functions as a slot antenna at short wavelengths.
The solution to this problem is to use electromagnetic band gap (EBG) / metamaterial structures. For customly machined box array of pins may be machined, which effectively blocks most of leakage. A cheaper approach is to use EBG mushroom-like structures on PCB: array of vias with metal patch on top. Good examples of using this technology:  ridge gap waveguides (RGW): machined or PCB substrate type; contactless rotary waveguide interconnections; contactless tunable filteres. EBG stucture can prevent leakage and simplify assembly process.

Another good example of using EBG to solve air gap problem: Contactless Air-Filled Substrate Integrated Waveguide
Attached image shows comparansion between conventional AFSIW assembly and contactless AFSIW. Just by adding EBG mushroom structure in middle layer of AFSIW it is possible to solve air gap problem. I've used EBGs in my designs too, and observed usable performance even with single row of EBG "mushrooms". Moreover, performance of single-row EBGs may be extended by adding second row of vialess EBG to reduce number vias for manufacturability and cost optimization. Also, EBG arrays may be designed to perform absorbing function, achieving good performance when using resistive interconnects.
« Last Edit: January 28, 2022, 03:54:07 am by »

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