Faraday cage from an old microwave oven

[jonovid]

gutting the electronics from an old microwave oven. you get this- .
the steel microwave oven case is lightweight without the hefty power transformer.
also the door catch mechanism remains intact.
removing the glass from door and the clear plastic film sheet from the inside of the door  see image.
with the steel door panel clear of its glass . modifications can be added to it.

as a faraday cage a plywood shelf can be added to the inside of the oven door to hold any RF project with any panel mount BNC connectors in the door itself.             
the steel microwave oven cover can go back on.  whatever works best.

[Ian.M]

The problem with that is the door EMI seal is a frequency specific 2.45 GHz half-wave choke. https://www.edn.com/your-microwave-oven-needs-a-choke-flange/

For frequencies where the door diagonal is a very small percentage of the wavelength, its probably good enough if the door is reliably grounded via its hinges, but unless you upgrade the door seal with a conductive elastomer gasket, on bare metal, contacting a bright-plated bare metal mating surface, it wont be a good Faraday cage for high frequency work.

Also why mess with the door to add feedthroughs etc. when you've already got the waveguide opening into the cavity? 

[jonovid]

The problem with that is the door EMI seal is a frequency specific 2.45 GHz half-wave choke. https://www.edn.com/your-microwave-oven-needs-a-choke-flange/

For frequencies where the door diagonal is a very small percentage of the wavelength, its probably good enough if the door is reliably grounded via its hinges, but unless you upgrade the door seal with a conductive elastomer gasket, on bare metal, contacting a bright-plated bare metal mating surface, it wont be a good Faraday cage for high frequency work.

Also why mess with the door to add feedthroughs etc. when you've already got the waveguide opening into the cavity? 

putting the steel microwave oven cover back on would give the faraday cage a flat top so
test equipment can sit on top of it saving bench space.
the waveguide opening into the cavity is on the side so hard to reach when setting up a project? so requiring a steel plate over the waveguide opening.
also requiring a hole to be cut in to the side of the steel oven cover with a rubber weather strip covering the sharp edges of the hole.
looks like your right this is not a cheap and simple DIY   

[Ian.M]

Hmmm.  Flexible conduit in a swept bend to the front control panel area from the waveguide opening, and add four studs round the waveguide opening to fit a plate with thru-connectors from the inside of the oven, clamped down on a conductive gasket using wing nuts?

You'd make up a project specific cable harness and plate, and hook the cables through the conduit using some sort of flexible grabber or similar.

That still leaves the problem of retrofitting an EMI gasket to the door.

[radiolistener]

what is purpose for that "Faraday cage"?

You can get large metal saucepan with lid for the same volume

[edpalmer42]

There are lots of things that can be used as the starting point for a Faraday cage.

- cookie tin
- tool box
- ammo box
- bread box
- lunch box

They'll all need work on the sealing surfaces and connectors to get signals in and out.

Decide what your requirements are and get busy.  Use your imagination!

Ed

[jonovid]

the original idea was
somewhere to test radiofrequency oscillator circuits without generating suburban radio interference
have 2 or more BNC passthrough panel mounts with end caps in the door screen for _
somewhere to twiddle trimpots & press buttons from behind a steel screen that lets one see the RF project at the same time.
using non conductive rods or trim tools inserted into holes in the steel screen.
to repurpose an old microwave oven at low or no cost. that was the idea.

[joeqsmith]

I keep an old cookie tin.  Easy to solder to.  Decent seal. 

[Ian.M]

the original idea was
somewhere to test radiofrequency oscillator circuits without generating suburban radio interference
...
to repurpose an old microwave oven at low or no cost. that was the idea.

As I said earlier, if the hinges ground the door effectively, it should be usable below about 15 MHz, maybe a little higher.

[CaptDon]

Strange fact about a microwave oven with the front door removed, The oven cavity doesn't launch the 2.45GHZ outside of the cavity very well. Not sure why but the RF field falls off really fast even at a few inches.

[MarkT]

Because of standing waves in the box?  I reckon it launches RF power to the outside perfectly well, but the field strength inside is even higher because of standing waves.

Not the sort of experiment to encourage, leaky microwaves have been known to cook the insides of eyeballs (which have no blood supply to remove the heat).

[EE-digger]

If not the best Faraday cage ... not at all, the microwave is a handy size, has a door, hinges, latch.

I've used them in a pinch with these mods:

( remove paint as needed to get a bare metal bond )

1.  copper tape over the microwave inlet to the chamber
2.  copper tape over the chamber light opening
3.  copper tape over the hole where the platter motor spindle "was"
4.  copper mesh over the door including the gasket / trap area
5.  wire mesh over foam (like Wurth products) gasket, centered in the trap width, do entire perimeter

Depending on how well the inner box seams are sealed (weld, spot weld, crimp), you may have a pretty good chamber at this point.

All that said, cookie tins work great as do larger containers that Christmas popcorn assortments come in.  Test for continuity and sand off the finish where necessary in the lid to can area.  Don't assume that a shiny gold or silver area is conductive.  I've found that frequently they are not.