How would I test for RF shielding?

[smashinator]

If I want to test different enclosures for their RF shielding abilities in the comfort of my own home, how could I do it?  I've tried google, and can't find any information.  It's possible I just don't know what to look for. 

Any ideas/suggestions?

[petemoore]

  Put a transmitter next to it, drive 5w via antenna booster and see what comes through !
  You could try a cordless drill. Not sure what is output as RF, EMI and the other one, but they put out strong fields of energy.
  VH used a cordless next to his pickups on 'Poundcake' [intro], but IIUC, strong magnetic fields can de-magnetize . ...possibly reducing magnetic pickup 1/2 life, might improve them...lol.

[darkseid]

Use a Tesla coil.....

This video shows 100% shielding lol.  A good Faraday cage for sure
http://socrates.berkeley.edu/~fajans/Teaching/cartoons/DrMegavolt/DrMegaVolt.avi

[Paul Perry (Frostwave)]

Seriously, it's pretty difficult to do this in a useful way. Very difficult indeed. Because, so many things make a difference. Including the frequency of the RF source. And the orientation of the box to the RF signal. And whether there are holes in the box, or gaps, and their size. And in any case, most RF problems are from pickup by leads going to the box.
Plus, sometimes it isn't RF at all, its an alternating MAGNETIC field! which isn't going to shield, except by a massive thickness of iron.
And then there is hash on the DC supply from the wart....

[smashinator]

Well, my intent is just to test enclosures (and various shielding methods) for stompbox use.  I was thinking I'd build a fuzz face, or something else radio-and-other-noise-pickup prone and put it in various enclosures and see what happens.  What I'm really concerned about is having an enclosure that will shield as well as an aluminum box (hammond-style).  My house is shockingly noise free, so I was wondering what sort of conditions I could introduce to test my box.

I happen to have a cordless drill.  I'll give that one a whirl (no pun intended).  I think a 5 watt transmitter would be too tempting, and Radio K-Smash would distract me from my stompboxes.

As for the Tesla coil.... Finally, an excuse to build one!   

[smashinator]

I forgot to say: Any other ideas out there for real world enclosure test scenarios?

[Hal]

Plus, sometimes it isn't RF at all, its an alternating MAGNETIC field! which isn't going to shield, except by a massive thickness of iron.

Don't all conductors sheild magnetic fields...or at least changing magnetic fields...?

I think they use copper sheets on transformers for magnetic shielding...

[Paul Perry (Frostwave)]

Don't all conductors sheild magnetic fields...or at least changing magnetic fields...?
I think they use copper sheets on transformers for magnetic shielding...

Well.......no. Conductors shield electric fields only. And the copper sheet inside some transformers, is just to prevent capactive transfer of electric spikes from one side of the transformer to the other.

[R.G.]

Unfortunately, you've hit on one of those things that is genuinely difficult.

Knowing that a conductive box is pretty good shielding is quite different from actually measuring HOW effective it is. Actually measusing shielding effectiveness requires several 6' racks of high dollar equipment to generate, then measure the RF. How would you, for instance, generate aircraft navigation radio signals? These happen at high power near airports, but where would you buy the equipment? In addition, you have to be able to see the RF when it gets into the box. I don't have an oscilloscope that will show me even FM radio carriers, and I have a pretty good one. To detect the RF inside the box you need to be able to receive it inside the box and detect it with equipment intended to detect it. Otherwise, it's a pretty porous test.

What makes it hard is that an enclosure that is a perfect shield at one RF frequency is a receiving antenna at another frequency. For instance - a slot, like a crack where a lid is not quite touching another part of the box, makes a dandy antenna at frequencies where the length of the crack is 1/4 wavelength. AM radio stations have LONG wavelengths, so any metallic stompbox is substantially impervious, but the same box may leak like a sieve at cordless telephone frequencies.

The other problem is that wires going into the box can also conduct RF in. AC power cords, the shield of a guitar cord that makes bad contact, things like that can make you a radio receiver.

About the best you can do is to use something like a broadband RF noise source, like a Jacob's ladder. Tesla coils are OK, but they are heavily monochromatic at the resonant frequency of the coil. And with these, you're looking for additional hiss, betting that your ears will hear the increase.

[smashinator]

Hmmm....  So I guess the best thing to do (in my case) would be to just use the enclosures and see what happens. 

Thanks for the info!

[jrem]

EMI shielding / testing etc is an expensive business.  In order to gain a CE mark on our equipment we need to do tesing, which we send out.  There are only a handfull of companies that do it, basically they have a large room that is lined (faraday cage), they bombard the equipment (which is under power) with frequencies and voltages, then measure what gets bounced back and/or leaked into the equpment ground, and reflected back to the mains.

Pass or fail, it's $10k on up per visit.

Also, the people that make the shields do lots of tests . . .  those little stamped cages that go into your cell phone, for example, get prototyped and prototyped.

[Paul Perry (Frostwave)]

There are two aspects to this problem (RF pickup):
1`. RF getting into the box (by holes and gaps, along in and out signal cables, along power cables).
2. Coupling of RF into the circuit & interference by the RF (or its demodulated audio).

#1 is why one uses ferrite beads & metal boxes.
#2 is a design problem, but for stompboxes, there isn't usually too much chance of signal pickup by the
components and wiring itself, because they are pretty small (makes a poor antenna).

An interesting book on this is mardiguian, "Controlling Radiated Emissions by Design". OK, he's looking at stopping stuff getting OUT of boxes, but it's the same problem, really!
usually, for FX, it's only a problem with fuzz (where the gain is ridiculously high). IF you get the RF out of the input signal, usually things are OK.

The worst thing about RF, is when house wiring or architecture acts as a tuned antenna and the field strength of some radio station gets very high. It's always worth testing a unit with RF problems in several different places.