RF Shielding for Studio

[DigitalDon]

Here's a topic I don't think anyone has covered yet but hopefully someone has some ideas.

I live in a rural area and want to build a small studio behind my house. This is great because I don't have to worry about disturbing neighbors. It would be built on an existing 30' x 30' concrete pad which already has electrical and water hookups available. Here's the problem. Across the street, approximately 1/4 mile away is a remote FM radio tower. Depending on atmospheric conditions, etc I can occasionally pick up the radio station on corded phones, on the television and during quiet sections while listening to a tape (especially with headphones). My concern is that I'll build this studio and won't be able to keep the radio station out of my recordings.

My day job is in electronics so I understand the theory and principles of radio frequency (RF) shielding. I've worked with shielding inside of electronic test equipment but never on the scale of keeping RF out of a small building. Does anyone have experience in this area? I'm assuming I could wrap the entire building in a metal mesh screen before attaching the outside covering (lapped boards, vinyl siding, or whatever) and ground the mesh. Would this help or would it create a big ground plane antenna? The grid size of the mesh would have to be the proper size to block the radio stations carrier frequency. Much like the "holes" you see in the mesh on your microwave oven door are designed to block that particular wavelength. Am I over-complicating this whole thing? My wife says I have a habit of doing that. I don't know. I'm just thinking about all the "antennae" like mic cables inside an unshield building. I'm also assuming the unwanted RF could enter the building through the electrical service since the power lines are aerial rather than buried.

John Sayers, any ideas here?

Thanks,
DD

[Track Rat]

What you're discribing is a Farraday shield(sp?). A friend of mine here in St.Louis lives damn near under one of the local TV transmitters and built one around his studio. He used sheet copper but that can get expensive fast. If you're trying to build something with the mesh size corresponding to wavelength, forget about it. The mesh size would veri with frequency and the for FM freqs the mesh would be quite large.
Wavelength in inches=11,811/f(MHz)
For quarter wave
1/4Wavelength= 2,775/f(MHz)
With the FM band starting around 87MHz and reaching to 109MHz,
wavelengths would range from 135.76" to 108.36". The quarter wavelenght would be from 31.9" to 25.46". I'd stick with chicken wire. Scrap the shit out of any junctions and solder it well and "star" ground it to where your studio power is grounded.

[Track Rat]

And for your power, there are power conditioners that can be installed on the circuit for your studio that will filter RF/EMI from the line.

[John Sayers]

DD - I haven't had much experience of these yet I've always thought you built one along the lines that TrackRat described. I've seen a big one in the science museum in Canberra where they let off huge arcs of lightning with a Van der Graff generator. It was built of 1" square mesh made of reasonably heavy wire.

I think it was discussed before on this site so I'd do a search.

I would say being in the situ you are in I'd try it. For the simple cost involved (i.e. chicken wire and a lot of solder) it could be a reasonable precaution to take but I'd wait to see what others think

cheers
john

[ola]

Does the aluminium fiol trick work? I.e. lining the entire room with regular kitchen aluminium foil and ground it? Or is it just like lining your walls with egg crates for diffusion?

Can anyone verify that it works?

[skippy]

It can work, but it doesn't always work as well as you might think: results can vary wildly. The aluminum oxidizes, and is hard to bond at the edges- or to the grounding straps that actually provide the conductive path to ground. This can lead to sections of the screen that may appear to be essentially floating at the frequencies of interest, and reduces the effectiveness dramatically. If you are going to do this, there is copper screen material available that solders well and will provide the best results. You can certainly do it with foil (or cheapo aluminum window screen wire!), but it's quite hard to consistently maintain the low impedance path to ground that you really need.

One real problem with building a Farady cage this large is that the inductance of your ground mechanism is *critical* for decent performance in the RF frequencies of interest. I posted some information about this some time back in this thread:

http://homerecording.com/bbs/showthr...threadid=18148

You _can_ do this, and it can have very beneficial effects- but done haphazardly, it can also have no effect at all. Do some research online before you spend money on it: a Google search on "Faraday cage"+audio+EMI led to a lot of interesting leads to investigate...

[atomictoyz]

There are vinyl sheet that you can purchase for lining your studio, its semi-expensive but works pretty well. Its loaded with a iron or lead particulate that absorbs the RF signals. Most placed that sell acoustic materials usually have this stuff in their product line somewhere. Its used alot where people do testing with radar and communication transmitters.

Aluminum is a conductor, more likely to turn your house into a giant antenna than RF sheild.

Peace,
Dennis

[ola]

The iron and lead in the vinyl are also conductives, that's the whole point. I don't think Mr Faraday would have survived had his cages been made from wood

I guess the hardest part with aluminium is the soldering. Anyone with experience who can tell us how you managed?

I don't need RF shielding myself as I cannot even get decent TV or radio reception where I live. I'm just over-curious.

[DigitalDon]

Skippy,
Thanks for the thread link. Picked up a lot from that lengthy discussion. I actually understood everything being said. One of our rooms at work is a copper shielded "vault" for precision AC/DC voltage calibration. It blocks Everything. Even sound doesn't seem to get in or out. Hmmmm - drum room???

DD