Using Aluminum Oxide Paint To Secure Wi-Fi

eldavojohn writes "The BBC reports on people using aluminum oxide in their paint to block Wi-Fi signals from leaving their home or business. Aluminum oxide resonates at the same frequency as Wi-Fi signals and other radio waves, blocking data from going outside a building. It's not a flawless solution, as it may also block AM/FM signals. You or your neighbors may be unwittingly using this already, as most pre-finished wood flooring uses aluminum oxide as a protective coating."

Comments

Cellphone reception?

[beef curtains]

Wouldn't keeping radio signals in also have the unfortunately side effect of keeping radio signals out? While having a neighborhood coffee shop offer free wifi to paying customers while being an of oasis of cellphone-free peace & quiet would be sweet, having no cellphone reception at home because one desperately wants to prevent neighbors from stealing one's wifi seems very inconvenient (especially when setting up even the most basic built-in wireless router security would successfully do the trick in 99.9% of cases).

Re:Cellphone reception?

[pz]

They said it, but I have to wonder. Resonant generally means you have a dimension of order a wavelength. The paint particles are much smaller than the wavelength, so it sounds to me as if they are simply building a Faraday Cage, but with metallic paint, not aluminum foil or metal sheeting.

Note, if you are going to do this, you need to avoid holes the size of a wavelength / 4 or bigger - a few cm for WiFi (12 cm waves). If you like windows you should also put a fine wire mesh on them.

Funny thing about electromagnetic resonance. The wavelength in vacuum / free air matters only ... in vacuum / free air. The wavelength of a signal in a different medium, with presumably different dielectric constant and impedance will be ... different! Water molecules are famously resonant at 2.45 GHz, that's where microwave ovens operate, despite the vacuum wavelength of 2.45 GHz photons being about 12 cm. The inter- and intra-molecular impedance makes H2O absorb those photons quite well. Water is quite rather opaque at those frequencies, despite being transparent at higher frequencies, say in the visible spectrum, and despite individual H2O molecules being many orders of magnitude smaller than the vacuum wavelength of 2.54 GHz photons.

Futhermore, bear in mind that aluminum oxide is not at all like aluminum. First off, it's an oxide. It's not a metal. It's a quite good insulator, and is used as a common abrasive because of its hardness. With impurities to give it color, it is the primary ingredient in sapphires and rubies. Coating something with aluminum oxide won't create a Faraday cage, but according to the news item, will provide a certain level of frequency-specific shielding.

Re:Cellphone reception?

[Anonymous Coward]

You are no physicist. Metalized paint can form a Faraday Cage. http://en.wikipedia.org/wiki/Faraday_cage

Re:Cellphone reception?

[Forge]

The paint could also provide some much-needed relief during nights out at the cinema.

Our largest local movie theater installed an electronic cellphone signal blocker some years ago. It worked very well and almost put them out of business.

You see people on call (like Sysadmins, Doctors etc...) and people who feel a need to be reached on short notice for personal reasons (parents of small children), no longer saw that theater as an option for dates. This might not be a problem in some places but because of the lower pay scales (and hence higher relative cost of movie tickets) here, those affected were a major proportion of the theater's customer base.

In short they had to turn off the signal blocker and announce to disgruntled customers that it's ok to come back.

I am trying to imagine the dilemma if they had used this Aluminum Oxide paint instead, how much would it cost them to scrape it all off?

Re:Cellphone reception?

[pz]

I stand corrected on the water resonance part.

The impedance part, well, perhaps Mr. Pink Corner needs to understand more where resistance comes from. The reason that microwave ovens work at all is that there is a resistive (ie dissipative, lossy, real, call it what you will) component to the impedance of water at those frequencies, dissipating EM energy into heat. Here's a quote from the nicely informative link Mr. Pink Corner provided:

Rather than interacting with the water molecules via a resonance, the microwaves in an oven heat the water by twisting its molecules rapidly back and forth so that they rub against one another. The molecules are heated by the molecular equivalent of sliding or dynamic friction. The choice of 2.45 gigahertz gives the water molecules about the right amount of time to twist in each direction. The precise frequency isn't important, but microwave ovens are required to operate at exactly 2.45 gigahertz so that they don't interfere with communication systems using nearby frequencies.

And here's another clue about physics, Mr. Pink Corner, when you have a complex impedance, like an RC circuit in a lumped model system, the speed of light through those components is determined by the RC time constant. The speed of light is slowed by the reactive portion of the impedance. In that system. Otherwise the movement of the current would not be impeded and the full voltage would appear at the far side of the C element instantaneously. But it does not: the speed of propagation is slowed, as reflected in the time constant. This becomes especially evident when you look at transmission lines and the impedance of the line determines the propagation speed (as well as the line loss due to heating of the real component of the impedance). Same thing in water, except, as I've been corrected, there is no resonance at 2.45 GHz.

Re:Cellphone reception?

[Forge]

This did not happen in the US but in Jamaica. Our rules are somewhat different. I.e. One of our maximum security prisons also installed a Jammer. It created problems for people living close to the prison and for people passing on the highway in front of the prison.

They were compelled to turn it off ontil they could contain the signal. within the walls. This fancy paint might actualy help them. I realy don't know how the open "play area" between the outer walls and the cell block will affect this. (That area is rather large and includes a Cricket Field.)

Re:Extra protection?

[ColdWetDog]

May not be better but it could be more, how shall we say, stylish. Maybe quieter as well. I do like his pandering to the 'think of the children' racket.

"We're assuming that excessive exposure could be bad for us. Therefore we're trying to make protective clothes for young children or pregnant women to help protect their bodies from such waves."

Give me a break. Next think he'll be trying to create is an Aluminum oxide / silver dip - keep away nasty radio waves and germs, all in one toxic package.

I think this guy is mostly looking for some investors.

Why not share wi-fi?

[presidenteloco]

We do know the world would be a better place if everyone shared their wi-fi securely using
a technology like FON, don't we. (No I'm not associated with the company. Just recognize a
great concept when I see one.)

I'm seriously tired of how, particularly in the US, sharing wi-fi gets implanted in peoples'
brains as a criminal, borderline terrorist activity, with terms such as
"theft of tele-communication resources" and similar Orwellian mindf**k terms.

Re:Why not share wi-fi?

[cheros]

Correct. And you can decide if you want to share free of get a kickback. If you share for free you are also entitled to access all other FON nodes in the world, if you share for a revenue share you have to pay. Pretty fair model IMHO.

However, I'm unhappy with a node on my personal network where someone can change code on the fly, which is why I took it offline a while back.

Is it safe ?

[CharlyFoxtrot]

We all know about the hazards of lead based paints, so is this safe ? I'm assuming the plastic lining in aluminum cans is there for a reason. Perhaps adding panelling containing a solid aluminum oxide mesh of some sort would be better.

Re:Is it safe ?

[CharlyFoxtrot]

I checked my deo, but no aluminum so I googled around and apparently it's an ingredient in antiperspirant which is in some deodorants (mine's the plain kind). Interestingly this turned up some debate on the safety of aluminum use :

"A small study in 17 women with breast cancer was quite widely reported in the news in 2007. It found higher levels of aluminium in the part of the breast nearest the skin, and the authors speculated that aluminium in deodorants might cause breast cancer. But the design of this study was not strong enough to draw that conclusion." From cancerresearchuk

And from wikipedia : "Aluminium, present most often in antiperspirants, but not usually present in non-antiperspirant deodorants, has been established as a neurotoxin in very high doses."

From what I can tell smearing it on your skin is probably OK and ingesting it in more than trace amounts should probably be discouraged :-) Thanks for the tip.

Re:Does not resonate with me

[_avs_007]

Seriously... I work with a WiFi lab at work... I have a Faraday Cage in our lab... One of the techs forgot to install an optical isolator on the network cables, so for a few days the ethernet cables went right into the cage... Well, first day I went to use it, I locked myself in the cage, and was surprised that my cellphone started ringing... Without the optical isolators on the network cable, the RF signals were able to find there way into the faraday cage through the ethernet cables....

with that being said, I highly doubt that simply painting your walls will keep RF signals at bay... Even when the grounding wire was simply loose on the door to the faraday cage, RF signals would leak in...

Usefull in heavily saturationed WIFI areas

[ACMENEWSLLC]

Where I am there is some WIFI (hidden or not) on every channel. There are large broadcast towers about half a mile from me which have various Internet over 802.11B from service providers.

My WIFI in my home has a hard time with all this. This paint would be a good way for me to improve my in home signal. A lot of my equipment doesn't support the new 5Ghz of 802.11N, so while I have 802.11N APs they do not help much.

limit my wifi? pah

[jackflap]

The number of times I've "borrowed" wi-fi from a neighbour, I felt obliged to leave mine open for everyone to use.

I did however install the Tomato firmware on my router, and use it's brilliant QoS to limit all machines but my own to 56k connections :)

My basement does a good job

[Jim Hall]

I happen to live in Minnesota, where most homes have basements. Our Comcast cable enters the house through the basement, so I figured it was a convenient place to hook up the WiFi access point. The WAP is sitting on the bottom shelf of a bookshelf I have there.

The net effect is that I have great WiFi signal throughout the house. I have a wood frame house, and WiFi signal is not retarded by normal wood construction. However, I get almost no signal as soon as I step out the back door onto the porch. A few feet away from the house, I can't pick up the signal at all. I've also tested it from the street, and it's like my WAP isn't even there. No, I'm not using any special paints or "Faraday cage" wallpaper.

That's because basements are constructed from cement blocks, which do a pretty good job of retarding WiFi signal. Not to mention my basement is 6-7 feet underground (there are windows at almost ceiling height) and all that dirt also helps block the signal. I figure I'm pretty safe from snooping. Sometimes it helps to just be incrementally harder to get to than the next guy. If you're a wardriver who's interested in free WiFi, my neighbor two doors down is an easier target (his WAP is on the second floor of his house, so he's essentially broadcasting to the whole neighborhood.) Or I suppose you could drive down a block and a half to the coffee shop who runs a completely open WAP.

* And yes, I do use good WAP security, with encryption, long passphrase, MAC filtering, only wired connections are allowed to connect to the admin screen.

Re:What is so hard about using WPA2?

[ElectricTurtle]

Encryption can be broken with less effort than a physical wall. It's also fundamentally naive to propose that one layer of security of any kind is the silver bullet that makes all other layers unnecessary. I use encryption and MAC address lists together because it means that if somebody wants to get in they have to do two things instead of just one. Can people still get in? Duh. Everybody should already know that wireless network security is about making a harder target than the one down the street.

Re:Does not resonate with me

[yurtinus]

I'm no EE or RF guy, so I don't know how far off base this is-- but aren't Ethernet interfaces typically transistor coupled on either end so you don't have an actual phsyical grounding on either side? I'm guessing the optical isolator is supposed to sit at the edge of the cage itself so you don't have an Ethernet cable antenna going out... ?

I'm still a bit skeptical as I've worked in EMI hardened facilities where we have solid wires leaving the structure to outside antennas and it's really not possible to get a cell signal with the door closed.

Disclaimers--
1) I'm what happens when you let a software guy look at too many circuit diagrams :)
2) Cell provider is Sprint, so I'm suprised when my phone rings in my garage at home...

i already have this on my house

[shadowrat]

the aluminum siding does a great job of keeping wifi in my house. i can hardly get a signal in my yard without LOS to the router through my window.

after installing a steel garage door, it did a pretty effective job of preventing my garage door opener from working either :(

Re:Does not resonate with me

[Anonymous Coward]

You took your *active* cellphone into a faraday cage used to shield your experiments from RF.... isn't that a bit self-defeating?

Re:Hey, it's good enough for the office...

[jonbryce]

What you need to be really careful about is the cops in helicopters doing thermal imaging of roofs. They do this to find people running cannabis farms. If you are releasing a lot more heat than normal, then expect a dawn raid. Also, if they don't see any heat being released, expect a dawn raid because they will think you are trying to shield it.