Why isn't my Faraday Cage blocking wifi signals?

[Joey B]

So I made this Faraday Cage to block wifi in all directions but 1, to block it from my neighbor. Each side has 6 Layers of aluminum foil on it with no gaps, It looks sloppy because when I tried it and it didn't work I quickly added more trying to make it work. If I put my router inside this, with the opening facing AWAY from me, and i stand 20 feet behind it, I still have full wifi signal, It does not affect my wifi at all. I even tried putting my cellphone inside the Faraday Cage facing away from my router and once again it did not affect my wifi signal. Why does my Faraday Cage not work? Everything I've researched says it should completely block it.

 

[Drakkith]

Try closing up the cage completely and see how that effects the signal. I'm not too familiar with blocking wi-fi signals, but our EMP shielding facility in the military had to have literally every little crack and gap blocked off in order to function properly. (Of course, when you're trying to block a high-altitude EMP even a small portion getting through can devastate your electronics inside) It's possible the cage is merely reflecting all of the signal out of the opening where it just diffracts and reflects around to get to you.

 

[tech99]

So I made this Faraday Cage to block wifi in all directions but 1, to block it from my neighbor. Each side has 6 Layers of aluminum foil on it with no gaps, It looks sloppy because when I tried it and it didn't work I quickly added more trying to make it work. If I put my router inside this, with the opening facing AWAY from me, and i stand 20 feet behind it, I still have full wifi signal, It does not affect my wifi at all. I even tried putting my cellphone inside the Faraday Cage facing away from my router and once again it did not affect my wifi signal. Why does my Faraday Cage not work? Everything I've researched says it should completely block it.

The WiFi signal will likely still reach you by three routes.
1. I can see gaps in the foil, and an EM wave will pass through the slightest crack, even a knife cut, provided the crack is more than half a wavelength long (about 7 cm long).
2. The signal emerging from the open end will be of normal strength (or maybe greater), so it will be reflected and scattered back in your direction by the walls and objects in the room.
3. The emerging wave will be diffracted in all directions at the edges of the screened box.

 

[anorlunda]

Have you tried choosing a different WiFi channel than your neighbor's.

 

[davenn]

If I put my router inside this, with the opening facing AWAY from me, and i stand 20 feet behind it, I still have full wifi signal, It does not affect my wifi at all. I even tried putting my cellphone inside the Faraday Cage facing away from my router and once again it did not affect my wifi signal. Why does my Faraday Cage not work? Everything I've researched says it should completely block it.

It cannot work if it has a big opening ! The RF will be pouring out and radiating everywhere

You don't have a Faraday cage ... you have a lined box with an opening on one side. A Faraday cage has NO OPENINGS that are bigger than around 1/8 wavelength at the frequency that needs to be blocked

You are NOT going to block your WiFi going to your neighbour's house unless you either put all of your or their house inside a really big Faraday cage

cheers
Dave

 

[berkeman]

Have you tried choosing a different WiFi channel than your neighbor's.

Yeah, what exactly are you trying to do? You would need to shield your entire apartment and filter all cables entering your apartment to keep your neighbor from seeing your WiFi signals. You have set a password, right?

 

[sophiecentaur]

Aluminium foil has a layer of oxide all over it. There resistance across the join between sheets will be very significant at any contact points where the sheets are not actually under pressure. What is the spacing between screws / rivets etc.? I think it should be less than λ/4 (that always seems to be a magic quantity in RF). Current can flow through the most insignificant cracks.
But what you describe is not, in fact a Faraday Cage. A 'Faraday Cage' does NOT have an open side; it is a closed surface / box.
If you want a bit of directivity then put your router behind a large plate of metal ( several wavelengths across) , between you and the interfering source.
But if you are trying to screen one digital signal from another, you should not need a high degree of isolation - 20dB ratio of signals should start to give you a pretty good advantage for the wanted signal. It has already been mentioned that choosing a different channel should remove the problem completely. (That is unless your neighbour is using an illegal power from his router - which is unlikely)

 

[davenn]

If you want a bit of directivity then put your router behind a large plate of metal ( several wavelengths across) , between you and the interfering source.
But if you are trying to screen one digital signal from another, you should not need a high degree of isolation - 20dB ratio of signals should start to give you a pretty good advantage for the wanted signal.

He's trying to stop his neighbour from getting his WiFi (internet access) for free

 

[anorlunda]

He's trying to stop his neighbour from getting his WiFi (internet access) for free

That's not clear from the OP. He could be trying to stop his devices from connecting to the neighbor's WiFi. Or the two WIFIs might be interfering with each other.

If all he wants to do is block his neighbor, then security and a password is the solution.

 

[davenn]

it seems very clear to me

but if you think not then let's wait till the Op respondsDave

 

[berkeman]

I think his neighbor is an alien, and he's worried about an invasion...

 

[davenn]

If I put my router inside this, with the opening facing AWAY from me, and i stand 20 feet behind it, I still have full wifi signal, It does not affect my wifi at all.

This was the key comment from Joey B

in other words he was expecting his screened box to stop his wifi signal going out the screened side
He doesn't understand
1) what a Faraday Cage is
2) once his wifi signal exits the open side of his screened box its going to be easily received anywhere around the box, the room and beyond
and the screening of the box between him and his wifi router is not going to be effective and hence his neighbours will still be able to access his wifi

ohh and ...
3) if he did succeed in making a proper Faraday cage with his WiFi inside, not even he would have access to his wifi rendering it useless

He doesn't need a different channel, he just needs proper security access setting on the router
Only then will his neighbours, if they are "aliens" computer wiz kids or the CIA will likely to be able to access it

Dave

 

[sophiecentaur]

It could be a case of RTFM, I think.

 

[Drakkith]

It could be a case of RTFM, I think.

RTFM?

 

[Vanadium 50]

RTFM = Read The Manual

 

[sophiecentaur]

When you think that the carrier frequency and the digital coding were chosen to ensure that signals get through in adverse circumstances inside buildings, the OP can hardly be surprised that it's hard to block physically.
I do have one blocking method, that works very well in my present house. You just need your internal walls to be built in the victorian era, thick and mostly dense brick. I need two wireless routers to cover the whole house and the back garden.

 

[rumborak]

I remember doing some experiments in my college's Faraday cage. The first few minutes were just left to showing how precise the thing has to be to do its job. We had a radio running, and even a millimeter crack in the door and the radio was running completely fine. Only when we closed it completely did the radio go dead.
Then, because the cage was made of a mesh, we took a small cable and stuck it through one of the holes of the mesh. Voilà, the radio started playing again.
Meaning, it is virtually impossible to build a Faraday cage that isn't just ... a cage.

Regarding the OP's motivation, I have a hard time imagining anybody who can use the internet doesn't know about WiFi passwords. I suspect the desire to block any kind of EM emission goes a bit farther.

 

[Fausto]

It seems to me that unless you have the cage grounded, that your cage is really an antenna in box form.

 

[sophiecentaur]

It seems to me that unless you have the cage grounded, that your cage is really an antenna in box form.

That's a false dichotomy. Antennae are not just bits of metal stuck up in the air. You can have a perfectly effective antenna which consists of a slot cut in a sheet of earthed (or not) metal. The only difference is the impedance that you need to match to.
His bits of tinfoil are merely acting as extra elements which are modifying the pattern of the router antenna.

 

[Fausto]

That's a false dichotomy. Antennae are not just bits of metal stuck up in the air. You can have a perfectly effective antenna which consists of a slot cut in a sheet of earthed (or not) metal. The only difference is the impedance that you need to match to.
His bits of tinfoil are merely acting as extra elements which are modifying the pattern of the router antenna.

My point is that unless the Faraday cage is grounded it is unlikely to block the OP's router signals.

http://en.wikipedia.org/wiki/Faraday_cage
"For example, certain computer forensic test procedures of electronic systems that require an environment free of electromagnetic interference can be carried out within a screened room. These rooms are spaces that are completely enclosed by one or more layers of a fine metal mesh or perforated sheet metal. The metal layers are grounded to dissipate any electric currents generated from external or internal electromagnetic fields, and thus they block a large amount of the electromagnetic interference."

I have personally worked in a laboratory wherein Faraday cages were utilized for testing UHF receiver and transmitter equipment. Extremely sensitive equipment was required to measure low level signal properties. Obviously, an open cage door would throw off measurements, both within and possibly beyond the cage.

 

[berkeman]

The metal layers are grounded to dissipate any electric currents generated from external or internal electromagnetic fields

That's a bit of a misstatement by wikipedia (imagine that!). The grounding is not part of the shielding of the interior from the exterior fields. Otherwise, how would shielding a space station work...?

 

[rumborak]

Yeah, a Faraday cage does not need to be grounded to work. You *do* want to ground so it doesn't build up static charges, but that's more of a practical matter.
Faraday cages work for the same reason that there is no electric field inside any conductor. The charges rearrange themselves close to the surface into an equilibrium that exactly counteracts the outside field.

 

[Fausto]

Yeah, a Faraday cage does not need to be grounded to work. You *do* want to ground so it doesn't build up static charges, but that's more of a practical matter.
Faraday cages work for the same reason that there is no electric field inside any conductor. The charges rearrange themselves close to the surface into an equilibrium that exactly counteracts the outside field.

Here is an alternate view:
"And oh yeah—yes, your Faraday cages DO need to be grounded. If it’s NOT grounded, then the Faraday cage merely becomes a reflector or an amplifier.

http://www.preparednesspro.com/emp-101-part-iv-faraday-cage
EMP 101: Part IV—Faraday Cages
By Kellene Bishop, Preparedness Pro on 18 June 2009 - 2:47pm

 

[rumborak]

No offense to that site, but it's a "prepper" site. I think I trust my Master's in EE on the matter a bit more. Faraday cages do not need to be grounded for the electrons to rearrange themselves. And even more so, they definitely do not become amplifiers. Just that sentence shows the author of that site has only a passing knowledge of physics at best. (Not surprising, preppers only have a passing knowledge of reality)

EDIT: For example, I can totally imagine how the guy assembled his FC from pieces, but didn't make sure the pieces were all well-connected. That of course means the electrons can't freely move around the surface of the cage. So, his first iteration of the cage doesn't work. He then decides to ground all the pieces, and then finds out it works. Well, that's because he now enabled the electrons to move between pieces. But, his conclusion is that the grounding was the cause.

 

[sophiecentaur]

My point is that unless the Faraday cage is grounded it is unlikely to block the OP's router signals.

That doesn't follow at all. The effect of an Earth is quite arbitrary; it may increase or decrease the currents flowing in certain parts of the structure. Antennas can be classified as 'balanced' or 'unbalanced', depending upon whether or not an Earth connection is used. If there is an Earth wire than it can just as easily radiate as any other part f the structure. The only reliable way to reduce the radiated power (if the box isn't closed) would be to use a dissipative load in a strategic place or use a resistive surface (like teledeltos paper).