Faraday Cage not blocking wifi

  • #1
In our school we made a submarine. The inner hull is a few cm thick metal, so it should be a good farady cage, but it does not act like one when blocking wifi from inside the hull. Wifi antenna is not touching the hull. The hull acts like a faraday cage when blocking a phone signal from the outside the cage. My question is, why does the hull not block wifi signal comming from inside the hull, but does block other signals comming from outside the hull.
 

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  • #2
davenn
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wifi is much higher frequency
how RF tight are the seals around the hull ?
water tight is one thing, RF tight is a whole different ball game
would take very little for 2.4 GHz or 5.8 GHz to leak out
 
  • #3
CWatters
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Gaps or slots in or between sheets of metal can sometimes act like aerials. As davenn said, it can be difficult to stop RF getting out.
 
  • #4
sophiecentaur
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so it should be a good farady cage,
'Fraid not. It's very hard to make a good faraday cage. When an EM wave hits the metal surface, it sets of currents all over. Where there is a join / seam, there will be a resistance and some current will flow in through the join (finite gap with a resistive layer on the metal surface). If only 1/10 of the surface current flows through then you will have something in the region of 1/100 of the incident power getting through. In RF terms, that's almost like a wide open window because receivers cope with a vast range of input signal levels. But, if it were a 'good' cage then you'd get no communications through it??
The reason why one way behaves differently from the other way could be the design of the receivers and transmitters inside and outside the box. Your receiver in the box may be just below threshold whilst the outside receiver may be 'better'. Also the transmit powers may be different. It would depend on the actual circumstances.
Why not fit a fibreglass window in the hull?
 
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  • #5
tech99
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In our school we made a submarine. The inner hull is a few cm thick metal, so it should be a good farady cage, but it does not act like one when blocking wifi from inside the hull. Wifi antenna is not touching the hull. The hull acts like a faraday cage when blocking a phone signal from the outside the cage. My question is, why does the hull not block wifi signal comming from inside the hull, but does block other signals comming from outside the hull.
If a radio signal originates inside the hull then it can propagate freely inside the hull. It just bounces around when it hits the metal. But when it originates outside, if the hull has no holes, it cannot penetrate.
 
  • #6
davenn
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If a radio signal originates inside the hull then it can propagate freely inside the hull. It just bounces around when it hits the metal. But when it originates outside, if the hull has no holes, it cannot penetrate.
that didn't really answer the Q :wink:
 
  • #7
arydberg
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I have made some receivers to detect static charges. I usually run a point out from a coil around the the antenna coil of a AM battery powered transistorized radio. The radio is then covered by aluminum foil. The interesting thing is that to shield the AM signals it is important to not only use aluminum foil but to ground the foil to the ground circuit on the radio.
 
  • #8
sophiecentaur
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why does the hull not block wifi signal comming from inside the hull, but does block other signals comming from outside the hull.
Whilst there is a principle called Reciprocity which says that a link should work the same in either direction, you can have local interference caused by equipment in the submarine. That will mean you have a worse Carrier to noise (/ interference) ratio inside the hull and worse performance, as a consequence.
 

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