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emf [2020/03/03 03:34]
bpaddock Changed note about Vehicle Occupation detection.
emf [2020/09/18 23:25] (current)
bpaddock Added blockquote around Clives Faraday Cage
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 https://​ieeexplore.ieee.org/​ielx7/​6287639/​6514899/​08732419.pdf https://​ieeexplore.ieee.org/​ielx7/​6287639/​6514899/​08732419.pdf
  
 +----
  
 +====== DIY Faraday Cage ======
 +
 +The message below if from Clive Robinson, who once was a Real World
 +'​Q'​ of 007 fame.  Well respected in the area of Security; Yet he can't
 +spell his way out of a paper bag, we've all gotten use to it..  This
 +is his recipe for a DIY Faraday Cage:
 +
 +<​blockquote>​
 +Clive Robinson January 21, 2014 8:14 PM
 +
 +@ Carl '​SAI'​ Mitchell,
 +
 +"... I'd purchase aluminium foil and line a closet/​small room width
 +it being sure to ground the foil. Obviously make all purchases with
 +cash and at various stores, etc. Jamming might also be possible, but
 +more easily detectable by the listening party"
 +
 +A single or double layer of aluminium foil is not going to be very
 +reliable for various reasons.
 +
 +If you are going to do this at home then you need to do it "flat pack"
 +style.
 +
 +You need to make up some light weight wooden frames just like those
 +artists do to stretch canvas for oil painting. However instead of
 +stretching canvas you stretch chicken wire such that it goes around
 +the frame. You take two of these and screw them together such that you
 +have both wooden frames inside an outer skin of chicken wire. It's
 +best to put thermal insulation of the fiber type in between as this
 +helps deaden sound. Don't rely on the compression contact between the
 +two sides of chicken wire lace the together with copper wire and
 +solder it to the chicken wire (it's usually galvanized so will take
 +solder if you clean it, flux it and use a hot enough iron). You can
 +make these flat pack frames up to 8x4 using standard DIY timber packs.
 +
 +Eight of them (1 floor, 1 ceiling, 2 end walls 4 for side walls) will
 +make a small box/room you can build in a shelf for a computer and an
 +office chair to sit on. However you need to make the floor frame much
 +stronger (unless you've over done the fasting diet craze ;-)
 +
 +Obviously one of the frames needs to have a door built in and this is
 +a little more complicated. Make a smaller frame of about 6x3 this is
 +the door, the frame holding it is made such that the Inner frame has a
 +smaller hole than the door to make a half to one inch jamb all around
 +the outer frame has a hole just large enough to take the door. You
 +then need to make a conductive door seal. The easiest way to do this
 +is with 75ohm TV coax, if you strip the outer plastic insulation you
 +have a soft foam plastic core with a woven braid outside it. Staple
 +this two the door and jamb such that you have two concentric circles
 +one on the jamb one on the door such that they don't touch each other.
 +You will need to come up with a door handle and bolt system such that
 +the braid makes good contact with the chicken-wire covering the door
 +and covering the jamb.
 +
 +Having bolted it together you then line the inside with aluminium foil
 +such that you have four layers. You put in the first layer horizontal
 +with the edges overlapping by an inch or so the second layer vertically,
 +the third horizontally the fourth vertically. Hold each layer in place
 +with small lengths of sticky tape. Using copper staples hang a layer
 +of thick hessian (sack) cloth over this then with copper plated marine
 +screws carefully put a layer of thin protective ply wood over this.
 +
 +There is a minor problem you next need to solve and that's ventilation
 +the easiest way to do this is when making the end wall frame that goes
 +over the computer desk make an internal frame the size of a small
 +bathroom/​kitchen surface mount extractor fan unit, don't put the fiber
 +insulation in here but do ensure the chicken wire covers both sides
 +of the frame. Mount the extractor on the outside after carefully
 +making holes in the aluminium foil. Make a similar arrangement in the
 +bottom of the door only don't add an extractor, put one of those
 +aluminium ventilation louvers with the sliding plate on either side,
 +make sure one has the louver slots vertically the other horizontally.
 +
 +Your next problem is getting power into the unit. To do this you need
 +two dicast aluminium boxes with screw on lids and two IEC (kettle
 +plug) EMI/EMC chassis mount filter connectors. Drill holes and mount
 +the IEC connectors one in each box, screw one box on the inside and
 +one on the outside with the holes to take the cable aligned wire the
 +two IEC's together. Connect a power strip board to a kettle lead and
 +plug it into the inside IEC connector, then for obvious safety reasons
 +screw a wooden block in behind it to stop it being pulled out and
 +allowing fingers to touch the pins in the IEC connector. On the outside
 +you need one of those "​garden lawn mower" earth leakage / residual
 +current trip devices in the wall socket and a suitable length kettle
 +lead to plug into the IEC connector on the external box. Remember it's
 +only good for about 5 Amps.
 +
 +You then need to get an EMC test receiver or appropriate spectrum
 +analyzer to do inside spectrum to outside spectrum comparison to see
 +how good a job you have done.
 +
 +If you know what you are doing you can fit a "​tracking generator feed
 +through bypass"​ basically it's a length of 50ohm coax mounted between
 +two glanded chassis mount N-type connectors one on the inside the
 +other on the outside. It enables you to have a broadband amp and
 +antenna on the inside of your box and the spectrum analyzer on the
 +outside with it's tracking generator output fed through the bypass to
 +the amp and antenna. When not in use you screw plugs onto the
 +connectors with hard shorts in them to prevent leakage.
 +
 +A less dangerous way to do it is to make up a temporary lead you have
 +fed through either the door louvers or extractor unit.
 +
 +If you can't see any leaks from the outside put the spectrum
 +analyzer inside and the amp and antenna outside and run the tests
 +again, remember to move the antenna from place to place outside. It
 +can be a quite time consuming effort and in some cases take longer
 +than it did to build the box...
 +
 +Usually if you are certifying a commercial RF cage, you are looking at
 +allowing four working days for a couple of engineers/​technicians.
 +
 +With appropriate antennas you can check how well your box screens. It
 +should be good for 60db or more of attenuation depending on how well
 +you fit the door and gaskets. With care you can get it to the point
 +where average test equipment is not sufficient to give readings above
 +it's noise floor.
 +
 +The real question is do you value your privacy to the point of a
 +capital outlay of around 1000USD and a week or two of time?
 +
 +...
 +
 +The most drastic I've had to do is with TEMPEST kit where the debug
 +port was a striped bare fiber optic cable through a pin hole in the
 +screened case, screened with RG174 coax with the inner pulled
 +out. Expensive and a right royal pain in the "sit upon".
 +
 +Oh one thing to remember when dealing with RF follow the "​single
 +earth" policy. Otherwise you will get "RF loops" which are the EM
 +equivalent of "audio howl around"​. RF thus gets in everywhere and
 +causes all sorts of nastiest in the EM spectrum as well as general
 +instability,​ messy messy messy...
 +
 +</​blockquote>​
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