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Need a working faraday cage product for legacy keyfob
- Thread starter njhtran
- Start date
they dont work because i put the fob in them and I walk up to my car and can still unlock it and drive away in it. Would that be considered not working?
Not necessarily. There's a lot of factors that come into play and black magic is generally considered one of them.
I'm looking at Amazon for "faraday key fob protector" and I don't seem to see one that I'd expect to really work. Many of them will reduce the signal, greatly reducing man in the middle attacks, but not completely shield it.
This is going to greatly attenuate the signal, but too many things tell me its still going to leak.
If I want to kill it, I'd be wrapping it many times in aluminum foil and then sticking it into a good faraday cage. And I'd be expecting to pay a few thousand for it. Commercial cages cost millions.
Actually, I'd pull the battery.
That's what I was thinking. Perhaps you could DIY something with layers of aluminum foil and foil tape (not duct tape, foil tape).
Um. Speaking as an EMI type..
Those baggies with the conductive coating that look silvery I wouldn't trust further than spit. Strictly attenuation, not cut-off.
I'm a bit surprised about the Mentos can, though. Although if I squint it's possible that the coating that's put on the can to keep it from corroding is likely insulative, so the top and the rest of the can are in electrical isolation. In which case things will get through.
However, if one is handy with a soldering iron, there's this stuff called "finger stock", comes in coils, and can be Soldered To Things. The tricky bit is soldering the extremely conductive but-it's-not-aluminum finger stock to a Mentos aluminum can.. unless the Mentos can is actually made of iron/steel, in which case one can totally do it.
In which case: grab some steel wool or emery cloth and sand the upper outside edge of the bottom Mentos can until it's shiny, put some flux on the steel, wrap a single layer of finger stock around the can, and solder the finger stock in three or four places, which should do the trick. On the lid, do the same bit with the steel wool/emery clock.. then try a thin layer of 3-in-1 oil. The oil will prevent corrosion, but is thin enough so the finger stock will go right on through the oil and make solid contact with the lid and, ta-da, Faraday cage.
Let's see, finger stock: You want the Really Thin Stuff, since the Mentos can is pretty tight tolerance in the first place..
Digikey carries lots: https://www.digikey.com/en/products/filter/rfi-and-emi-contacts-fingerstock-and-gaskets/945
Those baggies with the conductive coating that look silvery I wouldn't trust further than spit. Strictly attenuation, not cut-off.
I'm a bit surprised about the Mentos can, though. Although if I squint it's possible that the coating that's put on the can to keep it from corroding is likely insulative, so the top and the rest of the can are in electrical isolation. In which case things will get through.
However, if one is handy with a soldering iron, there's this stuff called "finger stock", comes in coils, and can be Soldered To Things. The tricky bit is soldering the extremely conductive but-it's-not-aluminum finger stock to a Mentos aluminum can.. unless the Mentos can is actually made of iron/steel, in which case one can totally do it.
In which case: grab some steel wool or emery cloth and sand the upper outside edge of the bottom Mentos can until it's shiny, put some flux on the steel, wrap a single layer of finger stock around the can, and solder the finger stock in three or four places, which should do the trick. On the lid, do the same bit with the steel wool/emery clock.. then try a thin layer of 3-in-1 oil. The oil will prevent corrosion, but is thin enough so the finger stock will go right on through the oil and make solid contact with the lid and, ta-da, Faraday cage.
Let's see, finger stock: You want the Really Thin Stuff, since the Mentos can is pretty tight tolerance in the first place..
Digikey carries lots: https://www.digikey.com/en/products/filter/rfi-and-emi-contacts-fingerstock-and-gaskets/945
ucmndd
Well-Known Member
I bought an amzon one a couple years ago and it seemed to work fine, but once I set the doors to not self open at home I didn’t need it.
I looked up the link: Amazon.com
I looked up the link: Amazon.com
ucmndd
Well-Known Member
joe.smith
Member
Wait, don't Faraday cages need to be grounded to really be effective? Isn't a metal 'box' simply acting like antenna in the OP's case?
Joe
Joe
Nope. It’s like that saying: Whatever happens in Vegas, stays in Vegas.
It’s like this: electromagnetic fields on the inside hit the inside of the cage, induce currents on the surface of the cage, which re-radiate back into the box. Fields on the outside cause currents on the outside surface which also re-radiate back into the outside of the box: but nothing in either case goes through.
Well, there is this thing called skin depth: When an E-field hits a metal surface, it causes the electrons on that surface to accelerate; the negatively charged electrons move towards the positive end of an E-field, since said electrons are free to move. Accelerating an electron causes it to generate an E-field in the opposite direction of the impinging E-field, causing another E-field to be generated that tends to cancel the impinging E-field; the new E-field thus generated moves away from the surface at Ye Speed Of Light; i.e., it's a reflection.
In fact, that's in general why one knows that one is looking at a metal, which is defined as having free-flowing electrons: It looks shiny because all those free-flowing electrons are making reflections like mad.
The skin effect is because, in a non-superconductor, the electrons aren't quite free to move: They generally do fly about, yes, but they also tend to bump into atom nuclei, causing said nuclei to bounce more in place. Said bouncing is, actually, what temperature is all about so (no surprise, here) some of the energy gets transformed into heat. Since the free-flowing isn't perfect at the surface, some of the incident wave penetrates a bit further, accelerates some more electrons which do their re-radiation bit, cancelling out more of the incident wave, which travels a bit further, and so on. This decay as one moves in is exponential in nature; the skin depth is defined as the distance the wave travels before it's reduced in magnitude by 1/e. Twice the distance results in the wave getting reduced by 1/(e^2), and so on.
This skin depth is related to wavelength: The smaller the wavelength, the smaller the skin depth. At frequencies above 50 MHz or so with a decent conductor (copper, silver, gold, iron) the distance is measured in the micrometers. So, at 100 MHz or so, a piece of aluminum foil is thousands of 1/e deep, the attenuation is in the trillions.. so nothing is getting through. Especially if one is talking about a Mentos can, which is a heck of a lot thicker than aluminum foil!
Interestingly, this skin depth is the reason that copper bus bars in 50/60Hz power plants that are more than, say, a couple of inches thick are built hollow: There's no current flowing in the center, so there's no point in putting (expensive) copper in there. That wouldn't be true for a DC power plant, though: At DC, the skin depth is infinite, and current flows throughout the conductor.
Finishing up: Suppose that one has a TV antenna, made out of metal rods, up on the roof. For this exercise, the metal rods are presumed to be capped and hollow. When moving electromagnetic fields (i.e., TV signals) hit those rods, the rods re-radiate, and the re-radiation hits other bars up there, which also re-radiate, and so on; and the panoply of all this re-radiation ends up being selective to the direction that one is pointing the antenna (i.e., one can point the antenna at a TV transmitter somewhere and get that TV transmitter really, really well, but none of the ones to either side). But the inside of the tubes? There's absolutely nothing going on in there.
Finally.. Back in Physics 169 (Electromag), the prof in the lecture hall had his fun: He had a copper-screened cage, about 7' high on the inside. (So long as the holes in the screen are much, much less than the wavelength of any signals on the outside, such a screen acts as a solid metal box.) He had a volunteer get into the box (it had a screen door, with finger gaskets around it). He then fired up this monster machine that consisted of a spinning conductive disk, about 7' high, that had a long, thick, heavily insulated wire on it, a long insulated handle, and the prof, himself, wore this monster rubber gloves, and fired up the apparatus. It threw, I kid you not, a 4' long continuous arc from the tip of that probe to the screen of the cage. The volunteer on the inside felt nothing, of course; in fact, the volunteer put his hands flat on the inside of the screen on the opposite side of where that arc was terminating and reported, "It tickles a little." Probably from the hot air circulating around.. but not anything shocking.
And this is the reason that sitting in a metal car during a lightning storm is the safest place that one can be. Even a Tesla, with all that glass, still has a metal framework that at the DC frequencies of the lightning shot acts like a pretty good Faraday cage. (Note: car is sitting on insulative, rubber tires. But a lightning bolt that just went through a kilometer of air and hit a car isn't going to have a problem going an additional foot from the bottom of the car to the Earth
.On the other hand.. At one time I used to use this Open Air Test Facility (OATS). We'd mount a piece of electronics whose radiation we wished to measure from DC to Daylight in the OATS, which had a metal floor about 50' square and had walls and a roof that were made out of fiberglass with no metal fasteners whatsoever. The gear was mounted on a small elevator that, when down, one could put a metal cover over the hole when no equipment was being used. The center hole could be rotated and there were antennas one could mount on the inside to measure the EMF being given off by the gear. All nice and good, and occasionally the FCC would come down and test some equipment or other that they were interested in.
But if there was a thunderstorm anywhere nearby, we'd retract the gear down, cover the hole, and Get Out Of Dodge: Because any lightning bolt that came down from on high would go straight through all that fiberglass and zap anything, including people, on the inside. Fun. An anti-Faraday cage, if you will.
KArnold
Active Member
I'm not trolling - this is a real question. Suppose you put your fob in a working faraday cage and there is an EMP. What are you going to do with it? Is your car also in a faraday cage? Might be a fun project to build.
Not sure I'm seeing your point.
Not sure I'm seeing your point.
Nope. It’s like that saying: Whatever happens in Vegas, stays in Vegas.
It’s like this: electromagnetic fields on the inside hit the inside of the cage, induce currents on the surface of the cage, which re-radiate back into the box. Fields on the outside cause currents on the outside surface which also re-radiate back into the outside of the box: but nothing in either case goes through.
Well, there is this thing called skin depth: When an E-field hits a metal surface, it causes the electrons on that surface to accelerate; the negatively charged electrons move towards the positive end of an E-field, since said electrons are free to move. Accelerating an electron causes it to generate an E-field in the opposite direction of the impinging E-field, causing another E-field to be generated that tends to cancel the impinging E-field; the new E-field thus generated moves away from the surface at Ye Speed Of Light; i.e., it's a reflection.
In fact, that's in general why one knows that one is looking at a metal, which is defined as having free-flowing electrons: It looks shiny because all those free-flowing electrons are making reflections like mad.
The skin effect is because, in a non-superconductor, the electrons aren't quite free to move: They generally do fly about, yes, but they also tend to bump into atom nuclei, causing said nuclei to bounce more in place. Said bouncing is, actually, what temperature is all about so (no surprise, here) some of the energy gets transformed into heat. Since the free-flowing isn't perfect at the surface, some of the incident wave penetrates a bit further, accelerates some more electrons which do their re-radiation bit, cancelling out more of the incident wave, which travels a bit further, and so on. This decay as one moves in is exponential in nature; the skin depth is defined as the distance the wave travels before it's reduced in magnitude by 1/e. Twice the distance results in the wave getting reduced by 1/(e^2), and so on.
This skin depth is related to wavelength: The smaller the wavelength, the smaller the skin depth. At frequencies above 50 MHz or so with a decent conductor (copper, silver, gold, iron) the distance is measured in the micrometers. So, at 100 MHz or so, a piece of aluminum foil is thousands of 1/e deep, the attenuation is in the trillions.. so nothing is getting through. Especially if one is talking about a Mentos can, which is a heck of a lot thicker than aluminum foil!
Interestingly, this skin depth is the reason that copper bus bars in 50/60Hz power plants that are more than, say, a couple of inches thick are built hollow: There's no current flowing in the center, so there's no point in putting (expensive) copper in there. That wouldn't be true for a DC power plant, though: At DC, the skin depth is infinite, and current flows throughout the conductor.
Finishing up: Suppose that one has a TV antenna, made out of metal rods, up on the roof. For this exercise, the metal rods are presumed to be capped and hollow. When moving electromagnetic fields (i.e., TV signals) hit those rods, the rods re-radiate, and the re-radiation hits other bars up there, which also re-radiate, and so on; and the panoply of all this re-radiation ends up being selective to the direction that one is pointing the antenna (i.e., one can point the antenna at a TV transmitter somewhere and get that TV transmitter really, really well, but none of the ones to either side). But the inside of the tubes? There's absolutely nothing going on in there.
Finally.. Back in Physics 169 (Electromag), the prof in the lecture hall had his fun: He had a copper-screened cage, about 7' high on the inside. (So long as the holes in the screen are much, much less than the wavelength of any signals on the outside, such a screen acts as a solid metal box.) He had a volunteer get into the box (it had a screen door, with finger gaskets around it). He then fired up this monster machine that consisted of a spinning conductive disk, about 7' high, that had a long, thick, heavily insulated wire on it, a long insulated handle, and the prof, himself, wore this monster rubber gloves, and fired up the apparatus. It threw, I kid you not, a 4' long continuous arc from the tip of that probe to the screen of the cage. The volunteer on the inside felt nothing, of course; in fact, the volunteer put his hands flat on the inside of the screen on the opposite side of where that arc was terminating and reported, "It tickles a little." Probably from the hot air circulating around.. but not anything shocking.
And this is the reason that sitting in a metal car during a lightning storm is the safest place that one can be. Even a Tesla, with all that glass, still has a metal framework that at the DC frequencies of the lightning shot acts like a pretty good Faraday cage. (Note: car is sitting on insulative, rubber tires. But a lightning bolt that just went through a kilometer of air and hit a car isn't going to have a problem going an additional foot from the bottom of the car to the Earth
On the other hand.. At one time I used to use this Open Air Test Facility (OATS). We'd mount a piece of electronics whose radiation we wished to measure from DC to Daylight in the OATS, which had a metal floor about 50' square and had walls and a roof that were made out of fiberglass with no metal fasteners whatsoever. The gear was mounted on a small elevator that, when down, one could put a metal cover over the hole when no equipment was being used. The center hole could be rotated and there were antennas one could mount on the inside to measure the EMF being given off by the gear. All nice and good, and occasionally the FCC would come down and test some equipment or other that they were interested in.
But if there was a thunderstorm anywhere nearby, we'd retract the gear down, cover the hole, and Get Out Of Dodge: Because any lightning bolt that came down from on high would go straight through all that fiberglass and zap anything, including people, on the inside. Fun. An anti-Faraday cage, if you will.
Wow, you really *are* "Tron" guy.
Oh great and knowing one, why don't they work?If I had to guess it’s probably because they don’t work.
i.e. what is the definition of "not working" In what situations.
Never mind, the OP answered hours before your worthless post.
Nope. Those have some very specific features built into them and operate at a very different frequency range.
That's a little like apples and oranges because we are talking different frequencies here.
So that part that makes a differnece is not that it is
I'm 99% sure that it they aren't using RFID, they are using 300-400 MHz classic key fob frequencies.
RFID often ranges in inches and are mostly used for identification tags. Frequencies can range from kHz to GHz.
One source, which reviewed the FCC registration information indicated that they were on 315 MHz.
Bluetooth is 2.4 GHz.
Took the image out. But, yeah, looks a lot like boards I've designed in the past. But there are engineers who've done lots more circuit boards than I have.
Well, I have been banging on electronics since the mid 1960's in my teens; and the last 30+ years doing actual engineering work, so, yeah.
Let's see. Automotive key fobs frequencies: A quick search says either 300-400 MHz for standard key fobs and in the 2.5 GHz for bluetooth.
Wavelength for 350 MHz is 3e8/350e6 = 0.857 meters. So, so long as the gap between adjacent grounding spots is 1/16 of that, or 5.3 cm, which would be one or two spots on the edge of a Mentos can, a standard Mentos can, with the insulating stuff emery clothed/steel wool'd off should work fine.
Bluetooth, as in Teslas, use the standard ear bud frequencies in the 2.4 GHz range. Wavelength for that would be 3e8/2.4e9 = 0.125 m. One 16th of that would be 7.8mm, or around 1/3 of an inch. Um. That last bit is problematic with a Mentos can, implying that the ground points that one gets with a Mentos can, as one goes around the rim, can't be more than 1/3 of an inch from each other.
Problem with stuff like that when one is putting two halves of a can together is that the can lid probably only makes contact in two or four places; if the can is, I dunno, four inches or so across the entire lid, it's likely that the "ground spots" are an inch or more apart; which means that bluetooth signals will tend to go right through without the 30dB or so of attenuation one would like.
So, in my first response to the OP, I suggested finger stock. Finger stock will make contact at every finger, giving one a solid shielding ground. The tricky bit is finding thin enough finger stock so that one can get the can over it. Image of finger stock:
There's tons of this stuff for sale. Digikey has a couple dozen manufacturers on its catalog and hundreds of types in total; stuff is a couple of bucks for a foot long piece, I think. Tricky bit is to get both sides electrically connected; the fingers should rest against a steel-wool'd shined up area; the base of the gasket should be solder-tacked down several places around the lid on the other half of the Mentos can, keeping in mind that 1/3 of an inch distance between the tack-downs.
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