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September 11

Induction in contactless cards

How are the wires oriented in contactless card? It seems to me that no matter how I hold it, the reader manages to connect to it. Are the antenna and the RFID in the card coils? Are they multiple wires, oriented in multiple direction, to guarantee that one will get induced?--B8-tome (talk) 07:52, 11 September 2017 (UTC)[reply]

Doesn't much matter. The design and position of the antenna in the reader is such that it'll get it somehow. Provided that there is some field which the card's antenna intercepts, then it'll read. If you carefully align the card to be vertical (relative to the reader antenna) then you can stop it, but this is a very narrow null position - you need to be watching a signal strength meter to feel for it and even then you're unlikely to hold it in place accurately enough.

These cards (ISO/IEC 14443) are at quite a low frequency - 13.56MHz So they work by loop antennae, as a dipole antenna would be too big to carry. As the "card" needs to be flat enough to fit in a wallet, they're almost all a loop antenna around the edge of the card, to make it as big as they can.

It's possible to read these from huge distances away, if you have a big enough loop on the reader and ignore the official power limits - fraudsters are already doing this. If they have access to a venue for installation, or can do it surreptitiously, then any doorway or closed-loop archway you can walk through is potentially a place for a reader.

This is also why it's hard to make "RF shields" to go around them. Tinfoil won't do it. Andy Dingley (talk) 11:04, 11 September 2017 (UTC)[reply]

Shocking. Does this mean that neither my RFID blocking wallet, nor my tin foil hat is protecting me? What could I use instead? Hofhof (talk) 12:22, 11 September 2017 (UTC)[reply]

You can buy RF shields. Some of them work. You can of course test them with a standard reader in many honest shops. But tinfoil and optimism won't do it. Andy Dingley (talk) 12:52, 11 September 2017 (UTC)[reply]

Can you cite some reference on "tinfoil won't do it"? Faraday_cage#History says his first demonstration of the eponymous cage used metal foil. SemanticMantis (talk) 14:41, 11 September 2017 (UTC)[reply]

A Faraday cage is a shield purely against an electrostatic field, rather than electromagnetic radiation. Particularly for frequencies this low, and with loop antennae (the size of which, relative to the wavelength, will influence the shape of this field) a Faraday cage makes a poor radio shield. Now of course you can say, "It's all just electromagnetism, so it's all the same" - which would be half right. Hopefully WP has a better article on RF shielding, because a Faraday cage is starting on the problem from the wrong end.

As to sources, then unfortunately I can't post the NFC and 14443 studies I have, as they're commercially confidential (and very expensive). The published books I have are all a bit old, so not covering this technology in specific detail. But it's core postgrad EE stuff and in all the usual books. 1980s sources (and in the UK is was the TEMPEST hoo-hah that set it off) start to look at issues of 1-100MHz shielding against close proximity devices.

As to making a shielded wallet, then forget tinfoil. Use copper tape instead, or else rigid metal sheet (the stuff stripped out of a scrap laptop is better than anything from a kitchen). But really, a carry-around shield ought to be made from properly made stainless mesh, woven as an RF shield (and not as an oil filter!). Expensive in middling quantities, but you only need a small piece and it's a pig to sew, so buy a good ready-made shield. If you do make a tinfoil shield, test it when new, then progressively as it ages from carrying around. Andy Dingley (talk) 15:08, 11 September 2017 (UTC)[reply]

Thanks. "But it's core postgrad EE stuff", sure, but I don't think I should need a MSc or PhD in EE to see why aluminum foil doesn't provide good RF shielding. The RF shielding article says "Copper is used for radio frequency (RF) shielding because it absorbs radio and magnetic waves.", but gives no further explanation. This is technically true of many metals, correct? It seems implied that copper is better than many alternatives, and that's fine, I don't mean to get in to the atomic structure of metals etc. However, I see plenty of reports online of e.g. cellphones wrapped in aluminum foil not ringing, cards wrapped in aluminum foil not reading at shops, etc. I grant that I have not done these tests personally, but in aggregate the do seem convincing, especially because what I know about Faraday cages. Now, the static vs/dynamic field is a good point, but it seems from first principles that time-varying fields will still be notably distorted when passing in/out of a Faraday cage. For practical protection of a credit card, it is not necessary to completely conceal a reflected ping, only to obscure the signal enough to prevent if from carrying all the same payment info (nb, I know this may require a fair amount of distortion, I'm sure the error detection/correction deployed in these technologies is quite advanced.

Perhaps what you're saying is more along the lines of "Casual application of aluminum foil is not a sufficiently general and robust method of RF shielding. Though it does have some effect on some signals/frequencies, I would not recommend it for serious (govt, engineering, security) applications."? SemanticMantis (talk) 15:46, 11 September 2017 (UTC) (PS: To be clear on the deleted cmoment, I know the phrase "tinfoil hat", and I was responding to you on matters of science, not Hofhof's joke comment, as per WP:THREAD. Also a good example of how jokes on the reference desk can impede communication and scientific discussion, even in small print.)[reply]

Cellphones and WiFi work at frequencies of a gigahertz or two. I'm old enough to believe that nothing cheap and plastic can possibly work at such frequencies and (like broadband over telephone wires) it must simply be demons instead. You can shield those with a thin layer of a conductive metal foil - even then though, it's not a Faraday cage in the simple electrostatic sense.

Card systems though are using a low frequency (13.56MHz), so their shielding needs to work differently. There is less difficulty in shielding tiny holes in an enclosure (WiFi goes through mouseholes), but it's much more difficult to find a bulk shielding material to make the "walls" of such an enclosure. Given the ability to make an antenna bigger than the "wallet" and unrestricted power, I cannot imagine any shield that you could fit in a pocket and that would still be guaranteed to shield it. It is a hard problem to make such a shield for such a frequency, against a large loop antenna.

Aluminium tinfoil is a problem though. Copper has better conductivity and aluminium has a surface oxide that stops it easily forming a single conductive wrapper. Also it's mechanically weak to fatigue and an aluminium foil carried in the pocket will simply fall apart quite quickly. If you want one, use stainless mesh, woven from a grade of stainless with adequate conductivity (hence no oil filters).

If you compare anti-theft electronic article surveillance, the 8.2MHz radio tag versions of this are possible to shield (shoplifters put them inside tinfoil bags, and the shop antennae are restricted in their interrogator power, a limit that card fraudsters don't follow). As a result, other systems are often favoured for high value stock, using magnetic systems rather than the 8.2MHz radio tags. Andy Dingley (talk) 16:16, 11 September 2017 (UTC)[reply]

DIY supercritical steam rocket

Would the following steam rocket design work? We take a thermos flask and fill it partially with hot oil (at about 600 C). We then put in boiling water in the flask using a plastic sheet so that the water initially does not make contact with the hot oil. We then close the thermos flask, turn it upside down at which point the water at 100 C will mix with the hot oil. The lid of the flask must be made such that it will break off at some high pressure that is well below the pressure at which the thermos flask would explode. Is there a simple DIY-way to construct a lid such that it will give way at the right pressure? Count Iblis (talk) 17:32, 11 September 2017 (UTC)[reply]

That's not a supercritical rocket. Instead it's a subcritical steam rocket, hoping to power itself by conduction from a mixture with heated oil.

There are several problems:

It's subcritical. Rather than being a supercritical rocket, where a slow process (heat transfer) is replaced by a fast one (vapourisation and expansion), it's stuck waiting for slow conduction from the mixed oil.
It relies on synchronising two things, mixing and releasing. This is why hypergolic rockets are easier [sic], they turn the two steps of propellant injection and ignition into just one, injection. Similarly a supercritical steam rocket needs to have its blowout disc blow open at some point, but it's less critical as to precisely when.
It assumes that the "plastic sheet" is also a thermal barrier.
It relies on the oil mixing with the water, such that heat conduction will be from small droplets with a great surface area. How does the oil get mixed or dispersed so quickly?
The reaction mass will be a mix of steam and liquid oil droplets, so probably not ideal for getting the most specific impulse from the heat energy.

Andy Dingley (talk) 17:51, 11 September 2017 (UTC)[reply]

Oh, and you don't need a Dewar or Thermos, maybe just a little bit of insulation. You have to heat this up, so you probably do that from outside anyway. Mostly you need a super-strong vessel, as the stronger it is, the higher the pressure you can use it at. This is why ex-WWII aircraft oxygen bottles (before the liquid oxygen systems, or the untestable composite pressure bottles) were used. If you're losing heat you can just add more, but a strength limit is a strength limit and Dewars are never very strong.Andy Dingley (talk) 18:04, 11 September 2017 (UTC)[reply]

The oil would all float above the water, so there wouldn't be much contact area. This would slow the heating of the water. But, it would work, to some degree. You could just use a thin material as the cap, or get a "pressure relief valve" if you need to control the temp better. Also, using a pressure cooker instead of a thermos might make sense, as modern ones have built-in pressure-relief valves. I'm skeptical that you could get it to launch, but you might get it to spin, provided the valve release if off-center, using an apparatus like Hero's engine. And beware that the hot oil spewing from the valve would pose an extreme burn hazard. StuRat (talk) 17:53, 11 September 2017 (UTC)[reply]

This maybe alright as a thought experiment but don't attempt to do it. Live steam is very dangerous and more (lethally) so, if you have to ask these sort of questions in the first place. Although, I do like the idea of mixing oil and water together – that would make a fireman’s day and the local funeral parlor very happy ! Hay, why stop at oil? Add some red fuming nitric acid the mix to archive criticality and maybe you'll be lucky enough to conflagrate your neighbor's home as well. Aspro (talk) 18:26, 11 September 2017 (UTC)[reply]

...or some metallic sodium ? StuRat (talk) 18:35, 11 September 2017 (UTC)[reply]

There is no "right pressure" that won't turn your rocket into a pipe bomb. You cannot hold the pressure of steam in a thermos, so any lid, no matter how weak, will work as well as any other lid. just put a square of heavy plastic like a zip lock freezer bag over the mouth and hold it in place with several rubber bands.

Let's look at the enthalpy curve for water:

http://www.kentchemistry.com/images/links/matter/Heat42.gif

A is -50 C ice heating up towards 0 C. As you can see, it doesn't take a lot of added heat to do that.

B is 0 C ice turning into 0 C water. It takes about 2.5 times more heat to do that than A took.

C is 0 C water turning into 100C water. It takes about 3 times more heat to do that than B took.

D is 100 C water turning into 100 C steam. It takes about 5 times more heat to do that than C took.

So, assuming equal parts oil and water, if your oil had the same specific heat as water, you would need to start with 100 C water and 600 C oil.

Alas, oil does not have the same specific heat as water, it has about 40% as much.

See http://www.engineeringtoolbox.com/specific-heat-fluids-d_151.html

So you would either have to up the temperature of the oil by a factor of 2.5 or use 2.5 times more oil than water.

Your next problem is that the water will not stay in the thermos long enough to turn into steam. The first 1% of the water tuning into steam will blow the rest out of the opening (or blow up your pipe bomb if you use a really strong lid). --Guy Macon (talk) 18:43, 11 September 2017 (UTC)[reply]

It strikes me the main thing missing from this model is a nozzle, e.g. the de Laval nozzle mentioned in the steam rocket article. The science of rockets is a science of nozzles. In theory, hot oil could be used as a reservoir of heat, though producing the heat with a chemical reaction would seem like a lighter option. My assumption is you want to pump your water (even with a simple air pressure mechanism) into a small space ('engine') heated with hot oil or other means, then have it turn to steam near where it exits the nozzle and gives you decent thrust. The hot oil possibly ought to have some way to dribble out too so it can be replaced by fresh hot oil from a reservoir. Maybe you could have it with fuel injectors, only a nozzle replaces the piston? I'm not a rocket scientist, but come on guys, at least try. ;) Wnt (talk) 22:50, 11 September 2017 (UTC)[reply]

P.S. Should the temptation to light that oil get the better of you, there's some discussion about diesel rockets (or alternatives) here. The RP-1 article says OTRAG actually launched a diesel rocket, and of course kerosene is used ... honestly, I don't know what oil you had in mind anyway. Wnt (talk) 23:01, 11 September 2017 (UTC)[reply]

Funny noone stumbled over the plastic sheet at 600 C yet! The best "plastic" industry can offer today can endure up to 600°F aka ~315°C for short periods of time. Additionally and generally please always keep a save distance to anything "supercritical" and dont even think about messing with it unless you have a professional licence for exactly that. You shure dont want to wake up in hospital, blind and feeling your marmorized face from 3rd degree skin burns. --Kharon (talk) 05:34, 12 September 2017 (UTC)[reply]

Hmmm, I've totally neglected the latent heat of water that Guy details above. External heat source will work better, and then we're back to the steam rocket as described on its wiki page. Count Iblis (talk) 19:46, 12 September 2017 (UTC)[reply]