5
$\begingroup$

I just did the following experiment:

Using a metal Whitman's chocolate box, I placed my phone it it, closed the lid then called it from another phone. To my surprise, it rang.

The normal parameters of a faraday cage is any gaps have to be small compared to the wavelength of the radiation. The edges of the box overlap by about 1 cm, and the gap is a fraction of a mm.

Phones operate at 1-2 GHz.  Skin depth for 1 GHz is about 2 microns Wavelength of 1 GHz is about 8 inches.

Even if it's 30 ga metal that's still 12 thou or about 300 microns.  Now it's steel, not copper, but steel isn't THAT much worse a conductor.

I don't understand what is going on.

Edit: More experiments:

I have phoning over wifi enabled, and I'm sitting 20 feet from the access point. Disabling the wifi on the phone and using the 2 bar local cell signal, no calls.

Re-enable wi-fi and I can reach the phone in the box intermittently. I can ping the phone from my local network which makes for faster tests. Curiously, the phone doesn't respond to pings when it's asleep.

My current working hypothesis is that the electrical connection between the box and the lid isn't reliable.

The box is laquered with red and gold Christmas colours which may be acting as an insulator.

After opening and closing the box several times, while running a ping to the device over my local I get erratic results. Sometimes closing the box gets me 'request timed out.' sometimes not.

Wrapping it with alumininum foil loosely gives more odd results.

At one point I had it loose enough that I could see the phone -- gap of maybe 1 cm, and got device unreachable, and didn't become reachable until it was open to about 2 inches. I suspect that my AP has some kind of adaptive power setting.

Right now it's sitting in it's metal box and I'll get a series of 20 unreachables, 4 pings with variable times, more timeouts.

$\endgroup$
5
  • $\begingroup$ What is the length of the longest seam in the box? $\endgroup$ – Bob D Oct 28 '19 at 16:19
  • $\begingroup$ Carbon steel has a resistivity 10x higher than copper, stainless steels more like 40x. Just to say just how much worse they are. A cheap box is likely just carbon steel. $\endgroup$ – Jon Custer Oct 28 '19 at 16:49
  • $\begingroup$ If stainless steel has a resistivity of $7\times10^{-7}$ ohm-m, then the skin depth at 9 GHz is only 4 $\mu$m. I would think that if the box is 400 $\mu$m thick, then there should be no way for microwaves to just penetrate it -- it would fall off by $e^{-100}$. $\endgroup$ – user4552 Oct 28 '19 at 20:12
  • $\begingroup$ Transmission through a circular hole of radius $a$ is $T=(a/\lambda)^4$. See physics.stackexchange.com/questions/141562/… . Not sure if this makes sense at all for a linear seam, but supposing that something like this does apply, then we basically expect the attenuation through the seam to vary like a power law, which is certainly going to be much gentler than the exponential attenuation we get from skin depth. $\endgroup$ – user4552 Oct 28 '19 at 20:24
  • $\begingroup$ Skin depth has a square root dependency on resistivity. So if it actually is 40 times the resistivity, then it would have a skin depth of roughly 12 microns instead of 2. Obviously, however there is enough leakage for the phone to reconstruct enough signal to ring. $\endgroup$ – Sherwood Botsford Oct 30 '19 at 15:18
1
$\begingroup$

The normal parameters of a faraday cage is any gaps have to be small compared to the wavelength of the radiation. The edges of the box overlap by about 1 cm, and the gap is a fraction of a mm.

The important dimension is the longest dimension of the gap. So if you have a seam 0.01 mm wide, but 10 cm long, it's the 10 cm dimension that matters when deciding if the gap is "small" compared to the wavelength.

So if the seal between the lid and the body of your candy box goes all the way around the box, as they typically do, then the critical dimension is the length of the box.

It's also possible the box parts have some coating (paint or a plastic film) that prevents the two metal part making a metal-to-metal contact anywhere in the seam.

The fact that the two parts overlap by 1 cm does mitigate this somewhat, so you would expect some blocking from this arrangement.

The point made by Jon Custer in comments about the high resistivity of the carbon steel used in this kind of box is also likely relevant.

$\endgroup$
3
  • $\begingroup$ This answer makes some assertions and speculations, but I don't see any theoretical or mathematical foundation here. The important dimension is the longest dimension of the gap How do you know? Is this based on some theory? On some calculation? $\endgroup$ – user4552 Oct 31 '19 at 4:27
  • $\begingroup$ @BenCrowell, the point about the gap dimensions was one small part of a long multi-part question. If someone wants to ask a question soley about that, I could give a longer explaining it in more detail. $\endgroup$ – The Photon Oct 31 '19 at 15:54
  • 1
    $\begingroup$ If you want to research it on your own, google "slot antenna" and "Babinet's principle". $\endgroup$ – The Photon Oct 31 '19 at 15:55
3
$\begingroup$

I'm not conversant in RF shielding, but it is possible that the simple friction fit of the cover on the box doesn't make a good RF shield. If it were that simple, microwave ovens would not require special RF shielding techniques to prevent microwave leakage from occurring where the inner perimeter of the door makes contact with the front surface of the oven. They would simply rely upon good (small gap) surface contact between the door and the oven.

Hope this helps.

$\endgroup$
1
  • $\begingroup$ Expanding a little on your reference to microwave ovens: indeed because it's so difficult to get a reliable electrical contact all the way around the perimeter of an openable door (especially with greasy food residues) they do not rely on a Faraday cage approach. Instead a carefully chosen gap forms a "quarter-wave choke" and acts like a short circuit for a narrow range of frequencies around the operating frequency. $\endgroup$ – pericynthion Nov 1 '19 at 5:22
-2
$\begingroup$

All light waves and all cell phone waves have reflections. Both light waves and cell phone waves reach inside of the box, if there is a possibility to come inside through reflections, regardless of the material by which the box was made up. Our eyes may not be able such reflected light but ordinary antenna sensors may be able to observe such reflected cell phone waves. If reflection is not possible it is very difficult even for cell phone waves to get inside. For example, it is always a challenging task to have radio communications with submarines which are immersed in sea water. This happens because there are no reflections of radio waves in sea water, except penetration of radio waves to reach submarines immersed, because sea water covers immersed submarines completely with no gaps.

$\endgroup$
1
  • $\begingroup$ I don't think this answer was helpful... the whole point of the OP's question was wondering how the waves could get inside the metal box, and just saying "reflections" doesn't really answer that. Some of the previous answers did explain where the gap might be. $\endgroup$ – Eric Smith Mar 22 at 12:22

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.