Why does a radio still work inside a metal box?

In summary, a commercial radio placed inside a metal tin with a tight lid may work when receiving an AM station (1 MHz) due to the frequency-dependent penetration depth of RF waves. However, it may only produce static when switched to FM (100 MHz) due to the smaller skin depth at higher frequencies. Grounding the tin may improve reception, but the use of a ferrite rod antenna may also play a role. The presence of a narrow slot around the lid's closure may allow the radio to work at MF, and the length of the slot is more important than its width. The hissy static heard may be caused by a variety of noise sources, including cosmic rays, solar wind, and RF radiation from nearby devices.
  • #1
Paul Uszak
84
7
I've put a small battery powered commercial radio inside a Quality Street tin. The tin is all metal with rolled joints and the lid is a tight fit. I'd be willing to bet that there are no holes greater than 0.1mm around the lid's closure.

Why does the radio work when receiving an AM station (1 MHz), but only produces static when switched to FM (100 MHz)?
 
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  • #2
The depth to which RF can penetrate an ungrounded metal shield is frequency dependent. Read about skin effect/depth.

I don't know for sure what will happen, but try grounding the tin to a water pipe.

AM is around 1Mhz, FM is around 100Mhz. Skin depth will be 10 times less at 100Mhz.

Minimum signal strength required for AM and FM receivers also differ (FM has capture ratio)
 
  • #3
Paul Uszak said:
I've put a small battery powered commercial radio inside a Quality Street tin. The tin is all metal with rolled joints and the lid is a tight fit. I'd be willing to bet that there are no holes greater than 0.1mm around the lid's closure.

Why does the radio work when receiving an AM station (1 MHz), but only produces static when switched to FM (100 MHz)?
If there is a narrow slot around the lid closure, this may allow the radio to work at MF. A ferrite rod antenna, as used in your receiver, can work when placed inside a metal container, such as a tube, having a longitudinal slot in it. It is sometimes said that the H-field of the wave can then enter, whereas the E- field cannot. The system is used for precision direction finders. However, the theory of how it works is subject to discussion, because in essence, E and H cannot exist separately. For the VHF case, if the slot is half a wavelength or more in length, the signal can enter. But in your case, the slot is perhaps 0.2m long, whereas half a wavelength is 1.5m. A slot can be extremely narrow, like a razor cut, and still work as an efficient antenna provided it is long enough.
 
  • #4
The tin is electrically small at lower AM frequencies so you can mainly treat the fields (IRT to shielding) as quasi-static and separate on it if the loss from skin effect is low. The ability to shield electostatic fields from the outside is related to the conductivity of the metal and the ability of free charges in that conductor to move and neutralize the field inside in response to the external field. The ability to shield static magnetic fields depends on the permeability of the metal and its ability to shunt or draw the field away from the inside of the enclosure not to block the magnetic field. A ferrite rod antenna receives the magnetic field conponent of the EM wave so it should have little problem at AM wavelengths in an enclosure with likely poor permeability at that frequency range like a thin metal tin. Even tiny electrical gaps in good shields would allow some flux to enter the box.
 
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  • #5
Is my answer technically wrong? If one assumes a wire antenna (not a loop, or ferrite), would skin depth be the primary difference?

I didn't think of the ferrite vs whip antenna possibility, which is likely the major difference, but I'm curious beyond that.

Assuming a wire antenna, would grounding the box make any difference?
 
  • #6
tech99 said:
If there is a narrow slot around the lid closure, this may allow the radio to work at MF. A ferrite rod antenna, as used in your receiver, can work when placed inside a metal container, such as a tube, having a longitudinal slot in it. It is sometimes said that the H-field of the wave can then enter, whereas the E- field cannot. The system is used for precision direction finders. However, the theory of how it works is subject to discussion, because in essence, E and H cannot exist separately. For the VHF case, if the slot is half a wavelength or more in length, the signal can enter. But in your case, the slot is perhaps 0.2m long, whereas half a wavelength is 1.5m. A slot can be extremely narrow, like a razor cut, and still work as an efficient antenna provided it is long enough.

The slot eh? The tin is approximately 300mm in diameter so that slot might actually be 900mmish long.

I'll ground the box tomorrow and see...
 
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  • #7
I have a battery powered radio (Sony ICFS22 FM/AM Pocket Radio) inside a fully enclosed metal tin. The radio just produces a hissy static, not receiving any particular FM station.

What specific noise source generates the majority of the hiss? I'm thinking of things like shot noise or Johnson–Nyquist noise etc. I guess this is dependent on which component does the primary reception. Or is it something else altogether?
 
  • #8
Hissy static is low level random white noise radiation comprised of multiple sources.
Everything from cosmic rays, and CMB, to solar wind, to your nieghbours microwave oven, and a lot more is likely to be in there.
 
  • #9
Ah, no.

This question has bled over from another. The radio is in a fully enclosed metal tin acting as an effective Faraday cage. This is confirmed as tuning the radio into Lady Gaga generates joy, but when the tin's lid is put on without retuning, she sadly goes away and is replaced by static. Consequently the only thing generating the hiss comes from within the tin and thus the radio components themselves...
 
  • #10
With closure of the tin, it is not the width of the gap that counts but the length of the slot. A long slot can support a peripheral RF current that emulates a dipole. A slot should work better when the length is closer to half a wavelength. Therefore 100MHz FM should work better than 1MHz AM through a shorter slot. There is also a rule of thumb that says a slot with a length less than one tenth of a wavelength will not radiate significantly.

I have known signals between 100kHz and 1MHz to pass through the welded forward hull of an ice strengthened ship. There was no local VHF FM detectable inside the forward hull. Any VHF FM would have had to enter through the gaps around the bulkhead doors that we closed to quieten the RF environment.

So I think in your case it has to be skin effect and the sensitivity of the receiver employed that is making the frequency sensitive difference.
 
  • #11
Paul Uszak said:
and thus the radio components themselves...

There is a certain amount of hiss generated by thermal noise from components ... this is a huge problem in radio astronomy
and similar research when extremely low noise levels are required so that the very weak signals can be heard
one of the significant things that is done is to cool down the receiver to very low temperatures using liquid nitrogen
This decreases thermal noise substantially
 
  • #12
FM detectors effectively measure phase shift, so the hiss is not necessarily the noise of the front end RF amplifier/mixer, but the phase noise of the first local oscillator amplified in the limiting amplifier chain.

Liquid N2 boils at −196°C = 77 K. Radio amateurs communicating by Earth–Moon–Earth reflection often use liquid nitrogen.
Liquid He boils at −269°C = 4 K. Radio astronomers use liquid Helium to cool their GaAs FET frontend RF amplifiers.
 
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  • #13
In addition to skin effect I think better signal strength is required for wideband FM to be demodulated (capture ratio). FM is there, or not there, sort of. AM on the other hand linearly fades into the noise.
 
  • #14
FM radios use a limiter, which essentially turns the (sometimes 10.7Mhz) IF into (essentially) a square or sine wave. The discriminator, through a complex process (for wideband FM, anyway) turns that into a demodulated signal (roughly akin to measuring the phase or frequency shift). When the IF "squarewave" is just random noise, it essentially produces loud hiss. It isn't really coming from anything in a classic signal sense. It's just the output of a discriminator trying to discriminate random input. As the signal to noise ratio increases (say you slowly increased the FM signal's strength) the "real" signal sort of takes over (a threshold effect) the process and you get demodulated FM. In an AM radio you would have heard the signal quality slowly improve "underneath" the noise until eventually the noise was mostly suppressed (by the AGC)

Generally FM capture ratio really refers to the ability of FM to lock on to 1 or the other of two signals on the same frequency, but I loosely (and maybe incorrectly) use it to refer to the wideband FM threshold effect also.
 
  • #15
Baluncore said:
With closure of the tin, it is not the width of the gap that counts but the length of the slot. A long slot can support a peripheral RF current that emulates a dipole. A slot should work better when the length is closer to half a wavelength. Therefore 100MHz FM should work better than 1MHz AM through a shorter slot. There is also a rule of thumb that says a slot with a length less than one tenth of a wavelength will not radiate significantly.

I have known signals between 100kHz and 1MHz to pass through the welded forward hull of an ice strengthened ship. There was no local VHF FM detectable inside the forward hull. Any VHF FM would have had to enter through the gaps around the bulkhead doors that we closed to quieten the RF environment.

So I think in your case it has to be skin effect and the sensitivity of the receiver employed that is making the frequency sensitive difference.
I looked up the skin depth for various metals at 1 MHz. For copper it is 65 um and for steel it is 7.5 um. If the material is 30 gauge, that is 305 um thick. Of course, the signal does not stop at the skin depth, but continues to fall off exponentially, so if the signal is very strong, reception might still be occurring. The receiver has Automatic Gain Control , so even a weak signal may sound quite normal.
 
  • #16
meBigGuy said:
Generally FM capture ratio really refers to the ability of FM to lock on to 1 or the other of two signals on the same frequency, but I loosely (and maybe incorrectly) use it to refer to the wideband FM threshold effect also.
If one FM signal is more than 12dB above all others, the stronger signal will be clearly demodulated without interference from other co-channel signals.
If one FM signal is 12dB above the noise floor it will be cleanly detected, but if below 12dB it will be audible with noise.
 
  • #18
tech99 said:
I looked up the skin depth for various metals at 1 MHz. For copper it is 65 um and for steel it is 7.5 um. If the material is 30 gauge, that is 305 um thick. Of course, the signal does not stop at the skin depth, but continues to fall off exponentially, so if the signal is very strong, reception might still be occurring. The receiver has Automatic Gain Control , so even a weak signal may sound quite normal.
Having just looked this up a couple of days ago, the skin depth is the depth which the currents degrade by 1 neper (about 8 and a half dB). A strong signal might be 60dB over the minimum needed. (Or considerably less; down to 0dB for a weak signal or a cheap radio.) It's surprising the radio works at 1 MHz, but not unheard of with a strong signal.
 
  • #19
I've never seen a quality street tin. But from a picture there looks to be some sort of enamel or paint on it. The paint could act as a dielectric/insulator, or not depending on its unknown electrical characteristics.

BTW, Grounding to Earth and grounding to the receiver ground are different things.
 
  • #20
Even inside a Faraday cage, cosmic waves and such will generate some noise. I would tend to agree that various thermal/shot noise from the electronics would tend to dominate in this case, yet there are many potential causes of noise. Sorting them out has been done, but you will need more than a cookie tin to do it.
 
  • #21
Baluncore said:
FM detectors effectively measure phase shift, so the hiss is not necessarily the noise of the front end RF amplifier/mixer, but the phase noise of the first local oscillator amplified in the limiting amplifier chain.

Liquid N2 boils at −196°C = 77 K. Radio amateurs communicating by Earth–Moon–Earth reflection often use liquid nitrogen.
Liquid He boils at −269°C = 4 K. Radio astronomers use liquid Helium to cool their GaAs FET frontend RF amplifiers.

yeah I almost added Helium as well LOL
thanks for the addition
 
  • #22
meBigGuy said:
Is my answer technically wrong? If one assumes a wire antenna (not a loop, or ferrite), would skin depth be the primary difference?

I didn't think of the ferrite vs whip antenna possibility, which is likely the major difference, but I'm curious beyond that.

Assuming a wire antenna, would grounding the box make any difference?

Not really, I think the primary factor is the tin is just a bad shield at low frequencies.
http://hollandshielding.com/109-Mu copper Faraday cages
 
  • #23
Does a magnet stick to your Quality Street Tin?

It probably will, tin plated steel is standard for these products. In the off chance that it is aluminum:

Skin depth of Al is 82um @ 1MHz. Food tins are typically 0.2mm thick or 2.4 skin depths, or 16dB attenuation (not much attenuation).
 
  • #24
I just want to point out that the VERY LOUD noise of an FM receiver with lack of signal is very different from the hiss of an AM receiver. That is because of the limiting IF amplifying everything to full level and the discriminator trying to detect FM. I'm pretty sure that, in the tin, various noise sources from the front end dominate. But their characteristic sound is changed by the limiter/discriminator.

If you listen to an AM radio at normal volume, and then remove the signal, there will be some increase in hiss due to the action of the AGC. In an FM receiver, without a mute, or squelch function, there will be a very loud, full volume "roaring" hiss.
 
  • #25
meBigGuy said:
When the IF "squarewave" is just random noise, it essentially produces loud hiss. It isn't really coming from anything in a classic signal sense. It's just the output of a discriminator trying to discriminate random input.

The radio is inside a fully enclosed metal tin acting as an effective Faraday cage, discussed earlier in this thread. There should be no input inside the tin. At least non at FM frequencies it seems.

I'm picky about the source of the hiss as I'm experimenting with using the (hopefully random) hiss as an entropy source for a hardware random number generator. It would be interesting to know the initial source of the hiss. If it's thermal then I can rely on it being random, but if it's just a phase difference there is the danger of harmonics, predictable wave interference and autocorrelation which are all bad for random numbers.
 
  • #26
Paul,
Just curious how have you determined that the tin is acting as an effective Faraday cage.

Your original post said the radio worked when tuned to an AM station. I assume that this means that you could hear the programming, is this correct?. This would suggest it is not an effective Faraday cage, at least at 1MHz. If you are hearing AM through this tin then it is either entering through seams or through skin depth penetration.

Have you tried the magnet experiment?
 
  • #27
Paul Uszak said:
The radio is inside a fully enclosed metal tin acting as an effective Faraday cage, discussed earlier in this thread. There should be no input inside the tin. At least non at FM frequencies it seems.

I'm picky about the source of the hiss as I'm experimenting with using the (hopefully random) hiss as an entropy source for a hardware random number generator. It would be interesting to know the initial source of the hiss. If it's thermal then I can rely on it being random, but if it's just a phase difference there is the danger of harmonics, predictable wave interference and autocorrelation which are all bad for random numbers.
The noise mainly comes from thermal noise in the receiver front end. With an FM receiver, the spectrum of the audio noise is triangular i.e. stronger at the high frequency end. To some extent this effect is reduced by the de-emphasis circuit in the receiver. Another problem is that it is possible that the largest peaks of noise are being accidentally clipped off by the receiver audio amplifier - you would not know. It is possible to make a noise generator using a Zener diode with an amplifier.
 
  • #28
there is no way I could comment on the power spectral density of the "mostly noise" output of a radio with "very little" input signal, be it AM or FM.
You need to look at it on a spectrum analyzer. I expect you will not like what you see.

Seems like a bad idea for a noise source. Won't be consistent over time (oxidation of metal at joints) or over location (nearby radio stations).

http://www.maximintegrated.com/en/app-notes/index.mvp/id/3469
http://www.experimentalistsanonymous.com/ve3wwg/doku.php?id=noise_generator
fetch.php?media=white_noise_stage.png
 
  • #29
  • #31
the_emi_guy said:
Have you tried the magnet experiment?

The magnet experiment, and some other actions suggested by contributors:-

The tin IS magnetic, attracting a magnet well
The walls are 0.25mm thick (including paint)
The inside of the tin is unpainted

This is the tin and radio (should have posted a photo earlier :frown: )

radio-tin.JPG

http://argos.scene7.com/is/image/Argos/5003820_R_Z002A_UC1051961?$TMB$&wid=312&hei=312

I grounded the tin to the Earth wire from a UK mains electrical socket, and repeated the reception test. I filed off the paint where the wire touched the tin. Results:-
FM: Total static. No discernible change with /without Earth wire. No Gaga.
AM: Tinny but recognisable radio station. No, repeat no discernible change in pitch or volume with /without Earth wire. Surprised.
 
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  • #32
Baluncore said:
FM detector noise is certainly not random

This vexes me. Some contributors have suggested that the hiss is thermal noise in the early stages of the receiver. I take this to be stochastic Nyquist noise. If so, surely its amplification should also be random...
 
  • #33
Doing the math:
0.25mm is 33 skin depths in steel which is 285dB attenuation. It's not getting in that way.

Seems like it must be a seal issue around the lid or bottom

When two smooth metal surfaces come into contact it is not uncommon for the actual contact to be limited to widely separated discrete points. Even if it "feels" tight you may only have a few point of actual contact. This is why we use fingerstock for electrical bonding, and why motors use brushes for contact with armature.
 
  • #34
The answer is to use an audio spectrum analyzer which can be done with a PC sound card's line input.

I never worked with one but it has 14 day eval and is inexpensive. http://www.zelscope.com/
Maybe someone else has a preferred application. For example, store the data and analyze it in matlab, if you want.
 

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