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Why does a radio still work inside a metal box?

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  1. Jun 6, 2015 #1
    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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  3. Jun 6, 2015 #2

    meBigGuy

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    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)
     
  4. Jun 7, 2015 #3

    tech99

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    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.
     
  5. Jun 7, 2015 #4

    nsaspook

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    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.
     
    Last edited: Jun 7, 2015
  6. Jun 7, 2015 #5

    meBigGuy

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    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 possibilty, which is likely the major difference, but I'm curious beyond that.

    Assuming a wire antenna, would grounding the box make any difference?
     
  7. Jun 7, 2015 #6
    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...
     
    Last edited: Jun 7, 2015
  8. Jun 7, 2015 #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?
     
  9. Jun 7, 2015 #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.
     
  10. Jun 7, 2015 #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...
     
  11. Jun 7, 2015 #10

    Baluncore

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    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.
     
  12. Jun 7, 2015 #11

    davenn

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    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
     
  13. Jun 7, 2015 #12

    Baluncore

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    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.
     
  14. Jun 8, 2015 #13

    meBigGuy

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    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.
     
  15. Jun 8, 2015 #14

    meBigGuy

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    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.
     
  16. Jun 8, 2015 #15

    tech99

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    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.
     
  17. Jun 8, 2015 #16

    Baluncore

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    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. Jun 8, 2015 #17

    meBigGuy

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  19. Jun 8, 2015 #18
    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.
     
  20. Jun 8, 2015 #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.
     
  21. Jun 8, 2015 #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.
     
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