I typically listen to NPR in the morning from a clock radio next to my bed. There are days when just moving my body, even my arm or leg, is enough to cause interference with the radio to dramatically decrease the volume, or it appears as if I lost the radio station altogether. I know that my body acts like an antenna because, from experience with an oscilloscope probe, I can grab one end of the probe and amplify the AC signal coming from the overhead fluorescent lights in the physics lab room. So I am assuming that this is the same effect as my clock radio.

  1. How is this interference created? That is, how is my body acting like an antenna, causing interference to occur with my clock radio?
  2. I assume that this interference of my body is (i) size- and (ii) distance-dependent as well as (iii) frequency-dependent. Can someone explain the connection between these things?
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    $\begingroup$ Check this post out: physics.stackexchange.com/q/101913 It might help you. If not its really interesting anyway. $\endgroup$ – jerk_dadt Mar 6 '14 at 1:59
  • $\begingroup$ @jerk_dadt: thanks for the link and it was really interesting. I believe that it doesn't answer my question but it has to be related somehow. $\endgroup$ – Carlos Mar 6 '14 at 3:35

This is something we must all have observed, but I don't know of any definitive study. In the absence of hard data I can think of three potentially relevant effects:

  1. Dielectrics, like the human body, deform electromagnetic fields in their vicinity

  2. The wavelength of FM radio is around 3m and therefore comparable to the size of a typical human. This means your body may be refracting the radio waves.

  3. The proximity of your body may be causing changes in the inductance of coils in your radio and therefore changing the tuning.

Re suggestion 3, I have noticed that with my old radio if I put my arm near it the station fades out but can be retuned. Then when I remove my arm the station fades and needs tuning back to the original setting. This suggests option 3 is the cause, but whether this applies to all radios or just this specific case I don't know.

Whatever the cause, I don't think it's your body generating interference. You lose the signal either because the presence of your body reduces the radio signal strength or in option 3 changes the tuning.

  • $\begingroup$ how is my body changing the inductance of the radio? $\endgroup$ – Carlos Mar 7 '14 at 1:23
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    $\begingroup$ @Carlos: Bringing any dielectric near an inductor will change its inductance because it changes the permittivity of its surroundings. I'd guess the effect is generally small, but if the radio reception is marginal it may be enough to cause the radio to lose the station. $\endgroup$ – John Rennie Mar 7 '14 at 7:05
  • $\begingroup$ I love the way your answers are always clear and well thought out. Thanks! $\endgroup$ – Carlos Mar 7 '14 at 10:55

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