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My sister asked me why her small radio sometimes has better reception when she places her hand nearby (without touching) the antenna. I did some research and wanted to ask a few questions.

Here are some of the ideas I came across:

  1. Electromagnetic radiation like radio waves can induce a current in conducting bodies, and that induced current in turn generates an electromagnetic field, or causes EM waves to be emitted. When the hand is near the radio, emitted EM waves from the hand interfere with the RF waves that the radio antenna receives.
  2. EM waves can be scattered. When placing the hand near the radio antenna, RF waves scattered off the hand cause interference patterns that may be constructive or destructive.

Q1. What is the cause of better radio reception? EM scattering or EM emission from the body due to an induced current?

Q2. Is there a difference between an electromagnetic field (say, generated by an induced current) and electromagnetic waves? I think of an electromagnetic field as attentuating rather quickly, but I don't have that same intuition for radio waves. They seem to travel quite far.

Q3. When touching a radio antenna, it's often said that the reception improves because the body acts as a larger antenna for the radio. Is this effect different than the scattering/emission discussion above?

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    $\begingroup$ Lots of options, depending on the wavelength. One example is radio direction finding for, e.g., search and rescue. The classic Emergency Locator Transmitter (ELT) broadcasts on 121.5 MHz, slightly above the FM band. Placing a handheld radio (my ham handheld does fine) tuned to that frequency with the antenna vertical right next to your body allows for rough direction finding - rotate in place, and the signal/volume will decrease if your body is between the antenna and the transmitter. A YAGI antenna is better, but more cumbersome. $\endgroup$ – Jon Custer Apr 7 at 21:20
  • $\begingroup$ I've been meaning to ask a similar question for a while. I have a shower radio that always gives excellent reception for one station, but my other favorite station is very dependent on where I am in the room. $\endgroup$ – Barmar Apr 8 at 15:39
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Small pocket radios usually have fairly poor antennas in them, because of their small size. But by placing your hand nearby the antenna of that small radio, you are creating a capacitor in which one terminal is the radio chassis and the other is your hand. Any radio frequency signal induced in your skin by radio broadcasts will be capacitively coupled from your skin to the radio by means of that capacitor, and since your body is a lot larger than the antenna in the radio, the radio reception will be strengthened.

Depending on the frequency the radio is tuned to and how sweaty your skin is, the strengthening effect may be greater when you are actually touching the antenna in the radio.

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    $\begingroup$ So the primary reason reception is improved is from the induced radio frequency emitted from our body, which becomes coupled with the attena. The RF scattering is a different effect that doesn't necessarily improve reception. Is that correct? $\endgroup$ – nwsteg Apr 7 at 22:00
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    $\begingroup$ yes, that is correct. $\endgroup$ – niels nielsen Apr 8 at 2:12
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    $\begingroup$ @StrugglingStudent42 Yes, the human body is effectively a 100pF capacitor in series with a ~1.5kohm resistor. $\endgroup$ – J... Apr 8 at 14:20
  • $\begingroup$ I observe a different effect that doesn't require me to be touching the antenna, it's just dependent on how close I am. There are a number of "sweet spots" in the vicinity of the radio. $\endgroup$ – Barmar Apr 8 at 15:42
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    $\begingroup$ @Barmar, if you are talking about FM radio, that is a common occurrence- your body can act as a reflector or a lens, making reception either better or worse depending on where you position yourself within a span of about 1 to 2 meters. $\endgroup$ – niels nielsen Apr 8 at 16:55
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The EM waves act on charges, and the frequency of the movement of that charges is selected by the LC reception circuit of the radio.

But without enough carries present in a good antenna, there is no much signal to select. The effect of the human body is to provide movable charges for the EM waves. When touching the antenna, it increases its role. But even without touching, there a good capacitive effect due to the high frequency of the waves.

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  • $\begingroup$ It's kinda weird that EM waves jiggle the electrons in our body but don't affect our brain function too much, which depends on ionized chemicals moving around. $\endgroup$ – nwsteg Apr 8 at 1:09
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    $\begingroup$ Well, the currents are very weak, and flow mostly through our skin. $\endgroup$ – Claudio Saspinski Apr 8 at 2:11
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    $\begingroup$ @StrugglingStudent42 In addition to the currents being weak, it's also worth noting that neurons don't have ion currents flowing down their length. Rather, there are special gated channels that open upon reaching threshold voltages, which then create a propagating wave of channel activation, causing ions to flow across the membrane. It then resets after the pulse (known as an action potential) by pumping them back out again. So in order for EMF to interfere with the nervous system, it needs to result in a sufficient voltage drop to either trigger or suppress this process. $\endgroup$ – Dan Bryant Apr 8 at 17:56
  • $\begingroup$ @DanBryant You make a good point about the gates along the axon length. I was thinking that the close range electrochemical forces must be much, much greater than any forces from the EMF. $\endgroup$ – nwsteg Apr 8 at 18:31

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