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:
- 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.
- 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?
Best Answer
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.