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I have a gizmo that broadcasts an FM radio signal from my phone's headphone socket to my car's radio antenna.

Sometimes, there's another station on the same frequency with a stronger signal and my car plays that signal instead. However I've noticed that if I hold my hand up near the transmitter, my signal takes over again.

What's going on when this happens? Am I somehow boosting the signal with my hand?

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up vote 7 down vote accepted

You're not boosting the signal; you're either acting as a reflector (capturing a bit more of it to feed to the antenna) or blocking a competing source, or perhaps a bit of both.

By analogy, when you hold your hand to your ear to help you hear something, your hand is acting a reflector for sound waves to direct a little more energy into your ear. It can also keep out other extraneous sounds coming from other directions.

Our bodies contain a lot of water and other chemicals which makes them fairly electrically conductive. Any electrically conductive material can serve as either an antenna or a radio-reflective element. So, the position of your body relative to the radio antenna will affect its reception.

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I've observed that people can alter the tuning, not just reflect or block. – JDługosz Nov 21 '14 at 10:28
@Anthony X What else could act as a reflector? Can I cover my transmitter with something to improve the signal? – Shane Aug 7 '15 at 18:17

A human body may reflect and absorb radio frequencies, though not very efficiently. It may as well act as a resonance chamber for certain frequencies. For a signal of 100 MHz, the involved wavelength is 3 m, and so it is possible that parts of your body are acting slightly as a resonant chamber. (for an optimal resonance, you should have 1.5 m diameter, too many cheeseburgers...)

You might be surprised if you have a look at the amazing Lydia Kavina and other theremin players videos around. This instrument is played without any physical contact, by merely doing hand movements near the antennas. The human body acts itself as a part of an L-C circuit.

enter image description here

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I cannot resist adding a couple of additional amazing links: another Lydia Kavina performance, and a curious amateur thereminist playing The Beatles in front of M51 – Eduardo Guerras Valera May 11 '13 at 20:13

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