I was watching my oscilloscope as I was holding a very short wire attached to the probe. It was showing about 0 voltage (small spikes) until I touched the metal. When I touched the metal, the scope immediately showed a 60.0Hz (peroid of 16.66ms), approximately sinusoidal (very noisy) waveform. Considering that the power in my area is a 60Hz AC, I think it most likely that it was picking up coupling from the power lines. This sine wave was immediately replaced with a flat, -200mV line when I touched an electrical ground, but resumed when I let go.

Was I acting as an antenna for electrical noise from the mains line? I was wearing rubber shoes on a concrete floor and touching nothing but the probe.

  • You can even make music this way with instruments like the theremin for example. – Magic Smoke Apr 18 '16 at 14:59
up vote 35 down vote accepted

Antennas are usually thought of as converting a radio wave into an electrical signal. I say this because you wouldn't name the plates of a capacitor as antennas. Capacitive coupling (not EM antenna reception) is the phenomena you witnessed when you touched the o-scope probe.

You might have heard about fluorescent lamps lighting up when pointed at overhead wires: -

enter image description here

The alternating electric field produced by the overhead lines cause a small current to flow through the lamps and light them - this is the same principle involved when touching the scope probe. Picture taken from here. If there were an earthed shield above the tops of the tubes and below the overhead wires, those lamps would not illuminate.

Your body has a large surface area and this massively increased the capacitance between probe tip and the local conductors supplying over one hundred volts RMS around your building.

When you also touched earth, this galvanic electrical connection would dominate the capacitive "connection" to your AC power wiring and the "picked up" signal reduced significantly.

It's all about potential dividers formed by capacitors and resistors and not really about antennas (RF electromagnetic wave devices).

And the presence of the human body can reduce signals reaching (say) a sensor plate: -

enter image description here

Here, the hand (and its capacitance to ground) diverts the electric field from hitting the "receive" plate and less current flows into that plate.

Your body can act as an antenna, but definitely not in this case. For a 60Hz signal, the value of λ/2 is 2500 km, so your body has not nearly the right size to act as one. You have picked the mains wave via the capacitance between your body and the electric wires.

Things change in VHF/UHF range, where your body can actually convert an electromagnetic wave to a signal. In analog TV era, this phenomenon could be easily seen when tuning the "rabbit ears" dipole antenna: the reception quality would sometimes improve while you're holding the antenna, and revert back when you let go.

  • 3
    This answer is the most accurate. Distance up to one wavelength is usually considered near-field, where capacitive coupling has larger effect than actual antennas: en.wikipedia.org/wiki/Near_and_far_field . And for 60Hz, that near field extends to 5000 km distance from the closest mains power cable. – jpa Apr 18 '16 at 9:22
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    Hm, so, according to the Internet, my mother could double as a 60Hz antenna? – Kroltan Apr 19 '16 at 10:03

Yes, definitely. Your body is partially conductive. It's enough for your skin to act as an 'antenna' and inductively couple your oscilloscope to the 60 Hz power line. Due to your body actually being a bad conductor, the induced energy does not produce much power. It's possible to see the voltages relative to power ground on your scope because it has a high impedance input. When you touch a conductor, you're applying a lower impedance 'load' and you're completing the circuit, dropping the voltage to that of the electrical ground.

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    So those cartoons where people or animals stand on top of old satellite TVs to boost the signal are based on real science? – intcreator Apr 18 '16 at 4:39
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    Sorry but this answer, despite the up-votes is wrong. There is no inductive coupling at work here -1 – Andy aka Apr 18 '16 at 7:54
  • @brandaemon Yes. (But with cartoonish exaggeration) One common problem in the real world was that you could adjust the TV perfectly as you stood next to it, but when you walked back to the couch, reception got bad again. – Stig Hemmer Apr 18 '16 at 8:20
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    Anecdote: We were playing with an old TV, and a friend was the antenna. He put his finger into the coaxial input and we got a signal. But the guy was into heavy metal, so that could have been a factor. not serious, but true – WalyKu Apr 18 '16 at 8:36
  • @andyaka I have to agree with you after reading the other answers. Despite all the TV anecdotes, TV signals are at a much higher frequency than mains AC. See Grigoryev's answer for the explanation. – dpdt Apr 21 '16 at 23:44

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