Human (And Dog) Antenna
May 25, 2008 11:01 AM   Subscribe

A question about how our bodies affect radio reception...

We have a stereo in our living room that has poor radio reception. It is plugged into a power strip that is in turn plugged into a wall outlet (an inner wall, if that's important). It doesn't have an antenna.

Whenever one of us walks by, the reception gets crystal clear, and significantly louder. It reminds me of when I was a kid and we could get clearer reception on the television by holding the "rabbit ears" in our fingertips. But, with the stereo, there's no one touching any part of it when the phenomenon occurs. It even happens, though not every time, when our dogs walk by (they're both small terrier mixes, if that matters), but not when the cats walk by.

I understand that our bodies can act as antennae, but how does this work, especially when simply walking by the stereo without touching it?
posted by amyms to Science & Nature (13 answers total) 5 users marked this as a favorite
I am very curious about this, too. The same thing happens with my tiny clock radio that has no external antenna.
posted by fructose at 11:32 AM on May 25, 2008

As I understand it, you are crossing the parabolic antenna pattern of electromagnetic radiation. This is generated by any receiver or transmitter. You will find that your stereo does have an antenna -- even if it is not connected to an external antenna, there are components in the stereo that perform the function of an antenna. The power cable frequently acts as an antenna when there is no external antenna connected and your stereo undoubtedly has an internal carbon rod or some other electrical component that is designed to act as an internal antenna.
What is happening is that there is induced current flow when a conductor (you or the dog) moves in a magnetic field (the internal antenna's parabolic field). This occurs because of Fleming's right hand rule of electromagnetic generation. If you don't believe that you are a conductor, just try sticking your hand in an electrical socket (don't try this at home kids, or we are going to need another fructose!). In this case, you or the dog are cutting through invisible lines of the internal antenna's generated magnetic field and in doing so, you are inducing an electrical current to flow through the antenna. This amplifies the radio reception. It is a well known phenomenon. When I was at school, we kids used to take it in turns to hold the rabbit-ears (TV aerial) on our laps, as this gave us a better TV reception ... :-)
posted by Susurration at 11:48 AM on May 25, 2008 [4 favorites]

PS - as well as moving through the electromagnetic field, just sitting in it generates inductance, which generates an induced current in the antenna. This resonates with the signal passing through the internal antenna, which could just be a track on a circuit board in the stereo or the clock radio (although the clock radio will definitely have a little carbon rod and a small internal wire or metal rod for the antenna, depending on the reception band).
So - amyms - the trick now is to persuade your dog to sit in the right place!
posted by Susurration at 11:53 AM on May 25, 2008 [1 favorite]

Thanks, Susurration.

So - amyms - the trick now is to persuade your dog to sit in the right place!

That'll be a fun project (and I'll be doing it for Science!).
posted by amyms at 12:16 PM on May 25, 2008

Nice answer, Susurration! Flagged as awesome.
posted by horsemuth at 12:50 PM on May 25, 2008

Susuuration's answer is comprehensive but unfortunately more-or-less entirely wrong.

Radio waves bounce off things, including walls, ceilings, and even your body.

A radio in a particular location receives the main broadcast signal, but also receives reflected signals. If these signals are significantly "out of phase" with the main broadcast signal, they will reduce the overall signal strength. If these signals are "in phase", they will boost the strength. Although the reflected signals are pretty weak compared to the main signal, the radio might be seeing reflections from lots of surfaces at once, and together, they might add up to a fair bit of interference.

For FM radio, the signal wavelength is around three meters. So if there is a reflector 1.5 meters further away from the transmitter than the radio is, it will arrive at the radio having travelled 3m further than the original signal, and hence in phase. On the other hand, if there is a reflector 75cm away from the radio, the signal from it will arrive out of phase. You can see that this is very much an issue in a regular-sized room.

The bottom line of all this is that your body is capable of both causing an in-phase reflection (which is good) and blocking an out-of-phase reflection (which is even better) if it's in the right position.

All of these issues are more common using cheap radios, and either in remote locations with very weak radio signals, or in cities with many very strong radio signals on adjacent frequencies.

There is another, related, issue. Have you ever noticed that the signal quality changes when you take your hand away from the tuning dial? Radio receivers depend on filter circuits made up of capacitors and inductor coils. The exact value of an inductor depends on what kind of core it's wound up on, how wide the core is, what metal objects are next to it on the circuit board, etc. Your hand, or any other nearby ground-like object, also makes a difference.

By moving your hand nearer or further away from the tuning circuit, you are actually affecting the precise inductance value of the coil, and hence the precise frequency that it tunes to. So you tune to the frequency, it sounds great, and you move your hand away - but now the frequency of the tuning circuit has shifted slightly because your hand isn't there any more, so the tuning is off! (Radio geeks will know that you also alter the overall Q of the circuit and hence the selectivity of the radio, which is why you sometimes can't correct for this problem by fiddling with the tuning even if you know what you're doing).

Again, this is primarily an issue for cheap, poorly-designed radios. Proper shielding makes this a non-issue.
posted by standbythree at 5:24 PM on May 25, 2008

IAAEE... and seconding standbythree. Susurration needs to lay off the crackpipe.
posted by polyglot at 6:21 PM on May 25, 2008

I am also an electrical engineer (by training, rather than current job). The second part of standbythree's explanation accords entirely with the second part of mine. I am happy to stand corrected over the first part, if this is current theory. However, an experimental study in which I was involved a few years ago (as part of a radio interference group) would bear out this explanation - we measured the inductance in the antenna of a receiver and found that it increased significantly with moving low-density objects (such as human or canine bodies!), supporting the generator rule theory. High-density objects (e.g. steel reinforced concrete beams) caused a more significant reflection effect. Our subsequent receiver design was based on these findings. It is still in use, as far as I know ... :-)
posted by Susurration at 7:04 PM on May 25, 2008

standbythree: I agree except that people are capacitive, not inductive. Also, there's an additional effect due to bodies providing an effective ground.

Susurration: Something is very wrong with your reasoning. The phrase "just sitting in it generates inductance, which generates an induced current in the antenna," makes no sense. For one, it violates energy conservation. Simply placing a coil of wire somewhere will not induce a current elsewhere.
posted by dsword at 10:24 PM on May 25, 2008

Susurration's explanation, while plausible-sounding ... just isn't correct. Even if the act of you moving near the antenna did induce some current in it, it wouldn't be of the same frequency that the radio's discriminator is tuned to (and it's not nearly enough to induce “front-end overload”). Also, it would only apply when you're moving in relation to the antenna, since non-moving objects do not induce current flows.

Standbythree's explanation — about the multipath interference and your body blocking or otherwise changing the reception pattern — is closer to the explanations I've heard from radio geeks over the years. The other one is that, just by being near the antenna, you end up capacitively or inductively coupled to its radiating/receiving elements, and change its fundamental frequency a little. I've heard this explanation more in reference to transmitting antennas, but I think it would apply to receiving ones as well.

If the behavior you're describing is the sometimes drastic changes in reception that can occur just by moving a few inches, or standing in a very particular place in the room, I think it's probably multipath.
posted by Kadin2048 at 7:45 AM on May 26, 2008

I just wanted to follow up by saying that I started typing that comment last night, when the only other ones in the thread were Susurration and Standbythree; I didn't mean to give the impression that I was ganging up on him. (And his second comment about inductance does seem correct.)
posted by Kadin2048 at 7:49 AM on May 26, 2008

Just wanted to pop back in and say Wow, I didn't know there were such divergent thoughts and opinions on the mechanics and science of radio transmission/reception. Thanks for the interesting answers.
posted by amyms at 11:11 AM on May 26, 2008

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