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The Amazing Radio Man
November 3, 2005 9:20 AM   Subscribe

RF.Filter: How come I can always increase the quality of a poor radio signal by grasping the antenna with my hand?

I've had this work with FM radios, cell phones, and most recently, my laptop's WiFi. But some things trouble me:
- I don't understand the physics
- You would think that antenna designers would have invented synthetic palms by now.
- No one else in internetland seems to have noticed

So am I really seeing something here, or is it just confirmation bias. If true, how does it work?
posted by Popular Ethics to Science & Nature (18 answers total)
 
Yes, I've noticed it. Human beings can act like a giant antenna. I doubt it has anything to do with your palms, more like "if you put a giant conductive thing in contact with the antenna, it becomes a bigger antenna, which will get better reception."
posted by muddgirl at 9:36 AM on November 3, 2005


I've noticed this too. I think it has something to do with using your whole body as an extention of the antenna, not just the grasping motion, so you'd need more than just a synthetic palm to recreate it..
posted by PinkStainlessTail at 9:36 AM on November 3, 2005


I do this with my TV and it has always bugged me, too.
posted by tristeza at 9:55 AM on November 3, 2005


The very facet that allows you to grasp the antenna (nervous system) is a series of the biological equivalent of electrical circuitry -- your whole body is a giant grid bursting with conductivity. If you alligator-clipped a huge piece of metal (a conductor) to it, you'd get the same result, as muddgirl notes.
posted by vanoakenfold at 10:19 AM on November 3, 2005


You are a giant resistor. Short wave buffs often add a small resistor in the line of a wire antenna to boost reception.
posted by Pollomacho at 10:24 AM on November 3, 2005


This was common knowledge for people of my generation (I'm 38) since we all grew up with TVs with antennas. Most antennas were collapsible and you'd extend them out as needed.

Besides holding it (or even better, having someone else hold it) people would also build aluminum foil antenna extensions for the same purpose.
posted by vacapinta at 10:24 AM on November 3, 2005


You are a giant resistor. Short wave buffs often add a small resistor in the line of a wire antenna to boost reception.

this isn't really an accurate description of what's going on. grabbing ahold of an antenna that's encased in an insulator doesn't put you inside the current path any more so than grabbing ahold of an extension cord that's plugged into the wall.

your body is mostly water, which has a lot of electrolytes like sodium, calcium etc dissolved in it - in solution these ions act as charge carriers. this is why electrical current will flow through your body. the most basic antenna is a piece of wire; this is a piece of metal that has free electrons that move whenever an electric field is nearby. an oscillating electric field, like that in a cellphone signal, will make the electrons in the antenna oscillate, and therefore oscillate in whatever circuit the antenna is attached to.

so, because you're a giant wet conductor, when you are just walking around in the world all of the various kinds of EM radiation around us - TV signals, radio stations, wifi, cell phones.. everything - is making the ions dissolved inside your body oscillate back and forth, as the sum of all of the radiation that passes through you.

when you get near another conductor, even if you're separated by an insulator like the plastic on your antenna, your body capacitively couples to the conductor and effectively increases the area of the antenna. since bigger antennas work better, the signal strength will increase.

you certainly could unscrew the little antenna in your cellphone and attach a giant metal antenna to your cell phone, but then you'd be carrying around a giant cell phone. much of antenna engineering is about trading off size (for portability or aesthetic or whatever reasons) vs. how well it picks signals up.
posted by sergeant sandwich at 11:30 AM on November 3, 2005


Thanks sarge, that sounds about right. Here's a follow up: if capacitive coupling is indeed the mehanism, would I get the same effect standing next to a lightpole?
posted by Popular Ethics at 11:40 AM on November 3, 2005


well, a light pole usually has a lightbulb at the top and wires connected to the power grid running down the hollow inside of the pole. if the pole were plastic or fiberglass, the wires which oscillate at 60 Hz would radiate at 60 Hz and your body would indeed pick it up. this is the source of the controversy over the health effects on people living underneath high-voltage transmission lines.

however if the pole were metal, as most are, the free electrons in the metal surrounding would shield you from most of the radiation. this is how shielded audio cables work; they are simply surrounded by a metal cladding like that braided wire stuff.
posted by sergeant sandwich at 11:54 AM on November 3, 2005


I notice this too, except in the case of my shortwave radio. The radio has a long (3 or 4 foot) retractable antenna, and if I touch it, the signal completely drops out. I always have to be careful not to brush it against me when I'm listening to it.
posted by knave at 12:00 PM on November 3, 2005


sorry sergeant sandwhich, I meant the question in the opposite context. If all conductive things are resonating to the ambient RF field, and these resonances can be transfered capacitively, shouldn't I see signal improvements by standing next to big metal objects. (Maybe I do, but I haven't noticed).
posted by Popular Ethics at 12:06 PM on November 3, 2005


shouldn't this be directed at NotchFilter?
posted by kcm at 12:30 PM on November 3, 2005


PE: yes - but the strength of capacitive coupling falls off as 1 over the distance between conductors (i think) and so you'd have to press your cell phone up to the lamppost for it to work very well.

and what i forgot to mention above was that the metal lamppost shields EM radiation if it's grounded, which most big metal things are, but if it's ungrounded it should act as an extension of your antenna. note too that the dimensions and shape of the object in question are relevant to the resonant frequency (and the coupling, for that matter); not all antennae are created equal.
posted by sergeant sandwich at 1:27 PM on November 3, 2005



The very facet that allows you to grasp the antenna (nervous system) is a series of the biological equivalent of electrical circuitry -- your whole body is a giant grid bursting with conductivity.


Just to clarify, the electrical conductivity of your body is not really related to your nervious system.
posted by delmoi at 1:39 PM on November 3, 2005


The first experiment we did in circuits lab was to measure the resistance of our bodies (current-limited low level DC of course). I think we are about a megohm from hand to hand.
posted by mbd1mbd1 at 3:35 PM on November 3, 2005


Err... This is off the top of my head, but...

The capacitance of a parallel plate capacitor falls off linearly with separation until the separation is on the order of the size of the plates.

The capacitance between parallel wires falls off by the square of the separation until the separation is on the order of the length of the wire.

The capacitance of a parallel plate capacitor also goes up linearly with surface areas.

So, when you grab an antenna and squish your hand around it you are not only minimizing the distance, but you are also increasing the area drastically. Think of two Plasticine ropes, lay them on each side of a piece of paper, and then squish them together. You are not only decreasing the distance, you are also increasing the surface area, but even further you are going from the parallel wire model to the parallel plate model. The end result is that the capacitance between you and the antenna spikes up a lot right at the end.
posted by Chuckles at 11:51 PM on November 3, 2005


mbd1mbd1, be careful... The contact surface area is critical in that calculation too.

Here is a great article, and subsequent discussion, about electrocution that discusses some of the subtleties of calculating the resistance of a body.
posted by Chuckles at 11:58 PM on November 3, 2005


And then I say "right at the end" I mean right at the end of the grasping motion... That was probably already clear...
posted by Chuckles at 12:00 AM on November 4, 2005


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