For seven chapters we've shaped waves every which way (stretched them taller, sped them up, slid them sideways), always taking for granted that the wave was simply out there in the open air, ready to be shaped. Time to face the thing we kept skipping: how does a wave get out there in the first place? How does a signal trapped inside a wire leap off the end and go racing across an empty room, an empty sky, the empty space between worlds?
The answer is a gadget so plain you'd never look twice at it: a piece of wire. We call it an antenna , and its whole job is to be the doorway between a wire and open space, on the way out, and on the way back.
A wiggle that won't stay put#
Back in chapter 3 we found the idea at the heart of every radio wave: take one of those tiny charges (an electron) and shake it, and a ripple of electric-and-magnetic field spreads out on its own, needing nothing to travel through. An antenna is simply a wire built to do that shaking on purpose. Push a current up and down it (a current is just a crowd of electrons all flowing together, here up and down, millions or even billions of times every second) and the electrons inside slosh along with it. How fast you shake it is the frequency you're sending. Each shake flings off a little more wave, and the whole thing streams outward at the speed of light.
Watch one do it. The charge bobs along the wire, and with every bob it sheds another ring of field that swells outward and never comes back. That is a radio wave being born — not stored in the wire, but thrown clear of it.
Throw it, then catch it#
Now run the whole thing backward. Far away, that wave washes over another plain piece of wire. Its electric push gives the electrons there a little shove, and because the wave is wiggling, it shoves them up, then down, then up again, in perfect time with itself. A wiggling wave makes a wiggling current. That faint current is the signal, arriving. And untangling it back into sound or pictures is the whole job of the radio it's plugged into.
Here's the quietly beautiful part: it's the same wire. An antenna that's good at flinging a wave off is exactly as good at catching one. Throwing and catching are a single skill, run in opposite directions. The little aerial on a walkie-talkie sends when you speak and receives when you listen, without changing a thing about itself.
The wave remembers which way you shook it#
There's one more secret hiding in that little launch scene, and it's easy to miss: the direction of the shake. Our transmitting wire stood upright, so its electrons sloshed up and down, and the wave that flew off carries that up-and-downness with it. All the way across the room, across the city, across the sky, its push keeps shoving up and down, never sideways. The wave remembers which way it was shaken, and that remembered direction is called polarization : a wave from an upright wire is vertically polarized, and a wave from a wire lying flat is horizontally polarized.
Why should anyone care? Because of the catching. An arriving wave can only push electrons along a wire; that is the only direction they're free to move. Stand the receiving wire upright, perfectly in line with the wiggle, and it feels the whole shove. Tilt it, and only part of the shove runs along the wire. Lay it fully crosswise and the shove pushes straight across the wire instead of along it, and the electrons barely stir. Try it:
perfectly lined up: the wire feels the whole shove
This is why the antennas of a town quietly agree with one another. A broadcaster picks a polarization and sticks to it, and everyone tilts to match: in many countries TV was sent horizontally, so whole rooflines of aerials lie flat, while handheld and car radio lean vertical, which is part of why a car's whip antenna stands up straight. Your phone can't promise any tilt at all (you hold it however you like), so it hides several little antennas at different angles and quietly listens with whichever is catching best.
Polarization even lets two signals share the very same frequency. Wiggle one up-and-down and the other side-to-side, and a receiver lined up with one barely notices the other; satellite TV really does this to fit twice the channels into the sky. And some satellites, GPS among them, play a subtler game for receivers that might be tilted any which way: they send a wave that spins as it flies, like a corkscrew, so that no tilt ever lines up exactly wrong. There is even an antenna shaped like a corkscrew to match; you'll meet it in the field guide.
You already own a polarization filter#
Here's the delightful part: you may own a polarization gadget already and wear it on your face. Remember from From ripples to radio that light is the same kind of wave as radio, just a wildly faster wiggle, so light has polarization too. Sunlight starts out wiggling every which way at once. But when it glances off something flat and shiny (a lake, a wet road, the hood of a car), the bounce mostly keeps the side-to-side wiggle. Glare, in other words, is largely horizontally polarized light.
Polarized sunglasses hide a filter that only lets the up-and-down wiggle through. Ordinary light loses about half its strength and merely dims, but glare, being mostly sideways, is nearly wiped out. That's the whole game. And you can catch the filter red-handed: tilt your head toward your shoulder while looking at glare on water and watch the shine blaze back, exactly like the crossed wire in the figure above. Or hold the sunglasses in front of a phone screen and slowly turn them; screens give off polarized light, so at one angle the screen looks normal and at another it fades toward black. Two crossed antennas, right there in your hands.
Why antennas come in so many sizes#
If an antenna is just a shaken wire, why is an AM radio tower taller than an office block, while the one tucked inside your phone is smaller than a fingernail? Because a wire shakes best at one particular length, and that length is set by the wave it's trying to send or catch.
Think of a child on a swing: push in time with its natural rhythm and tiny pushes pile up into a soaring arc; push at the wrong moments and you just fight it. A wire has a natural rhythm too, set by how long it is, and it trades energy with a wave most happily when its length is about half the wave's wavelength . Much shorter, and it can barely get a grip on the wave at all.
So the size of an antenna simply follows the size of the wave. Low-frequency waves are enormous (an AM signal's wave can stretch a few city blocks), so their antennas are towers (and for the very biggest waves engineers cheat, letting the ground stand in for the bottom half, so the mast need only be about a quarter of a wavelength tall). Crank the frequency up and the wavelength collapses; a Wi-Fi wave is a few inches long, so half of one is a sliver of metal that vanishes inside a phone. Slide the frequency up and watch the wavelength (and the antenna it calls for) shrink together; drag it back down and they grow.
FM & TV: a rooftop or car aerial
So that is the doorway: a wire, shaken at just the right pace and tilted just the right way, turning a current into a wave on the way out and a wave back into a current on the way in. The plain stick is only the beginning, of course. Antennas also come as rabbit ears, rooftop combs, dishes and corkscrews, and once you start noticing them they are everywhere; A field guide to antennas puts names to the shapes. Either way, the signal is finally off and running, tearing across open space at the speed of light.
But "across open space" hides a catch. A wave doesn't travel forever unchanged. The farther it roams, the fainter it grows, and it can fade alarmingly fast. Why a signal weakens with distance, and just how quickly, is where we head next.