A friendly guide to antennas · 11 short chapters · By Rohit Gupta

How does a stick of metal talk to the sky?

You can’t see them, but the air around you is busy. Your phone is chatting with a mast down the road. Your Wi-Fi router is talking to your laptop. Far overhead, GPS satellites are calling out the time.

All of it travels as radio waves. Every one of those waves starts at an antenna and ends at another one: a piece of metal, shaped just right.

Tap anything in the picture to find out what it’s saying.

Not to scale. Real radio waves are much bigger than these, and much faster.

This guide explains how that works, from scratch. No physics needed. The pictures move, and you can play with most of them.

Start with a pond ↓

1Waves

What is a wave, anyway?

Before we get to antennas, we need one simple idea: the wave.

Poke a stick into a pond, again and again, and ripples spread out across the water. Here’s a pond seen from the side. The stick on the left bobs up and down:

Watch the yellow ball.

Watch the yellow ball. When a ripple arrives, the ball bobs up and down, but it doesn’t ride along with the ripple. The water mostly stays where it is and moves up and down. What travels across the pond is the ripple.

Now slide the stick faster. The ripples squeeze closer together. Slow it down, and they spread apart.

Those two things have names. How often the stick bobs is the frequency. The gap from one ripple to the next is the wavelength. Faster bobbing, shorter waves. Keep that pair in mind, because it explains a lot about antennas.

Radio waves are ripples too. They aren’t ripples in water, though. They’re ripples in something invisible that surrounds every electric charge. So let’s meet the charges.

A wave is a ripple that travels. The stuff it travels through mostly just wiggles in place.

2Charges

The invisible strings

Rub a balloon on your hair and hold it over some scraps of paper. The scraps jump up to meet it. Nothing touched them. The balloon pulled them from a distance.

That pull comes from electric charge. Everything around you is made of tiny charged bits: electrons, which are negative, and the centers of atoms, which are positive. Usually they balance out, so you never notice them. Rubbing moves some electrons from your hair onto the balloon, and now the balloon has a few extra. That’s enough to pull on the paper.

Charges push and pull on each other, even across empty space. Drag the big charge and watch the small one:

Drag the big red charge toward the small one.

The closer they get, the harder the push. A lot harder: twice as close means four times the push.

Now switch on the lines. They show the push all around the charge. Each line shows which way a little charge would be shoved if you put it there, and where the lines crowd together, the push is strong.

Michael Faraday came up with these lines in the 1800s, and he pictured them almost like stretched strings tied to the charge. That turns out to be a very useful way to think about them.

So here’s a question. If you move the charge, its strings have to move too. How quickly does the far end of a string find out?

3Radio waves

Flick the string

Tie a long rope to something, hold the other end, and flick your wrist. A bump runs along the rope, away from your hand. Try it here. Drag the hand up and down, or press Flick:

Drag the hand, or press Flick.

The bump travels, but the rope doesn’t. Each bit of rope just goes up and back down as the bump passes, like the ball on the pond.

The strings around a charge behave the same way. Move a charge suddenly, and a bump runs out along every one of its strings. Nothing can travel faster than light, so far away, the strings don’t know yet that anything happened. Press Flick once:

Slowed down about a billion times.

The charge jumps up and back, and a ripple spreads out through its strings. I’ve slowed it down about a billion times. At real speed, it would cross this picture in a few billionths of a second.

Notice where the bump is biggest. It’s strongest off to the sides of the charge, and there’s no bump at all straight above or below it. That will matter later.

Now press Keep shaking. The bumps come out one after another and make a wave, just like the bobbing stick made ripples on the pond. Shake faster and the waves bunch up closer together.

That’s a radio wave. Nothing more mysterious than a charge being shaken, with the news of its shaking spreading out at the speed of light. Shake a charge a few hundred trillion times a second instead, and the same kind of wave is light.

There’s one more thing that makes these waves special. They fade much more slowly than the charge’s ordinary push. Go ten times farther away and the push is a hundred times weaker, but the wave is only ten times weaker. That’s why radio can cross a whole city, while the static on a sweater only reaches a few centimeters.

Shake a charge, and a wave runs out along its strings at the speed of light. That’s radio.

4Sizes

How big is a radio wave?

Radio waves come in every size. It all depends on how fast the charge shakes. Remember the pond: faster shaking, shorter waves. Tap through some you use every day:

FM radio.

  • AM radio shakes about a million times a second. Its waves are 300 meters long, as tall as the Eiffel Tower.
  • FM radio shakes 100 million times a second. Its waves are about 3 meters long.
  • Phones use a few hundred million to a few billion shakes a second. Their waves run from about 40 centimeters down to a few centimeters.
  • Wi-Fi shakes 2.4 billion times a second, with waves about 12 centimeters long. Your microwave oven heats food with waves almost exactly that size.

Light, by comparison, has waves less than a thousandth of a millimeter long.

Keep one of these in mind. FM waves are about 3 meters long, and the thin rod on an old car is about 75 centimeters (30 inches). That’s exactly a quarter of an FM wave. It isn’t a coincidence, and by chapter 8 you’ll know why.

5Metal

Why are antennas made of metal?

One shaking charge makes a wave far too weak to notice. To make a useful signal, you need to shake billions upon billions of charges together, all in step.

Metal is perfect for that. In a metal like copper, every atom lets one of its electrons wander off. So a metal wire is packed with loose electrons that can drift around freely. Push on them, and they all move together. That’s an electric current.

So here’s an antenna: a straight wire, cut in the middle, with a source that pushes the electrons one way, then the other.

The electrons slosh from end to end.

The electrons slosh from end to end, like water in a bathtub. One end fills up with extra electrons while the other runs short. Then they swap.

I’ve exaggerated how far they move, by a lot. In a real FM antenna, each electron moves back and forth by less than the width of an atom. Like the ball on the pond, the electrons hardly go anywhere. What travels is the push, passed from each electron to the next.

All those electrons shake together, so their little waves add up into one big one. That’s the wave that leaves the antenna.

6Length

Why does the length matter?

Have you ever pushed someone on a swing? It’s all about timing. Push while the swing is moving away from you, and every push adds a little height. Push while it’s coming toward you, and you fight it. Try it:

Tip: you can only reach the seat when it’s on your side.

Press Push to get the swing going.

An antenna is a lot like that swing. Each push from the source runs along the wire to the end, bounces back, and returns to the middle. If it gets back at just the right moment, it helps the next push instead of fighting it.

That happens when the wire is about half a wavelength long. Slide to change the length of the wire, and watch how much of the radio’s power actually leaves as waves:

Find the sweet spot.

There’s a clear sweet spot at about half a wave. It’s a little short of exactly half, because the ends of a real wire act as if it were slightly longer. Make the wire too short or too long, and most of the power bounces back to the radio instead of leaving.

That’s why antennas come in particular sizes. Each one is cut to fit the waves it’s meant for. A half-wave antenna for FM radio is about 1.5 meters long. One for Wi-Fi is about 6 centimeters.

But wait. The car antenna is only 75 centimeters, a quarter of an FM wave, not half. Hold on to that. We’ll get there in chapter 8.

An antenna works best when its size fits the wave, like a swing pushed in time.

7Direction

Where does the signal go?

Remember the first flicked charge? Its bumps were biggest off to its sides, and there were none straight above or below. An antenna is the same, because all of its electrons shake along the wire.

So an upright antenna sends its waves out sideways, all around, but nothing straight up or straight down. In 3D that makes a shape like a donut, with the antenna through the hole. Drag to turn it:

The farther the surface is from the antenna, the stronger the signal in that direction.

Here’s the same antenna from the side, with its waves spreading out. Carry the phone around it and watch the bars:

Carry the phone around the antenna.

Full bars out to the side. Fewer as you move toward the tip. None at all right off the end.

An antenna can’t make extra power. All it can do is choose where to send it. The donut sends it out sideways, toward the horizon, because that’s where the people are. That’s why antennas on masts usually stand upright. It’s also why radio hams tell beginners never to point the tip of a walkie-talkie’s antenna at the person they’re talking to.

An antenna can’t make power. It can only choose where to send it.

8Receiving

Catching it, and the missing half

So far we’ve been sending. Receiving is the same thing in reverse.

When a radio wave washes over a piece of metal, it pushes the loose electrons inside back and forth. That makes a tiny current, rocking in time with the electrons in the faraway antenna. A radio picks up that tiny current and turns it back into sound, or pictures, or messages.

Which way the metal points matters. Below, a wave is coming straight toward you, pushing up and down. Turn the rod:

Turn the rod by dragging its handle.

Lined up with the push, the rod catches the whole signal. Turned sideways, the push just shoves the electrons across the thin rod, where they can’t go anywhere, and the signal disappears. If you ever twisted the rabbit-ear antenna on an old TV to clean up the picture, this is part of what you were doing.

Now, back to the car. A proper antenna is two rods fed in the middle, but the car has only one. So where’s the other half?

It’s in the roof. To radio, a big sheet of metal acts like a mirror. A rod standing on a metal roof sees its own reflection in the roof, and the rod plus its reflection work just like a full half-wave antenna. That’s why the car’s rod only needs to be a quarter of a wave: 75 centimeters. The other 75 are in the mirror.

The mirror also explains why the rod stands straight up. Tilt it and watch:

Standing up, the rod and its reflection work together.

Standing up, the rod and its reflection work together. Lying flat, the reflection points the other way, and the two cancel out. A rod lying on the roof would barely work at all.

Big AM radio towers use the same trick. Their waves are about 300 meters long, so a quarter wave is 75 meters. The whole steel tower is the antenna, standing on a giant insulator, with the ground as its mirror.

A quarter-wave rod on a metal roof acts like a half-wave antenna. The other half is its reflection.

9Shields

Why does my phone lose signal in a lift?

If metal is a mirror for radio, then a metal box should keep radio out. It does. When you step into a lift, you’re standing in a metal box, and your phone’s bars drop.

The box doesn’t even have to be solid. Here, a radio wave hits a metal wall with holes in it. This is a real simulation of the wave, seen from above. Change the size of the holes:

Holes much smaller than the wave.

When the holes are much smaller than the wave, it can’t squeeze through, and almost nothing gets past. As the holes grow toward half a wave, more and more leaks through. Bigger than that, and the wave pours through.

Your microwave oven depends on this. Its waves are about 12 centimeters long, and the metal mesh in its door has holes a millimeter or two across. The microwaves stay inside. Light waves are thousands of times smaller than those holes, so the light gets out, and you can watch your food.

It’s also why a car’s antenna sits outside, on the roof. Inside, the metal body would block most of the signal.

Metal stops radio, as long as its holes are smaller than the waves.

10Teamwork

How do you aim a radio wave?

One antenna sends its donut in every direction around it. Often that’s exactly what you want: a radio station wants everyone in town to hear it. But sometimes you want to send your signal one way only.

One way is teamwork. Put several antennas in a row and send the same signal from all of them. In some directions their waves line up and add together. In others they cancel out. Then give each antenna a tiny delay after its neighbor, and the beam swings around, even though nothing moves. Here they are seen from above:

Six antennas working together.

That’s how a Starlink dish works. Inside its flat panel are hundreds of small antennas. By changing their delays, it steers its beam to follow satellites across the sky, with no motors at all. A row like this also sends a matching beam out the back. So real panels put a sheet of metal behind the antennas, and the mirror from chapter 8 turns that back beam around.

There’s an even cheaper trick, and it’s on rooftops everywhere. The helpers don’t need to be plugged in at all. A plain metal rod near an antenna catches some of its waves and sends them back out a moment later. A slightly longer rod behind the antenna bounces the signal forward. Slightly shorter rods in front pull it forward even more. Add some helpers and watch the bars:

Working it out…

That’s the rooftop TV antenna. The longest rod, at the back, bounces the signal forward. The rod with the cable is the real antenna. The row of short rods pulls the beam toward the TV tower. Look down a street of houses with these on the roofs, and you’ll see them all pointing the same way.

It was invented in Japan in 1926 by Shintaro Uda and his professor, Hidetsugu Yagi. Most people still call it a Yagi.

11Dishes

How do you call a spacecraft?

For the tightest beams of all, antennas borrow a trick from flashlights: a curved mirror. Put a small antenna in front of a dish, at just the right spot. Its waves bounce off the dish and leave side by side, in one narrow beam.

A narrow beam.

The wider the dish, the narrower the beam. A satellite TV dish sends and catches a beam only about 3 degrees wide, which is why the person installing it fusses so much over where it points.

The record holder is Voyager 1. It was launched in 1977, and it’s now more than 25 billion kilometers away, the farthest thing people have ever made. It still talks to us, through a 3.7-meter dish and a transmitter of about 22 watts, roughly what a fridge light uses.

By the time that signal reaches Earth, the beam has spread out to about the width of Earth’s whole orbit around the Sun. NASA catches it with dishes 70 meters across, and what they collect is less than a billionth of a billionth of a watt. It works because both ends use a dish: one to aim, and one to catch.

A dish is a mirror for radio. The bigger the mirror, the tighter the beam.

Now look around.

Next time you pass an old car, you’ll know why its antenna is 75 centimeters long. It’s a quarter of an FM wave, and the roof makes up the other half. It stands up because lying down, it would cancel its own reflection.

The TV antennas on your street all point the same way because their helper rods aim them at the tower. The satellite dish next door is a mirror. Your Wi-Fi router’s little antennas are cut to fit Wi-Fi waves. And your phone has several antennas tucked inside it, folded up small, listening and talking all day long.

Every one of them is a piece of metal, the right size and pointed the right way, with electrons shaking in time with the waves.

Shake a charge, and the news spreads at the speed of light. That’s all radio is.

Every moving picture on this page is worked out from the real physics as you watch, slowed down so you can see it. The figures are flat slices of a 3D world.

Made by Rohit Gupta with Claude.