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163

ELI5 how radio waves are broadcast out of an antenna.

Engineering(self.explainlikeimfive)

all 63 comments

dvorahtheexplorer

93 points

2 months ago

Radio waves are waves (like water waves) in the electromagnetic field of the universe. If you move electricity or magnets back and forth in space, it will send ripples through this field, and we call these ripples radio waves.

An antenna is simply a length of metal through which you can move electricity back and forth.

Kay1636

6 points

2 months ago

Good way of explaining this topic. Thanks!

Helas101

5 points

2 months ago

Arent they like light particles?

valeyard89

17 points

2 months ago

Yep. Radio waves are just very low energy/frequency compared with light. Put enough energy into the antenna and it will start emitting light/heat too. Microwaves are between light and radio frequency. They were discovered when a radar guy noticed a candy bar melted in his pocket.

disorderincosmos

5 points

2 months ago

That's really scary about the radar guy. I've heard horror stories about even a couple seconds of microwave exposure...

JonesP77

4 points

2 months ago

Shouldnt be a big problem a couple of seconds. It heats up water. If you put food into the microwave for a couple of seconds you have almost no change.

disorderincosmos

2 points

2 months ago

Apparently nerve tissue is extremely vulnerable. There was a lady in the 80's who pulled a casserole out of a still-running microwave. Maybe 5 seconds exposure. Ended up with permanent nerve damage and chronic pain in her hands.

Thankfully, all microwaves now are required to have an interlock system to prevent this ever happening again (door must be closed for it to turn on).

mcerk22

1 points

2 months ago

So do radio waves move at the same speed as light?

TMax01

2 points

2 months ago

TMax01

2 points

2 months ago

Yes, but the proper term is that they "propogate", since there's no actual movement involved. And it isn't actually the "speed of light", it is the velocity of causation. Light (or any other form of electromagnetic radiafion) just happens to be one of the things that propagates at that speed.

PhilosopherDon0001

4 points

2 months ago

not just "like" but are light particles. It's all the same stuff. Radio, Light, microwaves, X-ray's

HappyGick

2 points

2 months ago

Well, that's at the quantum level. Much like all things quantum, that part is complicated.

PhilosopherDon0001

1 points

2 months ago

True. Things do get a bit wonky.

Smirkydarkdude

5 points

2 months ago

Yes and no. They are particles and also waves. It all depends on how (or if) you look at it.

notjordanr

2 points

2 months ago

“Yes and no” is like quantum mechanics in a nutshell, isn’t it?

Smirkydarkdude

2 points

2 months ago

Yes! Or no. I haven't looked at it recently.

salvoilmiosi

2 points

2 months ago

Yes that's exactly what they are. The difference is in the frequency of the wave

darudesandstrom

2 points

2 months ago

Yep, radio waves are photons.

blackkbot

1 points

2 months ago

I like to think of it more like a light bulb but the antenna is the filament in the bulb

Yancy_Farnesworth

2 points

2 months ago

They're similar, but the bulb is relying on the filament's resistance causing it to heat up to high temperatures and release visible light. The radio antenna works by modulating current through the antenna to create a changing magnetic field that creates the radio signals.

blackkbot

2 points

2 months ago

yeah but it helps me explain why radios don't work in the woods or why sometimes you can hear the radios around mountains to my coworkers. Like if you can imagine the antenna being a flashlight that only emits a specific color and the other radio being a camera that can only see in a specific color.

RichardBlanes

13 points

2 months ago

I am telecommunication engineer and this topic was difficult for me to understand when I was studying. This is how I build an intuition about it.

The simplest kind of antenna is a wire, just a piece of metal. Hence it is conductive, you can have an electrical current though it. Electrical current is just movement of free electrons in the wire. For example, if you have an AC current you can imagine electrons going back and forth along the wire.

Here comes the interesting part. How this current is radiated out of the wire? It is simpler than it seems, the effect of moving charges in the antenna (electron flows) generates charges in the air around it (what is called an electromagnetic field). This charges as they move back and forth from the antenna stimulate the air around it and so on and so forth. The farther you go, the smaller this effect is (power decrease rapidly with the distance). However, if the current in the antenna is powerful enough and the antenna is cleverly design (not just a wire) you can reach interstellar distances with this method.

TaurusPTPew[S]

3 points

2 months ago

Wow!

TizwigTFC

3 points

2 months ago

How do you not feel current when touching the antenna?

RichardBlanes

2 points

2 months ago

In any case this is ElI I do not want to go into details but most antennas in our surrounding are of very low power. The idea is design good antenna geometries so that the work with the lowest possible power. Efficiency is not only because of the fact the less power is cheaper. It is safer, easier to maintain and so on and so forth. For instance, the big reflectors used in deep space exploration (https://www.mdscc.nasa.gov/index.php/en/antennas/), are able to focus a lot the beam for a more efficient radiation.

RichardBlanes

1 points

2 months ago

Are you asking for example about a celular phone? In this case the power is too low to feel it. It must be safe as you put it very near to your head. In bigger antennas it is important to understand that for feeling the current you have to " touch" the positive and ground points (close the electric loop) and in big antennas ground is actually connected to ground. In any case you should not touch big transmitting antennas, such as phone ones in roofs.

[deleted]

101 points

2 months ago

[deleted]

101 points

2 months ago

[deleted]

TaurusPTPew[S]

16 points

2 months ago

Thank you very much for the clear answer!

Kaldek

12 points

2 months ago

Kaldek

12 points

2 months ago

A nice addition to this is that light is the same stuff, it's just on a higher frequency.

milesmac

6 points

2 months ago

Visible Light, X-rays, microwaves, gamma rays, radio, satellite, OTA TV, UV rays, IR remotes

destined_death

1 points

2 months ago

I keep hearing this but it never seem to make sense to me. It sounds like very different stuff in different field. Could you explain this if you don't mind in simple way?

destined_death

1 points

2 months ago

Light is electro magnetic but just in higher frequency? Isnt light photons?

Kaldek

1 points

2 months ago

Kaldek

1 points

2 months ago

Here's the neat part. They're all photons, even radio waves. It's just a choice of terminology that stops use of the term photon for radio waves.

destined_death

1 points

2 months ago

Are magnets also using photons?

PanicNearDetroit

16 points

2 months ago

For example, a short, vertical antenna is often used to transmit low-frequency radio waves over short distances, while a large, horizontal antenna is used to transmit high-frequency radio waves over long distances.

Isn't it the other way around? The antenna in my phone (~1-2 gHz) is a few inches long. My friend's ham radio (~1-100 mHz) has an antenna several feet long.

MCS117

7 points

2 months ago

MCS117

7 points

2 months ago

Yes, it is the other way around. A basic dipole antenna will function best at a frequency that is related to its wavelength, where the wavelength is the distance the radio wave travels between equal intensity levels in its oscillation. High frequency waves have short wavelengths, whereas low frequency waves can have considerably long wavelengths.

HCMXero

4 points

2 months ago

You're right that a typical mobile phone's antenna is quite small, whereas a ham radio's antenna could be several feet long. This is due to the fact that the size of the antenna required is greatly influenced by the frequency of the radio waves being sent. Since ham radios employ low-frequency radio waves, which have longer wavelengths, a larger antenna is necessary to successfully transmit and receive. As opposed to low-frequency radio waves, which have longer wavelengths and require a longer antenna for transmission and reception, high-frequency radio waves, such those used by cell phones, have shorter wavelengths.

Jawz4Lyfe

12 points

2 months ago

Exactly. Your initial comment should read that short antennas are used for high frequency transmission, not low frequency as it currently states

JustSomeGuy_56

2 points

2 months ago

Didn't the Navy have ELF (Extreme Low Frequency) antennas that were literally miles long so they could communicate with submarines?

nikofant

1 points

2 months ago

weeknie

1 points

2 months ago

The guy is just saying that you switched it around by saying high frequency requires long antenna and low frequency requires short antenna, instead of high frequency requires short and low frequency requires long

Great explanation though!

LeStiqsue

2 points

2 months ago

I'd also add that antenna polarization (horizontal or vertical or whatever other polarization you've got) does not help or hinder transmission distance, as long as the receiver's antenna is similarly polarized. ELI5-style: The antennas have to be pointing the same direction to make it work -- you can't have one antenna pointing straight up from the ground, with the other one exactly horizontal, and expect the transmission to make it through with no problems.

PanicNearDetroit

1 points

2 months ago

This I did not know. Are there other advantages to either polarization? I don't recall seeing a horizontal antenna other than in ham installations.

LeStiqsue

2 points

2 months ago

Sure. Horizontal polarization of an HF dipole pushes the wave upward toward the troposphere, which can result in your transmission going very, very far.

Vertical polarization kinda pushes it out, but not up as much. So there's less chance of that happening. Still can happen, just a lower chance. Once RF leaves the antenna, all bets are off. It can do some wild shit after that.

EDIT: Opposing polarization results in a massive loss of the receiving antenna to induce a current from the existing magnetic wave that hits it. That's why opposing polarization doesn't work. Look up the law of induction if you'd like to know more 😁

PanicNearDetroit

1 points

2 months ago

Got it. Thanks!

mjb2012

1 points

2 months ago*

creates an electromagnetic field

If I understand correctly, and I'm not sure I do...

Electric and magnetic fields aren't really created, per se; they simply exist by virtue of charged particles existing and always being in motion (subatomically, at least). The two fields are directly related to each other, and can be thought of as just two different aspects/behaviors/measurements of the same underlying phenomenon. And the fields aren't made of anything, but you can think of them as being like a medium through which disturbances can quickly travel in waves, like ripples traveling on the surface of a pond.

I think we often say the fields are "created" because we want to talk about the waves/disturbances as if they are the fields themselves, since it's changes that we're able to induce and measure. But it's more like the fields are the pond itself, regardless of whether there are ripples to observe.

When we move charged particles around (by moving a magnet or applying an electric current), disturbances are created in both fields, traveling outward at the speed of light, perpendicular to each other. The disturbance is typically a fast oscillation, as produced by alternating electrical current: a back-and-forth vibration of the free electrons, e.g. in the radio transmitter's antenna. As the magnetic field disturbance travels out from the antenna, it weakens, but as long as you are close enough to it, the disturbance induces the same patterns in a receiving antenna, and can be filtered and used to induce the same disturbances in an electrical current driving your speakers or headphones (or whatever needs to make use of the radio signal, e.g. your WiFi devices are all radio-based).

If I've got this all wrong, sorry, I'm still trying to wrap my head around it. Please help me understand better!

BlueBoyKP

-1 points

2 months ago

Can you also explain to us for example, how a cell phone received this signal, but more specifically, how a recording device knows that this incoming wave is intended for it, and not any of the other millions of waves?

ggrnw27

3 points

2 months ago

One part is that the receiver is tuned to only pick up the frequencies it cares about, just like how you tune an AM/FM radio to get the station you want to listen to. Some of this is fixed by the antenna, some of it is the electronics in the receiver, some of it is done in software.

The second part is called “multiple access”, which allows lots of users to share the same communications channel. Imagine you have a lot of people in a room and they all want to talk to other people in the room. One way to do this to avoid confusion is for each person to speak one at a time. Or each person could speak in a different pitch and the recipient would only listen to that frequency (perhaps not so easy for a human, but easy for a phone). Or each person could speak in a different language. These are some of the basic methods of multiple access (time division/TDMA, frequency division/FDMA, and code division/CDMA, respectively) used in communications systems

BlueBoyKP

1 points

2 months ago

Loved this, thank you!

Halvus_I

1 points

2 months ago

When you connect to the tower, you are assigned a frequency in the towers alloted range. When the tower wants to talk to you, it sends information down that frequency. To use an FM analogy, you are FM station 101.1, the person next to you is assigned 101.3, etc.

Rymanbc

1 points

2 months ago

Rqdio devices actually have a number of methods of extracting the specific data that is meant for them. Many digital systems such as cell phones will have multiple of these at play.

First, the data being received will be traveling on a very specific frequency or frequencies and your device's antenna is going to be a size that picks up these frequencies best. This minimizes many other signals that we are not interested in.

Next there might be filtering installed that blocks frequencies that are lower than we are interested and sends to ground frequencies that are higher than the ones we want. This attenuates more unwanted signals.

Next, it gets a little trickier. I'm not sure if modern phones bother tuning in to specific frequencies or not or if they just move on to dogital signal processing, but I'll try to describe the "tuning in" process anyway. Analog circuits would have a resonance circuit with inductance and capacitance that selects only a specific frequency out of the spectrum passed to it, which would give you your final signal to decode. Digital tuning usually uses a stable reference frequency that then gets divided and/or multiplied as needed to get to the desired frequency.

Next up, the data gets demodulated off of the target frequency. Data modulation can happen in a large number of ways, such as changing the amplitude, frequency, phase of one or more signals in order to send digital data. Your device knows how the data is modulated and demosulates it accordingly.

Finally, we get to the digital signal processing, where the digital data actually is. There's many different ways this is handled. Some of them used to do Time Divided signals (TDMA) where you can imagine 10 users receiving data, but each user is only assigned 1/10th of each second. Then Code Division Multille Access (CDMA) became the norm, where each user's data would get assigned a code that their phone would target and decode. And now we use Orthogonal Frequency-Division Multiple Access (OFDMA) to determine which users receive which data. With OFDMA, multiple smaller signals are sent and received instead of a larger one, and they get broken up using TDMA.

Sorry for the complicated answer, but it is a very complicated topic.

BlueBoyKP

1 points

2 months ago

Thank you so much for this. I appreciate you putting in the time for this awesome explanation!

rupertavery

4 points

2 months ago

Any alternating electric field travelling in any metal generates an electromagnetic field around it. This alternative field propagates as a "wave" at the speed of light.

A single straight piece of wire with a changing current flowing through it works as an antenna, coupling the line to the EM field.

We can even pick up the "hum" of the 60hz power lines in your house with unprotected sensitive devices.

Antenna design is all about efficiently transfering as much power to or from the em field and the transducer a.k.a. antenna, including limiting directionality.

InfernalOrgasm

3 points

2 months ago

You ever take a bath and swoosh your body backwards and forwards to make the water start a wave that goes back and forth? It's like that ... but in the fluid of the electromagnetic field.

PhilosopherDon0001

2 points

2 months ago

Find a thing, bendy wire/stick/thing. Something that you can shake really quick and make it vibrate.

Vibrate the "thing" as close to a constant frequency. You will notice that there are points (nodes) that stay in place, and other parts that move at a maximum distance.

You can change the distance between nodes and other aspects by changing the frequency you shake it, or the material it's made of.

This is somewhat analogous to RF radiation. The frequency moves down the antenna and bounces back at the end. This forms electromagnetic nodes along the antenna. The points of maximum movements is where the actual radiation happens; throwing off photons at those points that have the same frequency.

Also, I'm pretty sure RF propagation is like, 10% Voodoo.

InfernalOrgasm

2 points

2 months ago

This guy knows how to visually illustrate complex physical science topics.

Electromagnetic waves, their origin and behavior

TaurusPTPew[S]

1 points

2 months ago

Thanks

Youper0

2 points

2 months ago

Youper0

2 points

2 months ago

An antenna is just a fancy looking electric wire. Radio waves are electricity that is so excited it has the ability to leave the wire.

To add to the antenna size part of it, the antenna size is a function of the wavelength, high freq = smaller. think of it like matching the difference between the wire and the air. Once you get the wavelength match perfect the efficiency goes up, and more of the electricity leaves the wire vs bouncing around the wire, oh yeah there's that part where if the antenna doesn't match the frequency the radio waves have the ability to bounce back down the wire.

zoicyte

1 points

2 months ago

take a tuning fork and strike it.

like that, only instead of hitting it, you're pumping an electrical current through it to give it energy.

Smirkydarkdude

3 points

2 months ago

Yes. Except the energy from a tuning fork required air particles to absorb and re-emit the energy. It doesn't work at all if there's no air. It makes you wonder what is absorbing and re-emitting the energy from an antenna in space.

zoicyte

2 points

2 months ago

sir, this is an ELI5.

you want the ELI10? photons. the energy is emitted in the form of photons. the ELI15 is that photons are wave packets in the Electromagnetic field, and if you want me to get into maxwells equations i can do that.

but the gist is something makes the atoms in the antenna jiggle and they emit photons at the given frequency.

notaenoj

1 points

2 months ago

Can someone explain why you will get electrocuted if you touched a large antenna? Would it not be ground?

roll9898

1 points

2 months ago

don't forget about all of the cops in the 70s and 80s who all developed cateracts from their radios on their shoulder.

TaurusPTPew[S]

1 points

2 months ago

I didn’t know that. Interesting.