Like it picks up a signal and processes it and from that I can hear music. But is it just the variation in frequency? And if so, then how do stations work?
In: Technology
Whenever an electrically charged thing moves (more accurately, when it *accelerates*, i.e. changes its speed) it will momentarily generate a magnetic field. A little pulse of magnetism that spreads out like a 3D ripple through the fabric of space itself. That pulse, when it washes over other electrically charged things, will cause a tug on those things, forcing them to also move a little bit.
You can think of the effect as something like taking two strong magnets, putting one on top of a table, and holding the other below the table with your hand. When you move the magnet below the table, the magnetism propagates through the table and causes the other one to move with it. You are causing that second magnet to move, but you’re doing it remotely. You’re putting your energy into the first magnet, and due to ripples of magnetism in space, it causes the other magnet to get tugged along.
What radio antennas are are essentially long tubes full of electrons, which are electrically charged things. When you send a signal with an antenna, what you are doing is sloshing the electrons in the antenna up and down in unison in a very specific way that encodes what you’re trying to transmit. All that sloshing in unison causes big ripples in the electromagnetic field, which spread out in all directions**.
Then, there are receivers that contain smaller antennas that can “feel” these ripples. The electrons in them start to slosh around in tune with the ripples, which can be detected as a signal and converted into things like sound.
So, tuning and frequencies. I think this is best explained through an analogy that’s easier to see.
Consider [this YouTube video demonstration](https://www.youtube.com/watch?v=uFlIbujTuIY). It’s about the way buildings of different heights react to earthquakes. There are three dowels with masses on the end that represent buildings at different heights all on a block that can be moved back and forth. When the guy shakes the block slowly, the tallest building will start to sway violently, while the two other buildings barely shake at all. When he starts shaking the block quickly, the shortest building sways violently, while the other two are largely unaffected. And when he shakes it with a frequency somewhere in between, the medium-sized building is the one that shakes violently.
The electrons inside wires have a somewhat similar behavior. When you wash over an antenna with electromagnetic ripples, only ripples at or close to a very specific frequency will actually cause the electrons inside to resonate in unison. Most others will cause the sloshing to be chaotic, causing it to destructively interfere and mostly cancel itself out, leading to no signal beyond some white noise.
A radio receiver, called a tuner, is built in such a way that this “magic frequency” that causes the electrons inside of it to slosh is variable. That’s what you’re doing when you “tune” your radio. You’re changing which frequency of ripples causes the receiving antenna to react. With this technology, you can bombard the tuner with radio signals on all frequencies, but only the frequency that it is sensitive to will cause the electrons in it to slosh around, meaning you can auto-filter out everything except a single radio station. All you have to do is set the sensitive frequency of your tuner to match the frequency the radio station you’re interested in is broadcasting on.
There are a couple major ways to encode your information into radio waves, and have them decoded on the other end. The two most popular methods for public radio broadcast are AM and FM.
AM, or amplitude modulation, is the easier of the two to understand. In a nutshell, the way AM works is the broadcast antenna will constantly “hum” on its designated frequency, but it will constantly change how “loud” it’s humming, getting “louder” and “softer” back and forth very quickly. You can use this rapidly changing “loudness” to encode useful information.
FM, or frequency modulation, is trickier. In this setup, the radio tower hums on its designated frequency just like before, but instead of changing how loud it hums, it instead will hum “off-pitch” from its regular frequency, just a little bit. This will cause the ripples coming off of the tower to be emitted faster or slower than usual. A radio trying to detect that frequency can notice those ripples arriving earlier or later than they should. Information is encoded by strategically creating early and late pulses in a very specific way that the radio on the other end can piece back together.
^(** It doesn’t go in all directions equally. It’s possible to build your antenna with just the right shape that ripples cancel themselves out in certain directions, thus creating antennas that can beam most of their energy in a specific direction.)
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