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How is an antenna able to receive and transmit data? And how is the antenna able to convert that data to something the radio can read? |TTI

How is a metal rod able to receive radiowaves. It is because of its ability to receive EM waves easily due to its material and texture? In a radio, we can listen to the DJ, but how are we able to hear so audibly when the audio message is sent through radiowaves or microwaves?

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  1. Yeah, most antennas are just a piece of metal. The shape is the most important part. Some antennas (lots, actually) have other materials in them as well, and I’ll mention that after.

    So: an EM wave is basically a voltage traveling through the air at the speed of light. That’s pretty unintuitive, but it becomes more obvious when you think about it. If you could freeze time and measure the voltage (from the EM field) at any one spot, you could follow an EM wave by watching the voltage go up and down.

    Now freeze time around an antenna. Picture a normal car whip antenna, just a wire sticking straight up. The antenna is inside the wave, and takes on that voltage. The voltage goes up and down as the wave passes, so electrons are sucked into and out of the antenna.

    The next simplest antenna is a [dipole](https://upload.wikimedia.org/wikipedia/commons/thumb/d/dd/Dipole_receiving_antenna_animation_6_800x394x150ms.gif/400px-Dipole_receiving_antenna_animation_6_800x394x150ms.gif). The voltage induced in the antenna is the red curve that moves up and down. See how it causes electrons to flow back and forth across the resistor? A radio measures that flow.

    All the antenna does is produce that flow. It’s up to the radio to measure and amplify that flow. In fact the antenna doesn’t even provide the resistor- it’s up to the radio to have the right resistance, and match exactly what the antenna is tuned to expect. Normally this is 50 ohms. The reason why is complicated, which is to say people argue about it a *lot*.

    So, if the antenna was too big, or the wrong shape, or facing the wrong way, it might end up overlapping another part of the wave. If that happened, electrons would flow between that part of the antenna and the other part instead of through the resistor, and you wouldn’t be able to measure anything. That’s why metal heats up in a microwave. Microwaves use very similar frequencies to a household router, but are hundreds or thousands of times more powerful. When you expose a fork to that, it sits over a couple different waves at the same time- so the middle may be at -5 V, while the forks and handle are at +5V. Because metal has such a low resistance (WAY lower than water), that low voltage can still cause a LOT of current to flow, and the fork gets very hot. [This is even more complicated than it sounds, and you can also get *high* voltages in this situation- those cause sparks.]

    Modern antennas take advantage of that fact by carefully designing antennas to not cancel out. You can only measure so much of the wave before it passes, so they cheat and measure several parts of the wave at once- each time removing a as much energy as possible, without cancelling anything out. [Yagi-Uda antennas](https://en.wikipedia.org/wiki/Yagi%E2%80%93Uda_antenna) are one of the most commonly-seen antennas, and work basically like this [Note: many people will call this a lie. They are mostly right, but its a decent explanation anyways.] Yagis are one of the special antennas- some of the cross-wires are electrically insulated, and only serve to reflect EM waves- exactly like the dish on a [parabolic antenna](https://upload.wikimedia.org/wikipedia/commons/4/40/Erdfunkstelle_Raisting_2.jpg). Incidentally, parabolic antennas work exactly like you would think they do.

    Now we’re getting into the really weird antennas. Some of them used to be really common, and some still are, but antennas nowadays are mostly just metal shapes- we got a lot better at making them very sneaky. Cell phone antennas, and famously iphone antennas, are often built into the metal chassis- [you can see where each end is insulated from the rest of the chassis](http://photos2.insidercdn.com/iphone5.081012.jpg). One extremely popular and now totally obsolete antenna is the [ferrite loop](https://en.wikipedia.org/wiki/Loop_antenna#Ferrite). These were used for AM radio, which is a much lower frequency than WiFi, cellular or even FM. Lower frequency means longer wavelength- more distance between the high and low voltages. For medium-wave [Heard of shortwave? Same idea.] that distance can be hundreds of meters, and you can’t hope to make an antenna that long. Ferrite loop antennas wrapped dozens of meters (or more!) of wire around a [ferrite rod](https://upload.wikimedia.org/wikipedia/commons/6/62/Ferrite_antenna.jpg). The ferrite acts like a funnel for EM waves, increasing the antennas sensitivity by a huge amount. You know how you can [pick up a bunch of paperclips with a magnet](https://i.pinimg.com/originals/37/f1/05/37f105b897436b51f17f411c9bc20ed7.jpg)? Same deal. Each paperclip funnels the magnetic field through itself, so it’s still strong when it reaches the other end and weaker right beside the paperclip. The ferrite loop antenna works the exact same way.

    [Helical antennas](https://en.wikipedia.org/wiki/Helical_antenna) are another weird one: they’re extremely directional, kind of like a laser for EM waves. Usually used to track satellites, since they can be hundreds or thousands of miles away, and transmit at low power compared to earth transmitters. Now, these things are black magic, but given the basics you can at least see *why* they work if not exactly how. The magnetic field at different places on the helix is different, and reinforces or cancels itself out. If the wave comes from a different direction, different spots will be cancelling.

    Every bit of metal on earth acts like a little antenna, but if they aren’t plugged into a radio you’d never notice. For the most part they just absorb waves and immediately emit them back. If the wave is much bigger than the metal, the whole object will be at the same voltage and no electricity will flow inside it. For big metal things, electricity may flow inside them, but not enough to matter.

    > In a radio, we can listen to the DJ, but how are we able to hear so audibly when the audio message is sent through radiowaves or microwaves?

    Math, mostly. AM radio is the simplest: sound controls whether or not the transmitter is on. So [when the sound wave is high](http://www.mediacollege.com/audio/images/loudspeaker-waveform.gif), the transmitter is busily sending lots of waves out the antenna. When its low, the transmitter turns off. Your car radio receives those waves and turns them into another on-off signal, and uses that to control the speakers.

    The exact way you do it is called [envelope detection](https://en.wikipedia.org/wiki/Envelope_detector). There are a lot of ways to do this. One way is to use a rectifier, which is a device that turns AC waves into DC. When the rectifier is fed fully-on 800 kHz radio, it outputs, say, ~3 V. When it’s not getting any waves it outputs 0 V. That signal is fed from the rectifier through an audio amplifier that increases the voltage to 10+ volts and sends it to your speakers.

    FM radio is ~~worse~~ more complicated. *Hella* complicated. So goddamn complicated we don’t totally understand some things about it- like, when you lose reception on a station, why does it go from clear to static so fast? Why doesn’t the static slowly increase? [It’s called the lock-on effect, but most of the reason is that the way you hear sound is weird: loud noises all sound about the same volume while quiet noises sound VERY quiet. That’s why sound is in dB (Decibels) instead of Pascals.] Anyway, FM (Frequency Modulated) means that instead to the transmitter turning on or off, the frequency of the transmitter gets higher or lower. Radios are very clever and can translate that very easily, and even use it to tune to stations automatically.

    WiFi, cellular, and all digital radio (including the way that FM radio stations can transmit the station name, song title and artist) are all much more complex. I could go on for hours about just [fountain codes](http://blog.notdot.net/2012/01/Damn-Cool-Algorithms-Fountain-Codes). [The folks who do things like this](http://www.aholme.co.uk/GPS/Main.htm) have long beards (mandatory for the women too), fancy degrees and pointy hats with wide brims. They are to be treated with respect and discouraged from talking. Once they start going it sounds like they’re casting a spell [sleep, usually].

  2. https://en.wikipedia.org/wiki/Antenna_(radio)

    When radio waves hit an antenna, an alternating voltage will be produced across it as the varying electromotive force of the photons moves electrons in the antenna back and forth. This voltage can be detected by electronic circuitry, and converted into some human-readable form.

    To transmit radio waves you do the opposite, apply a varying voltage to the antenna to move the electrons batch and forth, and then radio waves will emanate from it.

    To transmit something like audio, you can modulate the amplitude envelope of the varying voltage. If you modulate it with a voltage signal from a microphone, and then apply that modulation to a speaker when you receive it, you’ll have a voice-over-radio system.

    You can also modulate by frequency, and you can send digital information by modulating high and low for ones and zeros.

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