How do radio signals work

In an AM signal the height of the waves varies; in FM it is the space between the waves that changes. WGPC Albany had to relocate its transmitter tower in because of overlap with a station in Valdosta which broadcast on the same frequency. Radio works by transmitting and receiving electromagnetic waves. The radio signal is an electronic current moving back and forth very quickly.

All wireless communication systems, from the home remote control up to the satellite, are based on this principle, even though increasingly complex technologies are of course used to encode these electromagnetic signals, improve their quality, increase the amount of information or make transmissions secure. We use radio waves in almost every aspect of our daily lives. In the morning we get the latest news from an AM or FM radio, newsfeeds on our mobiles, the television; mobiles help us pay for small purchases throughout the day such as our coffee, baguette or parking meter using NFC contactless technology, they enable us to alert the authorities in case of an emergency police, fire brigade, ambulance ; and they also enable connected objects to communicate thanks to consumer devices such as Wi-Fi, Bluetooth, LORA, DECT… and of course private and professional communication.

They have become indispensable. One characteristic of a sine wave is its frequency. The frequency of a sine wave is the number of times it oscillates up and down per second. When you listen to an AM radio broadcast, your radio is tuning in to a sine wave with a frequency of around 1,, cycles per second cycles per second is also known as hertz.

For example, on the AM dial is , cycles per second. FM radio signals are operating in the range of ,, hertz, so See How the Radio Spectrum Works for details.

Here's a real world example. When you tune your car's AM radio to a station -- for example, on the AM dial -- the transmitter's sine wave is transmitting at , hertz the sine wave repeats , times per second.

The DJ's voice is modulated onto that carrier wave by varying the amplitude of the transmitter's sine wave. An amplifier amplifies the signal to something like 50, watts for a large AM station. Then the antenna sends the radio waves out into space. So how does your car's AM radio -- a receiver -- receive the ,hertz signal that the transmitter sent and extract the information the DJ's voice from it?

Here are the steps:. In an FM radio, the detector is different, but everything else is the same. In FM, the detector turns the changes in frequency into sound, but the antenna, tuner and amplifier are largely the same. In the case of a strong AM signal, it turns out that you can create a simple radio receiver with just two parts and some wire! The process is extremely simple -- here's what you need:. Here's what you do:. Now if you put the earplug in your ear, you will hear the radio station -- that is the simplest possible radio receiver!

This super-simple project will not work if you are very far from the station, but it does demonstrate how simple a radio receiver can be. Here's how it works. Your wire antenna is receiving all sorts of radio signals, but because you are so close to a particular transmitter it doesn't really matter. The nearby signal overwhelms everything else by a factor of millions. Because you are so close to the transmitter, the antenna is also receiving lots of energy -- enough to drive an earphone! Therefore, you don't need a tuner or batteries or anything else.

The diode acts as a detector for the AM signal as described in the previous section. So you can hear the station despite the lack of a tuner and an amplifier! The Crystal Radio Kit that Radio Shack sells contains two extra parts: an inductor and a capacitor. These two parts create a tuner that gives the radio extra range. See How Oscillators Work for details. You have probably noticed that almost every radio you see like your cell phone, the radio in your car, etc.

Antennas come in all shapes and sizes, depending on the frequency the antenna is trying to receive. Radio transmitters also use extremely tall antenna towers to transmit their signals. The idea behind an antenna in a radio transmitter is to launch the radio waves into space. In a receiver, the idea is to pick up as much of the transmitter's power as possible and supply it to the tuner. For satellites that are millions of miles away, NASA uses huge dish antennas up to feet 60 meters in diameter!

The size of an optimum radio antenna is related to the frequency of the signal that the antenna is trying to transmit or receive. The reason for this relationship has to do with the speed of light , and the distance electrons can travel as a result. The speed of light is , miles per second , kilometers per second. On the next page, we'll use this number to calculate a real-life antenna size. Let's say that you are trying to build a radio tower for radio station AM.

It is transmitting a sine wave with a frequency of , hertz. In one cycle of the sine wave, the transmitter is going to move electrons in the antenna in one direction, switch and pull them back, switch and push them out and switch and move them back again. In other words, the electrons will change direction four times during one cycle of the sine wave.

One quarter of that is 0. At the speed of light, electrons can travel 0. That means the optimal antenna size for the transmitter at , hertz is about feet meters. So AM radio stations need very tall towers. For a cell phone working at ,, MHz , on the other hand, the optimum antenna size is about 8. This is why cell phones can have such short antennas. You might have noticed that the AM radio antenna in your car is not feet long -- it is only a couple of feet long.

If you made the antenna longer it would receive better, but AM stations are so strong in cities that it doesn't really matter if your antenna is the optimal length. You might wonder why, when a radio transmitter transmits something, radio waves want to propagate through space away from the antenna at the speed of light. Why can radio waves travel millions of miles? Why doesn't the antenna just have a magnetic field around it, close to the antenna, as you see with a wire attached to a battery?

One simple way to think about it is this: When current enters the antenna, it does create a magnetic field around the antenna. We have also seen that the magnetic field will create an electric field voltage and current in another wire placed close to the transmitter.

It turns out that, in space, the magnetic field created by the antenna induces an electric field in space. This electric field in turn induces another magnetic field in space, which induces another electric field, which induces another magnetic field, and so on. These electric and magnetic fields electromagnetic fields induce each other in space at the speed of light, traveling outward away from the antenna.

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