HOW RADIO WORKS
Radio waves transmit music, conversations, pictures and data invisibly through the air, often over millions of miles -- it happens every day in thousands of different ways! Even though radio waves are invisible and completely undetectable to humans, they have totally changed our lives. Whether we are talking about mobile phones, two way radios or the many thousands of other wireless products we use every day, all of them use radio waves to communicate with each other.
When we think about radio waves [radiation] we think about walkie talkies or two way radio. In fact radio waves appear in many other products such as baby monitors, radar and microwave ovens. In addition satellites depend upon radio waves to communicate with earth stations to relay TV broadcasts for example and aircraft would not be able to operate safely without many different types of radio equipment. The current trend towards wireless internet access is yet another example of the power of radio.
Radio can be incredibly simple, and around the turn of the century this simplicity made early experimentation possible for just about anyone.
All radios today, however, use continuous sine waves to transmit information audio, video, data. The reason that we use continuous sine waves today is because there are so many different people and devices that want to use radio waves at the same time. If you had some way to see them, you would find that there are literally thousands of different radio waves (in the form of sine waves) around you right now -- TV broadcasts, FM radio broadcasts, police and fire radios, satellite transmissions, mobile phone conversations, GPS signals etc.. It is amazing how many uses there are for radio waves today. Each different radio signal uses a different sine wave frequency, and that is how they are all separated.
All two way radios have two parts:
- The transmitter
- The receiver
Transmitting
The transmitter takes the audio [speech] message and, encodes it onto a sine wave and transmits it with radio waves. The receiver receives the radio waves and decodes the message from the sine wave it receives. Both the transmitter and receiver use antennas to radiate and capture the radio signal.
A mobile phone is also a radio and is a much more sophisticated device A mobile phone contains both a transmitter and a receiver, can use both of them simultaneously, can understand hundreds of different frequencies, and can automatically switch between frequencies. This is known as a duplex radio where you can speak and listen simultaneously. A two way radio or Private Mobile Radio [PMR] works in simplex mode where you press to talk and release to listen. Even this is now changing with the introduction of digital radio technology for the emergency services [TETRA] which allows the features of both a walkie talkie and a mobile phone to be incorporated into one handset. Here are some of the important characteristics of a typical analogue two way radio:
- Modulation: Frequency Modulation (FM)
- Frequency range: 66-88Mhz; 136-174MHz; 403-470MHz
- Transmitter power: 4/5 watts but upto 25 watts for a vehicle mounted [mobile] radio and upto 150 watts for a base station or repeater
If you have a sine wave and a transmitter that is transmitting the sine wave into space with an antenna, you have a radio station. The only problem is that the sine wave doesn't contain any information. You need to modulate the wave in some way to encode information on it. There are three common ways to modulate a sine wave:
Pulse modulation, amplitude modulation and for two way radios Frequency Modulation. Unlike pulse and amplitude modulation frequency modulation is almost immune to static and the transmitters sine wave frequency changes very slightly based on the information signal.
Once you modulate a sine wave with information, you can transmit the information!
Receiving
When you speak into the microphone of your walkie talkie your voice is modulated onto the carrier wave that the radio is tuned to. This is achieved by varying the amplitude of the transmitters sine wave An amplifier amplifies the signal to 4 or 5 watts if you are using a portable radio and then the antenna sends the signal out into space.
In order for your two way radio to receive a signal it needs an antenna to help pick up the transmitters radio waves from space. In addition your walkie talkie needs a tuner as the antenna will pick up thousands of sine waves. The job of a tuner is to separate one sine wave from the thousands of radio signals that the antenna receives. Tuners work using a principle called resonance. That is, tuners resonate at, and amplify one particular frequency and ignore all the other frequencies in the air. The tuner causes the radio to receive just one sine wave frequency.
Now the radio has to extract the audio out of the sine wave. This is done with a part of the radio called a demodulator. The radio next amplifies the signal and sends it to the speaker or other audio accessory. The amplifier is made of one or more transistors (more transistors means more amplification and therefore more power to the speakers).
You have probably noticed that almost every radio you see (like your mobile phone, the radio in your car, etc.) has an antenna. Antennas come in all shapes and sizes, depending on the frequency the antenna is trying to receive. The antenna can be anything from a long, stiff wire (as in the AM/FM radio antennas on most cars) to concave dish shaped antennas such as those used for satellite communications. 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.
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 186,000 miles per second (300,000 kilometres per second).
Antennas
Antennas are optimized for the frequency they operate at so for lower frequencies a longer antenna is required. You will notice that on some two way radios antenna lengths vary, even when operating on the same band. This to allow the radios to be carried more easily but shortening the antenna does impact on the performance of the radio. For a mobile phone working at 900 MHz, on the other hand, the optimum antenna size is about 8.3 cm or 3 inches. This is why cell phones can have such short antennas.
Radio Frequencies
A radio wave is an electromagnetic wave propagated by an antenna. Radio waves have different frequencies, and by tuning a radio receiver to a specific frequency you can pick up a specific signal.
In the United Kingdom, OFCOM (Office of Communications) decides who is able to use which frequencies for which purposes, and it issues licenses to radio users.