About Radio the fundamentals of wireless audio and video transmission, Abbr. EM


Since Nov. 18, 2003

"You see, wire telegraph is a kind of a very, very long cat. You pull his tail in New York and his head is meowing in Los Angeles.  Do you understand this?  And radio operates exactly the same way: you send signals here, they receive them there. The only difference is that there is no cat."      Albert Einstein

Thousands of times a day, all around you, "Radio waves" are transmitting music, conversations, pictures and data invisibly through the air, often over millions of miles.  Even though radio waves are invisible and completely undetectable to humans, they have totally changed our society. Cell and cordless phones, pagers, wireless networking, satellite data and video, Broadcast radio and TV are only a few of the  RF devices that use electromagnetic waves to communicate.

Any radio transmission requires two basic components: the transmitter and the receiver.  To get a better understanding of just how we can send information invisibly through the air we will take a look at these seemingly mysterious devices.

Transmitter: A device that takes some sort of message (it could be the sound of someone's voice, video from a camera or data from a computer), encodes it onto a radio frequency carrier wave ( a pure sine wave signal with a frequency high enough to be transmitted efficiently ) amplifies the encoded wave to the required power level and feeds it to an antenna where it is transmitted through space in the form of waves of electromagnetic energy. 

It may be helpful to think of this transmission as being similar to throwing a rock in to a pond.  Undetectable to humans, all of space contains a continuous field of electromagnetic energy.  This invisible field is affected ( warped, bent or shook ) by any object with changing magnetic or electrical properties.  When we talk about the Earth's magnetic field we are actually talking about the earth's magnetic properties formed by the alignments of charged particles deep in the Earth's core.  The result of this magnet force is the wrapping and warping of the portion of the continuous Electromagnetic field near the Earth.  As you can see this continuous field is not stiff and ridged.  It can be bent or warped by the affect of charged or magnetic particles.  In this regard is is rather like an ocean of energy ( in our example the water in the pond ).

The transmitter works by causing sufficient quantities of negatively charged electrons to rush into and out of the antenna to, in effect, grab the electromagnet field surrounding the antenna and shake it.   This is just like the rock striking the pond. The water vibrates as a result of the stone hitting the surface of the water and transferring some of the energy from the fast moving stone to the water in the form of waves in the water.  Radio transmitters cause waves in the continuous electromagnet field of space that radiate in every direction from the antenna.

In the early days of radio, the only type of transmitter was called spark generators, and they created a continuous stream of sparks at very high voltages (up to 20,000 volts). Much the same as high voltage coils in autos today, the high voltage created big fat sparks like you see in a spark plug. With each spark stored energy was released into the antenna causing electrons to accelerate in and out of the circuit.  Each time a electron is accelerated or decelerated it gives off energy in the form of electromagnetic waves ( vibrations ).  By using a telegraph key to turn the spark generator on and off Morris code could be transmitted. Suddenly short messages could be transmitted wirelessly over grate distances.

Today, operating a spark transmitter is illegal because it splatters the entire radio spectrum with RF energy, but in the early days it worked fine and was very common because there were not many people using radio waves  To allow more people to use wireless services it became necessary to divide the total available radio spectrum up in to small slices for each user.  Each transmitter must confine the majority of is RF energy radiation to an assigned channel.  This allows each receiver to select a channel of interest and ignore the others.  What separates one transmission form another is the frequency of its radio frequency carrier wave. 

If you have a RF sine wave generator at the assigned frequency, a RF amplifier to increase the output power, a tuned antenna to convert the electrical power into electromagnetic waves and the sine wave is radiating into space, you have a radio transmitter. The only problem is that the sine wave doesn't contain any information. To broadcast any information you need to modulate ( vary ) the wave in some way to encode the information on it. There are three common ways to modulate a carrier wave:

Pulse Modulation - In PM, you simply turn the sine wave on and off. This is an easy way to send Morse code. PM is not that common today, but is still useful when conditions are not favorable for voice transmission ( such as long distances or emergency operations ). One PM transmitter such as WWV ( US time standard transmission using low frequency and high power ) is able to cover the entire United States!

Amplitude Modulation - Both AM radio stations and the picture part of a TV signal use amplitude modulation to encode information. In amplitude modulation, the amplitude of the sine wave (its peak-to-peak voltage) changes. So, for example, the sine wave produced by a person's voice is overlaid onto the transmitter's sine wave to vary its amplitude. ( You may get an argument form radio engineers about how accurate the varying amplitude analogy is.  If you examine the output of a AM transmitter with a oscilloscope you will see the peak-to-peak voltage varying with time.  In truth though, because of side bands of RF energy that are created [ slightly above and below the carrier frequency ] in the modulation process it may be more accurate to say that the total radiated power is varying but the voltage analogy is easer to understand and works well for most purposes. )

Frequency Modulation - FM radio stations and hundreds of other wireless technologies (including the sound portion of a TV signal, cordless phones, cell phones, etc.) use frequency modulation. The advantage to FM is that it is largely immune to naturally occurring AM static and interference. In a FM transmitter the carrier's sine wave frequency changes very slightly based on the information signal

Once you modulate a sine wave with information, you can transmit the information!

AM Receivers - So, how about a real world example. Here in Atlanta we have a large clear channel AM station that can be clearly received from the Alabama border to the Atlantic Ocean ( even further with a good antenna). When you want receive WSB you tune your radio to 750 on the AM dial, WSB's FCC assigned transmission frequency.  WSB's transmitter produces a sine wave carrier osculating at 750 kHz. (the sine wave pattern of positive and negative pulses repeats 750,000 times per second). The stations audio program is then modulated onto that 750 kHz carrier wave by varying the amplitude of the transmitter's sine wave ( AM modulation ). An amplifier amplifies the signal to produce 50,000 watts of radiated power ( When you consider the station's  24 - 7 operation and the fact that there transmitter cannot be 100% efficient, I can't help but wonder how much there power bill is ). The station's 600 feet antenna tower stands on a large glass insulator to separating it from the ground.  The 50,000 watts of power generated by the transmitter radiates from the tower as a strong electromagnet wave lunching the radio stations signal into space.

So how does your AM radio -- a receiver -- receive the 750 kHz signal that the transmitter sent beaming through the air and extract the information ( audio ) from it? Here are the steps:

  1. Unless you are sitting in the parking lot next to the transmitter (WSBs antenna is located in the middle of a shopping center), your radio receiver needs an antenna to help it pick up the transmitter's radio waves.   An AM antenna is simply a wire or a metal stick that increases the amount of metal the transmitter's waves can interact with. The receiver's antenna generates a small voltage when the radio waves pass by.  If your antenna is not the correct length, orientation or location it may pick up more noise that the desired 750 kHz signal

  2. The receiver selects ( tunes ) the one radio signal of interest and rejects the rest.  Your radio receiver needs a tuner. The antenna will receive thousands of different carrier waves. The job of a tuner is to separate one sine wave from the thousands of radio signals that the antenna receives. In this case, the tuner is a filter tuned to pass the 750 kHz. signal while rejecting the ones that are higher or lower in frequency.

  3. Once the 750 kHz. carrier is selected by the tuner it must be amplified to the level needed by the next stage. The amplifier is made of one or more transistors. Some more advance receiver circuits perform an additional step at this stage.  The original carrier signal is converted to a lower frequency copy that still contains all the modulated signal.  This conversion is preformed to make the filters of the next stage easier to build an less expensive.  This is referred to as the Intermediate Frequency ( IF ) stage.

  4. The radio now has to extract the stations audio program out of the modulated carrier signal. This is done with a part of the radio called a detector ( or demodulator ). In the case of an AM radio, the detector is not very complicated.  You may have made a crystal radio set from a kit.  The major component is a simple diode.  A diode is a one way gate for electrons, so it  allows current to flow through the diode in one direction but not the other.  In doing so it clips off one side of the wave. The radio next filters the clipped ( positive going half of the carrier ) signal to remove the high frequency carrier (and any DC offset).  It then sends the remaining audio to the speakers (or a headphone) by way of the audio amplifier.

  5. What you hear coming out the speakers is the radio station's host voice!

Under Construction - Continue working hear - Coming All About FM Receivers:

FM Receivers - In an FM radio, the detector is different and more complicated, but the basic principles remain the same. In FM, the detector turns the changes in frequency not amplitude into sound, but the antenna, tuner and IF amplifier are largely unchanged.