Radio Basics

A Data Communication Historical Series

By Bob Pollard

RADIO Basics: (Click the pictures to enlarge; then use the back arrow)

Radio waves provide the medium to transmit music, conversations, pictures and data invisibly through the air, often over millions of miles. Radio waves are invisible and completely undetectable to humans. Cell phones, baby monitors, cordless phones, television or any one of the thousands of wireless devices all use radio waves to communicate.

 

In the early days of radio, the transmitters were called spark coils, and they created a continuous stream of sparks at a very high voltage (e.g. 20,000 volts), which allowed signal transmission for a considerable distance. Today, a transmitter like that is illegal because it would create interference across the entire radio spectrum, but in the early days it worked fine and was very common because there were not many people using radio communications.

 

All radios today use continuous sine waves to transmit information (audio, video, data, etc). If there was some way to see radio waves it would be discovered there are literally thousands of different radio waves all around us. These radio waves would include TV broadcasts, AM and FM radio broadcasts, police and fire radio conversations, satellite signals, cell phone conversations, GPS (Global Positioning Satellite) signals, etc. Each different radio signal uses a different sine wave (carrier) frequency, which allows the various signals to remain separated from each other.

 

Any radio device has one of two parts or both: The transmitter and the receiver.

 

The transmitter takes some sort of input signal, which could be the sound of someone's voice, pictures for a TV set, data for MODEM, etc, and encodes (modulates) it onto a sine wave (carrier) and transmits it as radio waves.

 

The receiving device receives the radio waves and decodes (demodulates) the information from the sine wave (carrier). Both the transmitter and receiver use antennas to radiate (send) and capture (receive) the radio signal.

 

A baby monitor is a typical simple radio. There is a transmitter that sits in the baby's room and a receiver that the parents use to listen to any sounds originated by the baby. A typical baby monitor description would include the following:

 

·         Modulation: Amplitude Modulation (AM)

·         Frequency (carrier): 49 MHz (49,000,000 cycles per second)

·         Number of variable frequencies: 1 or 2

·         Transmitter power: 0.25 watts

 

A cell phone is also a radio, but is a much more sophisticated device than a baby monitor. A cell phone contains both a transmitter and a receiver and can use both of them simultaneously; can utilize hundreds of different frequencies and can automatically switch between frequencies.

 

A typical cell phone (right) communicates via a cell phone tower and can transmit 2 to 3 miles (3-5 km), or more depending on external conditions.

 

The simplest form of a continuously varying wave is a sine wave like the one shown right: A sine wave fluctuates between, for example: +10 volts and -10 volts.

 

Frequency:

The frequency of a sine wave is the number of times it oscillates up and down (wave length) per second, measured in cycles per second (cps). When listening to an AM radio broadcast and the dial is set on 680 that means the radio is tuned to a frequency of 680,000 cycles per second (cps). The 680,000 cycles per second frequency is referred to as the carrier frequency, which is modulated by an input signal, such as a voice, at the transmitting radio station. The term Hertz (Hz) is normally used as the designation for frequency instead of cycles per second. FM radio signals operate in the range of 100,000,000 Hertz (100 Mega-Hz / MHz), so 101.5 on the FM dial is tuned to a transmitter generating a sine wave (carrier) at 101,500,000 cycles per second.

 

Transmitting:

A radio station has a transmitter that transmits the sine wave (signal) into space through an antenna, and in order to transmit useful information the sine wave must be encoded (modulated) in some manner. An amplifier at the transmitter amplifies the signal to something like 50,000 watts for a large AM station. Then the AM modulated radio waves are sent out into space via an antenna. A few common modulation schemes use by radio stations or other simple radio devices would include the following:

 

Pulse Modulation (PM): In PM the sine wave (carrier) is turned on and off. This is an easy way to send Morse code or International code. PM is not that common, but it can be used for controlling devices, such as the radio system that sends signals to radio-controlled clocks in the United States. One PM clock setting transmitter is able to cover the entire United States.

Amplitude Modulation (AM): Both AM radio stations and, presently, the picture part of a TV signal use amplitude modulation to encode information. In amplitude modulation, the amplitude of the sine wave (carrier) is varied up and down based on the signal imposed upon it. For example, the sine wave produced by a person's voice is overlaid onto the transmitter’s sine wave (carrier) to vary its amplitude; the voice signal (variable sine wave) modulates the transmitter carrier sine wave.

Frequency Modulation (FM): 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 of FM is that it is largely immune to static and other unwanted amplitude distortion signals. In FM, the transmitter's sine wave frequency changes based on the information signal that is imposed (modulated) upon it.

 

Once a carrier (sine wave) signal is modulated by an information signal it is transmitted to the atmosphere via an antenna where the transmitted signal can be received by an antenna.

 

Receiving a signal (AM example):

When an AM radio receiver is tuned to a particular station, 680 on the AM dial, it will receive the transmitted sine wave, which is being transmitted at 680,000 Hertz (680 KHz). The music or voices are modulated onto that carrier sine wave (680 KHz) by varying the amplitude.

 

The AM radio receives the 680,000-Hertz modulated signal that the transmitter sent, via an antenna, and extracts (demodulates) the information from the signal.

 

An AM antenna can be a length of wire or a metal stick as long as an adequate amount of interactive metal is provided. The interactive metal provides a surface (medium) for capturing the transmitter's sine waves.

The radio receiver uses a tuner to select individual stations, such as the dial being set to 680 in the above examples. The antenna attached to the radio will receive thousands of different frequency sine waves, and the function of a tuner is to separate one signal (sine wave) from the other thousands of radio signals that the antenna receives. In this example, the tuner is tuned to receive the 680,000-Hertz signal.

 

Tuners work using a principle called resonance. That is, tuners resonate at one particular frequency and ignore all the other frequencies being received from the antenna. A simple basic tuner is constructed with capacitors and inductors (coils).

The tuner enables the radio to receive just one sine wave frequency (example: 680,000 Hertz). Now the radio has to extract the music or voice out of that modulated sine wave. This is partially done with a part of the radio called a detector. In the case of an AM radio, the detector utilizes an electronic component called a diode. A diode allows current to flow through in one direction but not the other, so it clips off one side of the wave, as shown below:

 

The radio next smoothes out and amplifies the clipped signal (Direct Current) and sends it to the speakers, usually via an amplifier in order to create additional power for the speakers.

 

Antennas:

The size of an optimum radio antenna is related to the frequency of the signal.

For example: A radio tower for an AM radio station operating (transmitting) at 680 KHz. It is transmitting a sine wave with a frequency of 680,000 hertz. In one cycle of the sine wave, the transmitter is going to move electrons in the antenna in one direction, then switch and move electrons in the opposite direction during one cycle. The electrons will change direction two times during one cycle of the sine wave. If the transmitter is running at 680,000 hertz, that means that every cycle completes in 1/680,000 or 0.00000147 seconds.

 

When calculated out, the optimal antenna size for the transmitter at 680,000 hertz is about 361 feet (110 meters). So AM radio stations need very tall towers. For a cell phone working at 900,000,000 (900 MHz), on the other hand, the antenna size is about 8.3 cm or 3 inches. This is why cell phones can have such short antennas.

 

Optimal antenna physical sizes would be those designed for ¼, ½, or one full cycle of the sine wave.

 

Examples of Radio Station antenna

It is obvious that the receiving AM radio antenna in a car is not 300 feet long; it is only a few feet long. If the antenna was longer it would receive better, but car antennas are designed to receive both AM and FM radio signals at a multitude of frequencies, so the antenna length must be designed for optimum reception at all the AM and FM frequencies; a happy medium.

 

It should be noted that coils (inductors) can be added to a receiving antenna, which effectively changes the electrical length of an antenna, without changing the physical length

 

When current enters the transmission antenna it creates a magnetic field around the antenna. 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 (186,000 miles per second), traveling outward away from the antenna. Finally these magnetic fields are induced (received) into the receiving antenna.

 

Comment:

In communications different terms may be used when discussing AC landline / cable circuits or wireless (radio) circuits. The terms carrier, analog, frequency, Hertz (Hz), sine wave, band, sub-band and bandwidth (broadband) are normally used when discussing non-wireless circuits. When discussing wire-less (radio) circuits the terms frequency, hertz (Hz), band, tone, radio frequency (RF), frequency spectrum, wavelength and wave are normally used.