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Radio Radio Waves
Physics of Modern Devices February 18, 2009
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Intro to radio waves
"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 society. Whether we are talking about a cell phone, a baby monitor, a cordless phone or any one of the thousands of other wireless technologies, all of them use radio waves to communicate.
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Everyday technologies depend on radio waves!
AM and FM radio broadcasts Cordless phones Garage door openers Wireless networks Radio-controlled toys Television broadcasts Cell phones GPS receivers Ham radios Satellite communications Police radios Wireless clocks
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Radio Waves
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.
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Radio Parts
Transmitter
The transmitter takes some sort of message (it could be the sound of someone's voice, pictures for a TV set, data for a radio modem or whatever), 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.
Receiver
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Example of a simple transmitter
1.
Start with a battery and a piece of wire.
Battery sends electricity through wire if you connect the wire between the two terminals of the battery.
We did this on Monday!
The moving electrons create a magnetic field surrounding the wire, and that field is strong enough to affect a compass.
2.
Take another wire and place it parallel to the battery's wire but several inches (5 cm) away from it.
If you connect a very sensitive voltmeter to the wire, then:
Every time you connect or disconnect the first wire from the battery, you will sense a very small voltage and current in the second wire. Any changing magnetic field can induce an electric field in a conductor. this is the basic principle behind any electrical generator.
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Example in action
The battery creates electron flow in the first wire. The moving electrons create a magnetic field around the wire. The magnetic field stretches out to the second wire. Electrons begin to flow in the second wire whenever the magnetic field in the first wire changes.
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Electromagnets and Energy
Electric and magnetic fields contain energy An electromagnet stores magnetic energy Electromagnet consumes energy as it turns on
Current temporarily experiences a voltage drop Current temporarily experiences a voltage rise
Electromagnet releases energy as it turns off
Electromagnet opposes current charges
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Transmitters, cont
When you connect the battery, the voltage in the wire is 1.5 volts, and when you disconnect it, the voltage is zero volts.
Create a continuously varying electric current in a wire: sine wave.
A capacitor and an inductor can create the sine wave.
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Inductors
Inductors are electromagnets Inductors store magnetic energy Inductors oppose changes in current
Unit is a Henry (V*s/A) A H inductor is common in radio
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Tank Circuit
Resonant electric circuit
Tank circuit is an electronic harmonic oscillator
Inductor & Capacitor share energy Charge flows back and forth through inductor Energy shifts back and forth between the two devices
Energy oscillates between the capacitor and the inductor being stored as an electric field in the capacitor and then as a magnetic field around the inductor.
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Tank Circuit Oscillation
The capacitor is charged and it discharges into the inductor, which builds up a magnetic field as the current through it increases. Once the capacitor is out of charge, the magnetic field keeps the current flowing, causing the capacitor to charge in reverse. This reverse voltage reduces the inductor current until it stops. At this point the cycle begins again with the capacitor charged in the opposite sense.
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Capacitor and Inductor
The larger the capacitors capacitance, the more separated charge it can hold with a given amount of energy and the longer it takes that charge to move through the circuit as current. The larger the inductors inductance (opposition to current changes) the longer that current takes to start and stop. A tank circuit with a large capacitor and a large inductor may have a period of a thousandth of a second or more. One with a small capacitor and a small inductor may have a period of a billionth of a second or less.
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Tank Circuits in Radio
Tanks are resonant devices Tanks build up energy at a specific frequency Tanks help radios emit radio waves Tanks help radios detect radio waves
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Question
Why doesnt the radio transmitter simply push electric charge directly on and off the antenna, without using a tank circuit?
Amount of charge would be too small to create a strong radio signal. Tank circuit allows transmitter to move much more charge. Think of a tuning fork.
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Emitting Radio Waves (Part 1)
A transmitter uses a tank circuit to move charge up and down its antenna A receiver uses a tank circuit to detect charge move on its antenna Transmitter antenna charge affects receiver antenna charge
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A problem
FM radio stations use radio waves with frequencies of 88-108MHz. Assume and inductance of 0.6 and determine the range of capacitance values that are needed to pick up all the radio waves broadcasted by FM stations.
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Emitting Radio Waves (Part 1)
Accelerating charge emits radio waves
Charge produces electric field Current produces magnetic field Changing current produces changing magnetic field, produces changing electric field, prod
A radio wave consists only of an electric and magnetic field A radio wave travels through empty space at the speed of light
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Structure of a Radio Wave
Electric field is perpendicular to magnetic field Electric field creates magnetic field and vice versa Electric field determines polarization of the wave
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Transmitting Information
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 contain doesn't 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 Frequency Modulation
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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, but one good example of it is the radio system that sends signals to radio-controlled clocks in the United States.
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AM Modulation
Both AM radio stations and the picture part of a TV signal use amplitude modulation to encode information. Information is encoded in the fluctuating amplitude of the wave The amplitude of the sine wave (its peak-to-peak voltage) changes. Pressure variations cause changes in the amount ofcharge moving on the antenna.
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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. In FM, the transmitter's sine wave frequency changes very slightly based on the information signal. Information is encoded in the exact frequency of the charge motion Pressure variations cause slight shifts in the frequency of charge motion on the antenna The advantage to FM is that it is largely immune to static.
FM Modulation
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A question
If you listen to AM radio in your car and drive through a tunnel, why does the volume become very low?
The tunnel blocks most of the radio wave. Only a small fluctuating wave reaches the radio, so the radio produces only small fluctuations in the air density with the speaker. AM radio has trouble distinguishing between a distant transmission representing loud music and a nearby transmission representing soft music. In both cases, the receiver detects only small variations in the current moving up and down the antenna. So you must turn the volume up.
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FM/AM Radio
When you listen to a radio station, say 91.5 FM , what does that mean? You are listening to a radio station broadcasting an FM radio signal at a frequency of 91.5 MHz The transmitter at the radio station is oscillating at a frequency of 91,500,000 cycles per second. Your FM (frequency modulated) radio can tune in to that specific frequency and give you clear reception of that station. All FM radio stations transmit in a band of frequencies between 88 MHz and 108 MHz. This band of the radio spectrum is used for no other purpose but FM radio broadcasts. AM radio is confined to a band from 535 kHz to 1,700 kHz
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A problem
A station broadcasts an AM radio wave which is 1230 kHz on the dial and an FM radio wave which is 91.9MHz on the dial. Find the distance between adjacent crests in each wave.
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Common frequency bands
AM radio - 535 kHz to 1.7 MHz Short wave radio - bands from 5.9 MHz to 26.1 MHz Citizens band (CB) radio - 26.96 MHz to 27.41 MHz Television stations - 54 to 88 MHz for channels 2 through 6 FM radio - 88 MHz to 108 MHz Television stations - 174 to 220 MHz for channels 7 through 13
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History
Why is AM radio in a band at 550 kHz to 1,700 kHz, while FM radio is in a band at 88 to 108 MHz? It is all completely arbitrary, and a lot of it has to do with history.
AM radio has been around a lot longer than FM radio. The first radio broadcasts occurred in 1906 or so, and frequency allocation for AM radio occurred during the 1920s. In the 1920s, radio and electronic capabilities were fairly limited, hence the relatively low frequencies for AM radio.
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Wireless technology
Garage door openers, alarm systems, etc. - Around 40 MHz Standard cordless phones: Bands from 40 to 50 MHz Baby monitors: 49 MHz Radio controlled airplanes: Around 72 MHz, which is different from... Radio controlled cars: Around 75 MHz Wildlife tracking collars: 215 to 220 MHz MIR space station: 145 MHz and 437 MHz Cell phones: 824 to 849 MHz New 900-MHz cordless phones: Obviously around 900 MHz! Air traffic control radar: 960 to 1,215 MHz Global Positioning System: 1,227 and 1,575 MHz Deep space radio communications: 2290 MHz to 2300 MHz
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Antenna
Every radio you see (like your cell 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 something as bizarre as a satellite dish. Radio transmitters also use extremely tall antenna towers to transmit their signals.
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Antennas, cont
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 200 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.
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A problem
Why do AM radio stations need very tall towers and a cell phone antenna is only a few inches long (at most)?
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Cochlear Implants
Cochlear Implants also use radio waves!
microphone (worn behind the user's ear) thin cord (connects microphone to speech processor) speech processor (codes sounds electronically) transmitting coil (sends code as radio waves) receiver/stimulator (converts code into electrical signals) electrode array implanted in the cochlea (stimulates auditory nerve fibres when electrical signal is received)
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Cochlear implants
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A question
The ignition system of an automobile produces sparks to ignite the fuel engine. During each spark process, charges suddenly accelerate through a spark plug wire and across a spark plugs narrow gap. Sometimes this process introduces noise in your radios reception. Why?
As charges accelerate in the wires, they emit radio waves.
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Another question
The Empire State Building has several FM antennas on top, added in part to increase its overall height. These antennas arent very tall. Why do short antennas, located high in the air, do such a good job of transmitting FM radio?
Television transmission involves high-frequency, short-wavelength radio waves. Since a good antenna is 1/4 wavelength long, television transmission requires relatively short antennas.
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Summary
Radio waves are used for many modern devices.
HW#4 due Monday Feb. 23