Radio Links
Components of a radio link TX antenna RX antenna Radio waves Transmitter (TX) Receiver (RX) What are some different kinds of radio links? What determines the performance (usefulness) of a radio link?
Some radio links AM radio, FM radio Television (broadcast)
Link properties Information transmitted Information received Antenna TX, RX Cost TX, RX Size TX, RX Power available TX, RX
Electronic Article Surveillance Another type of radio link.
Electromagnetic waves Acceleration of electrical charge (e.g. electrons) creates electromagnetic waves Energy Energy These waves carry energy away from the source Also works the other way: electromagnetic waves cause acceleration of electrical charge
Any acceleration of electrons creates radio waves Receiver Transmitter
Most basic radiator: Electrical dipole Charge moving back and forth Charge movement Sinusoidal variation of charge position with time Charge position Period, T Frequency, f = 1/T Time
Structure of radio waves Close to source Near field is complicated Far from source Far field has simple plane wave structure periodic in space and time, travelling at the speed of light Wavelength, λ Electric Field, Magnetic Field Distance Moving at the speed of light --> Receiver
Receiving radio waves Radio waves cause voltage & current oscillations in receiving antenna with a characteristic frequency f = c/λ (c = speed of light = 300,000,000 m/s) Both size (wavelength) and frequency of radio waves are important for radio link design Period, T Antenna Voltage or Current Time Frequency, f = 1/T
Frequency choices
Transmitting radio waves Radiation of radio waves consumes power in a circuit, just as if a resistor were present Need to have right antenna at TX to maximize radiation (and at RX to get best reception!) One simple choice: Dipole antenna Wire Length = λ/4 AC signal source Wire Length = λ/4 = Radiation Resistance
Link budget Where does this power go? For communication, radiated power must be received and interpreted How much of the radiated power (signal) is received? How much interference is also received (noise)? What is the signal to noise ratio (SNR)? Higher SNR better ability to transmit information
Voyager spacecraft 23 W transmitter in deep space 70 m dish antenna on earth How much power is received?
Inverse square law Suppose transmitter radiates power equally in all directions ( isotropic radiator ) At a distance r, power is spread over the surface of a sphere, area 4πr 2 Antenna intercepts a portion of that power, according to its area
Message from Pluto Say we re radiating 23 W from Pluto: About 5.9 x 10 12 meters from earth (5.9 trillion) Receiving dish: 70 m diameter P r = P t (A e / 4πr 2 ) = 23 (π(35 2 )/ 4π(5.9 x 10 12 ) 2 ) = 2 x 10-22 W! Less than a billionth of a trillionth of a watt how can we do better?
Improving signal to noise ratio Decrease noise Decrease distance Increase transmitter power Increase antenna area Direct radiated power more efficiently
Antenna patterns No antenna is an isotropic radiator Dipole antenna has maximum radiation in direction perpendicular to charge motion Increases effective radiated power by 2x Dipole antenna pattern
Directional antennas Dipole omnidirectional 3-element Yagi Rhombic Antennas can be designed to concentrate power in a particular direction by many orders of magnitude Transmit and receive antennas can both be directional generally true for satellite links Imposes pointing requirements
Antennas for long-distance radio links Voyager highly directional antennas on transmitter and receiver What about other systems? Satellite television, GPS, Balloons, Rockets
Direct broadcast satellite (DBS) TV High-power (>1000 W at 12 GHz) satellites broadcast to small fixed dishes Satellites in geostationary orbit
Orbits Over 7000 man-made objects* orbit the earth Kepler s third law: orbit time T = kr 3/2 Geostationary satellites orbit above the equator, have R = 35,700 km, T = 24 hours * Greater than 10 cm diameter. Also 50,000 smaller objects and 10-100 billion paint chips
GPS 24 satellites in lowearth-orbit about 20,000 km not geostationary ~ 50 W transmit power at 1.5 GHz Ground antennas moderately directional (Not to scale)
Balloon & Rocket Telemetry Difficult to control orientation of transmit antenna Use omnidirectional transmit antenna, directional receiver antenna Balloon telemetry tracking system
Sounding rocket telemetry Poker Flat telemetry dish
Other telemetry design choices Frequency where (in frequency space ) is information transmitted Technological constraints: what can be built? Natural constraints: how do different frequencies behave in the environment? Bandwidth how much information is transmitted?
Frequency choices
Propagation of radio waves
Line of sight propagation About 400 miles at 100,000 feet
Atmospheric transmission Transmission window in GHz range
Regulations
Bandwidth Need more than one frequency to carry information need a band of frequencies Full range audio: 20 khz Telephone: 3 khz Morse code: 500 Hz Television: 5.5 MHz Ethernet (10 Mb): 10 MHz DBS TV: 33 MHz