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 These waves carry energy away from the source Also works the other way: electromagnetic waves cause acceleration of electrical charge Energy Energy

Any acceleration of electrons creates radio waves Receiver Transmitter

Most basic radiator: Electrical dipole Charge movement Charge moving back and forth Sinusoidal variation of charge position with 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 Receiver

Receiving radio waves Radio waves cause voltage & current oscillations in receiving antenna with a characteristic frequency f = c/l (c = speed of light = 300,000,000 m/s) Both size (wavelength) and frequency of radio waves are important for radio link design

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

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 4pr2 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 1012 meters from earth (5.9 trillion) Receiving dish: 70 m diameter Pr = Pt (Ae/ 4pr2) = 23 (p(352)/ 4p(5.9 x 1012)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 = kR3/2 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 low-earth-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