1. Radio Links

2. 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?

3. Some radio links
AM radio, FM radio
Television (broadcast)

4. Link properties Information transmitted Information received
Antenna – TX, RX
Cost – TX, RX
Size – TX, RX
Power available – TX, RX

5. “Electronic Article Surveillance”
… Another type of radio link.

6. 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

7. Any acceleration of electrons creates radio waves
Receiver
Transmitter

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

9. 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

10. 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

11. Frequency choices

12. 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

13. 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

14. Voyager spacecraft 23 W transmitter in deep space
70 m dish antenna on earth
How much power is received?

15. 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

16. 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 W !
Less than a billionth of a trillionth of a watt… how can we do better?

17. Improving signal to noise ratio
Decrease noise
Decrease distance
Increase transmitter power
Increase antenna area
Direct radiated power more efficiently

18. 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

19. Directional antennas
Dipole – “omnidirectional” 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

20. Antennas for long-distance radio links
Voyager – highly directional antennas on transmitter and receiver
What about other systems? Satellite television, GPS, Balloons, Rockets

21. Direct broadcast satellite (DBS) TV
High-power (>1000 W at 12 GHz) satellites broadcast to small fixed dishes
Satellites in geostationary orbit

22. 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 billion paint chips

23. 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)

24. Balloon & Rocket Telemetry
Difficult to control orientation of transmit antenna
Use omnidirectional transmit antenna, directional receiver antenna
Balloon telemetry tracking system

25. Sounding rocket telemetry
Poker Flat telemetry dish

26. 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?

27. Frequency choices

28. Propagation of radio waves

29. Line of sight propagation
About 400 miles at 100,000 feet

30. Atmospheric transmission
Transmission “window” in GHz range

31. Regulations

32. 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