Radio%20Communications

1
Radio Communications

• J. C. Sprott
• Department of Physics
• University of Wisconsin - Madison
• Presented to
• Physics 208
• on April 6, 2006

http//sprott.physics.wisc.edu/lectures.htm
2
Outline

• Properties of EM Waves
• Choice of Frequencies
• Radio Wave Production
• Radio Wave Propagation
• Radio Wave Reception
• Television
• Radar

3
Properties of EM Waves

• Transverse
• Propagates in Vacuum
• Any Frequency Possible
• Frequency X Wavelength c
• c 3x108 m/s (in vacuum)

E
B
4
Transverse EM Wave
5
The Electromagnetic Spectrum
6
Choice of Frequencies

• Lower Limit (105 Hz, 3 km)
• Bandwidth (data rate)
• Antenna size
• Project ELF
• Upper Limit (109 Hz, 30 cm)
• Propagation - line of sight
• Sources and Detectors
• Radar / Microwaves

7
Radio Wave Production

• Transmitter
• Spectral purity (min interference)
• Power level (W to kW)
• Antenna
• Size ( wavelength)
• Types
• Polarization
• Modulator
• CW, AM, FM, SSB, TV, Digital

8
Dipole Antenna
9
Types of Modulation
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Radio Wave Propagation

• Diffraction versus Line-of-Sight
• Low Frequency / High Frequency
• Role of Ionosphere
• MUF (f 9n1/2, n in particles/m3)
• Day/Night Variation (Skip)
• Sunspots (11-year Cycle)
• Magnetic Storms
• Multihop Propagation
• Long-Delayed Echoes (?)

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Ionosphere
Ionosphere
12
Density versus Height
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Role of the Ionosphere
100 MHz
10 MHz
1 MHz
Earth
14
Annual Sunspot Numbers
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Radio Wave Reception

• Antenna
• Reciprocity Theorem
• Size (Practical Limits)
• Receiver
• Superhetrodyne
• IF, AGC, AFC
• Demodulator (Detector)
• Stereo (Subcarrier - sum/difference)
• Signal-to-Noise Ratio

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Superhetrodyne Receiver
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Demodulation
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Television

• 50-890 MHz (6 MHz Channels)
• FM Audio, SSB Video
• 525 Lines _at_ 30 Frames / second
• Interlacing (Alternating Lines)
• Synch Pulses
• Luminance / Chrominance
• High-Definition TV (HDTV)
• Cable versus On-Air

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Radar

• Pulsed (Air Traffic Control)
• Doppler (Police)
• Microwave Ovens (2.45 GHz / 12 cm wavelength)
• Weather Radar