收音机介绍概要.ppt

Radio Communications J. C. Sprott Department of Physics University of Wisconsin - Madison Presented to Physics 208 on April 6, 2006 Outline Properties of EM Waves Choice of Frequencies Radio Wave Production Radio Wave Propagation Radio Wave Reception Television Radar Properties of EM Waves Transverse Propagates in Vacuum Any Frequency Possible Frequency X Wavelength = c c = 3x108 m/s (in vacuum) Transverse EM Wave 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 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 Dipole Antenna Types of Modulation 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 (?) Role of the Ionosphere Annual Sunspot Numbers Radio Wave Reception Antenna Reciprocity Theorem Size (Practical Limits) Receiver Superhetrodyne IF, AGC, AFC Demodulator (Detector) Stereo (Subcarrier - sum/difference) Signal-to-Noise Ratio Superhetrodyne Receiver Demodulation Television 50-890 MHz (6 MHz Channels) FM Audio, SSB Video 525 Lines @ 30 Frames / second Interlacing (Alternating Lines) Synch Pulses Luminance / Chrominance High-Definition TV (HDTV) Cable versus On-Air Radar Pulsed (Air Traffic Control) Doppler (Police) Microwave Ovens (2.45 GHz / 12 cm wavelength) Weather Radar * * /lectures.htm E B The Electromagnetic Spectrum Ionosphere Ionosphere Density versus Height 100 MHz 1 MHz 10 MHz Earth * * *