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Unformatted text preview: Radio Waves
Electromagnetic Radiation Radio Transmission and Reception Modulation Techniques UCSD: Physics 8; 2007 Electromagnetism Electricity and magnetism are different facets of electromagnetism recall that a static distribution of charges produces an electric field charge in motion (an electrical current) produce a magnetic field a changing magnetic field produces an electric field, which wants to move charge Electric and Magnetic fields produce forces on charge An accelerating charge produces electromagnetic waves (radiation) Both electric and magnetic fields can transport energy Electric field energy used in electrical circuits & released in lightning Magnetic field carries energy through transformer 2 UCSD: Physics 8; 2007 Electromagnetic Radiation Transverse electric and magnetic waves traveling through space Electric and magnetic fields are self sustaining Can travel for ever without loosing energy All electromagnetic radiation travels at c = 3 10 m/s in 3 UCSD: Physics 8; 2007 Examples of Electromagnetic Radiation AM and FM radio waves (including TV signals) Cell phone communication links Microwaves Infrared radiation Light X-rays Gamma rays What distinguishes these from one another? Electromagnetic waves are.. 4 Wavelength and Frequency UCSD: Physics 8; 2007 Color of light is related to.. 5 UCSD: Physics 8; 2007 The Electromagnetic Spectrum Relationship between frequency, speed and wavelength f = c f is frequency, is wavelength, c is speed of light Energy of photon = hf, h = Planck's constant Sunlight carries energy from sun to Earth Different frequencies of electromagnetic radiation are Made in different ways Travel differently through media (air, glass, metal) Are detected in different ways Have different uses in technology If you know the frequency and want to know the wavelength.. 6 UCSD: Physics 8; 2007 Generation of Radio Waves Accelerating charges radiate EM energy If charges oscillate back and forth, get time-varying fields The electric field arrow tells ..
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If charges oscillate back and forth, get time-varying magnetic fields too. Note that the magnetic fields are perpendicular to the electric field vectors Right hand rule: current flows up along right hand thumb, fingers curl to show the magnetic field. + + + - - - - - - + + + + - B - + Charges make em waves when they are..
8 UCSD: Physics 8; 2007 Polarization of Radio Waves Transmitting antenna
B E Demonstrate microwave polarization
9 UCSD: Physics 8; 2007 Reception of Radio Waves
E Receiving antenna works best when `tuned' to the wavelength of the signal, and has proper polarization Electrons in antenna are "jiggled" by passage of electromagnetic wave Optimum antenna length is /4: one-quarter wavelength. When longer, more electrons moving up when other moving down
10 B UCSD: Physics 8; 2007 Encoding Information on Radio Waves What quantities characterize a radio wave? Two common ways to carry analog information with radio waves Amplitude Modulation (AM) Frequency Modulation (FM): "static free" What do em waves carry? 11 UCSD: Physics 8; 2007 AM Radio Amplitude Modulation (AM) uses changes in the signal strength to convey information pressure modulation (sound) electromagnetic wave modulation 12 UCSD: Physics 8; 2007 AM Radio in Practice Uses frequency range from 530 kHz to 1700 kHz each station uses 9 kHz spacing is 10 kHz (only small gap) 117 channels 9 kHz of bandwidth means 4.5 kHz is highest audio frequency that can be encoded Far less than 20 kHz capability of human ear Previous diagram is exaggerated: audio signal changes slowly with respect to radio carrier typical speech sound of 500 Hz varies 1000 times slower than carrier thus will see 1000 cycles of carrier to every one cycle of audio Demonstrate AM transmitter
13 UCSD: Physics 8; 2007 FM Radio Frequency Modulation (FM) uses changes in the wave's frequency to convey information pressure modulation (sound) electromagnetic wave modulation 14 FM Radio in Practice UCSD: Physics 8; 2007 Spans 87.8 MHz to 108.0 MHz in 200 kHz intervals 101 possible stations example: 91X runs from 91.091.2 MHz (centered at 91.1) Nominally uses 150 kHz around center 75 kHz on each side 30 kHz for L + R (mono) 15 kHz audio capability 30 kHz offset for stereo difference signal (L - R) Again: figure exaggerated 75 kHz from band center, modulation is > 1000 times slower than carrier, so many cycles go by before frequency noticeably changes 15 UCSD: Physics 8; 2007 AM vs. FM FM is not inherently higher frequency than AM these are just choices aviation band is 108136 MHz uses AM technique Besides the greater bandwidth (leading to stereo and higher audio frequencies), FM is superior in immunity to environmental influences there are lots of ways to mess with an EM-wave's amplitude pass under a bridge re-orient the antenna natural processes do not change the frequency What is the wavelength of 91.1 Mhz.. 16 UCSD: Physics 8; 2007 US Frequency Allocation in MHz 17 UCSD: Physics 8; 2007 Converting back to sound: AM AM is easy: just pass the AC signal from the antenna into a diode or better yet, a diode bridge then use capacitor to smooth out bumps but not so much as to smooth out audio bumps radio signal B D amplifier/ speaker 18 UCSD: Physics 8; 2007 Converting back to sound: FM More sophisticated need to compare instantaneous frequency to that of a reference source then produce a voltage proportional to the difference Compute L = [(L+R) + (L-R)]/2; R = [(L+R) - (L-R)]/2 amplify the L and R voltages to send to speakers Amplification is common to both schemes intrinsic signal is far too weak to drive speaker What electrical component do we use to amplify a signal? What electrical components do we need to convert AM to a voltage
that can power a speaker? 19 ...
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