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Telecommunications Networking IWireless Point-to-Point LinkThe Electromagnetic SpectrumSlide 4Transmitter SubsystemsSlide 6The AntennaSlide 8A Directional RadiatorThe Parabolic Dish AntennaSlide 11The Receiver SubsystemSources of Thermal NoiseSlide 14The Receiver SubsystemThe End-To-End SystemThe End-to-End SystemSlide 18Calculating the Propagation LossSlide 20Why Do Broadcast Transmitters Have Such High Power?Copyright 1999, S.D. Personick. All Rights Reserved.Telecommunications Networking ILectures 14 & 15Wireless Transmission SystemsCopyright 1999, S.D. Personick. All Rights Reserved.Wireless Point-to-Point LinkRadioTransmitterRadioReceiverFeed Line (e.g., coaxial cable)AntennaCopyright 1999, S.D. Personick. All Rights Reserved.The Electromagnetic Spectrum•30-300Hz: SLF 3GHz-30GHz: SHF (DBS)•300Hz - 3kHz ULF 30GHz-300GHz: EHF•3kHz - 30kHz: VLF 300,000GHz: 1um light•30kHz-300kHz: LF•300kHz-3MHz: MF (AM Radio)•3MHz-30MHz: HF (Short Wave Radio)•30MHz-300MHz: VHF (FM, TV)•300MHz-3GHz: UHF (TV, Digital Cordless, …)Copyright 1999, S.D. Personick. All Rights Reserved.Wireless Transmitter Modulator MixerRF OscillatorAntennaRFAmpInformationIF OscillatorCopyright 1999, S.D. Personick. All Rights Reserved.Transmitter Subsystems•1 milliwatt -100 milliwatts: low r.f. exposure with hand held appliances, low battery drain (some hand held appliances radiate ~5 watts…but I wouldn’t hold one of these near my head!)•10 watts - 100 watts: okay for consumer and small business applications, with 115 volt or 12 volt automobile power. Stay 10-30 feet from the antenna. •5000 watts-50,000 watts : broadcast applicationsCopyright 1999, S.D. Personick. All Rights Reserved.Transmitter SubsystemsR.F. AmplifierCoaxAntennaFETR.F. Power = (V**2)/ZV~ 1 voltZ~50 ohmsR.F. Power (launched into the coaxial cable) = 1/50 Watt = 20 mWR.F. Power radiated = ?Copyright 1999, S.D. Personick. All Rights Reserved.The AntennaForward PowerReflected PowerRadiated PowerCopyright 1999, S.D. Personick. All Rights Reserved.The Antenna•If the antenna is much smaller in length than the wavelength of the radiation (c/f), then the antenna will be a very inefficient radiator (most of the forward power is reflected)•If the antenna is 1/4 wavelength or larger in size, it can be an efficient radiator•If the antenna is significantly larger in size than 1 wavelength, it can be an efficient and directional radiatorCopyright 1999, S.D. Personick. All Rights Reserved.A Directional Radiator1/4 wavelength spacing between two dipoles. 3/4 wavelength delay in crossover cableForward: fields addReverse: fields cancelIt’s actually not quite that simple: dipole interactionsCopyright 1999, S.D. Personick. All Rights Reserved.The Parabolic Dish AntennaDivergence angle~Lambda/DCopyright 1999, S.D. Personick. All Rights Reserved.Wireless Receiver Demodulator MixerRF OscillatorAntennaRFAmpInformationIF OscillatorCopyright 1999, S.D. Personick. All Rights Reserved.The Receiver SubsystemLow Noise AmplifierSignal + Background Noise + InterferenceCable attenuation + thermal noise + Amplifier noiseCopyright 1999, S.D. Personick. All Rights Reserved.Sources of Thermal NoiseHot object (333K)Background (27K)SignalField of ViewMirrorSunCopyright 1999, S.D. Personick. All Rights Reserved.The Receiver SubsystemExample:Equivalent background temperature: T = 100K (Kelvins)Equivalent background noise = kTB (watts)Coupling loss of antenna into cable ~ 0dBCable loss = 3 dBCable temperature = 293KPreamplifier Noise Temperature = 30KEquivalent amplifier total input noise = ???Copyright 1999, S.D. Personick. All Rights Reserved.The Receiver Subsystem Example (continued):Equivalent input noise = kB [100/2 + 293/2 + 30]=226.5kBI.e., half of the background noise + half of the cable noise + the preamplifier noiseCopyright 1999, S.D. Personick. All Rights Reserved.The End-To-End SystemExample (continued from prior example):Assume that the transmitter power amplifier produces 20 milliwatts. The coupling loss from the transmitter into its coaxial antenna feed cable is 0 dB. The transmitter antenna feed cable has 1.5 dB of loss. The antenna radiates 90% of the power that arrives from the transmitter antenna feed cable, and reflects the rest back into the cable. How much power is radiated? Assume that the bandwidth, B = 6 MHz. How much total propagation loss can we allow if the required signal-to-noise ratio is 40 dB?Copyright 1999, S.D. Personick. All Rights Reserved.The End-to-End SystemExample: (continued)The preamplifier produces 20 mW = +13 dBmThe loss of the cable, plus the impact of a 10% reflection (90% radiation) is:1.5 dB (cable loss) - 10 log (0.9) = 1.95 dBThe total radiated power = +11.05 dBm = 12.7 mWThe required power at the preamplifier input = 226.5 x k x 6 x 10**6 x 10**4 (watts)... I.e., 40 dB larger than the noiseCopyright 1999, S.D. Personick. All Rights Reserved.The End-to-End System•The radiated power = 12.7 mW •The required power at the receiver preamplifier input = 1.85 x 10**-7 mW•The required power at the receiver antenna is 2 x 1.85 x 10**-7 mW = 3.7 x 10**-7 mW•The allowed propagation loss is the ratio of these ~3.4 x 10**7 ~ 75.3 dBCopyright 1999, S.D. Personick. All Rights Reserved.Calculating the Propagation LossArea of surface= 4 r**2Antenna equivalentarea = ACopyright 1999, S.D. Personick. All Rights Reserved.Calculating the Propagation LossSuppose the frequency is 100 MHz, and the wavelength = 3 meters. Assume that the equivalent area of the antenna is 2.25 square meters. If the allowable loss is 75.3 dB, then the distance from the transmitter to the receiver, r , can be derived from:2.25/[4 r**2) > 1/[3.4 x 10**7]; r < 2.5 km (Line-Of-Sight)Copyright 1999, S.D. Personick. All Rights Reserved.Why Do Broadcast Transmitters Have Such High Power?In the previous example, we could cover 2.5 km with 20 mW. If we want to cover a 100 km radius, we would need 40**2 or 1600 times the power. That would correspond to 32 watts << 10 kWBut, what about: high antenna-feeder-cable losses, splitters, noisy preamplifiers, inadequate antennas (low effective area, and poor matching to the feeder cable), attenuation through buildings, and


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DREXEL ECES 490 - Lec14&amp;15

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