102L4prop - Chapter 4 Section A Physical Principles of...

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October, 1997 RF Engineering 102 v1.0 (c)1997 Scott Baxter 4 -1 Physical Principles of Propagation Physical Principles of Propagation Chapter 4 Section A
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October, 1997 RF Engineering 102 v1.0 (c)1997 Scott Baxter 4 -2 Introduction to Propagation n Propagation is a key process within every radio link. During propagation, many processes act on the radio signal. attenuation – the signal amplitude is reduced by various natural mechanisms; if there is too much attenuation, the signal will fall below the reliable detection threshold at the receiver. Attenuation is the most important single factor in propagation. multipath and group delay distortions – the signal diffracts and reflects off irregularly shaped objects, producing a host of components which arrive in random timings and random RF phases at the receiver. This blurs pulses and also produces intermittent signal cancellation and reinforcement. These effects are combatted through a variety of special techniques time variability - signal strength and quality varies with time, often dramatically space variability - signal strength and quality varies with location and distance frequency variability - signal strength and quality differs on different frequencies n Effective masteryof propagation relies on Physics: understand the basic propagation processes Measurement: obtain data on propagation behavior in area of interest Statistics: characterize what is known, extrapolate to predict the unknown Modelmaking: formalize all the above into useful models
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October, 1997 RF Engineering 102 v1.0 (c)1997 Scott Baxter 4 -3 Propagation Effects of Earth’s Atmosphere n Earth’s unique atmosphere supports life (ours included) and also introduces many propagation effects -- some useful, some troublesome n Skywave Propagation: reflection from Ionized Layers LF and HF frequencies (below roughly 50 MHz.) are routinely reflected off layers of the upper atmosphere which become ionized by the sun this phenomena produces intermittent world- wide propagation and occasional total outages this phenomena is strongly correlated with frequency, day/night cycles, variations in earth’s magnetic field, 11-year sunspot cycle these effects are negligible for wireless systems at their much-higher frequencies
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October, 1997 RF Engineering 102 v1.0 (c)1997 Scott Baxter 4 -4 More Atmospheric Propagation Effects n Attenuation at Microwave Frequencies rain droplets can substantially attenuate RF signals whose wavelengths are comparable to, or smaller than, droplet size rain attenuations of 20 dB. or more per km. are possible troublesome mainly above 10 GHz., and in tropical areas must be considered in reliability calculations during path design not major factor in wireless systems propagation n Diffraction, Wave Bending, Ducting signals 50-2000 MHz. can be bent or reflected at boundaries of different air density or humidity phenomena: very sporadic unexpected long- distance propagation beyond the horizon. May last minutes or hours
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102L4prop - Chapter 4 Section A Physical Principles of...

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