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Lec29Noise - ECE 488 RF Circuits and Systems Spring 2009...

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1 Signal Strength Usable Signal ECE 488 RF Circuits and Systems Spring 2009 Lecture 29 2009/4/8 ECE 488 – RF Circuits and Systems – Spring 2009 Swartz 08/4/2 Lecture 29: 1 Quantifying Noise Noise Figures Transmitter Hypothetical isotropic radiation from an antenna: Spherically symmetric radiation from center. Total radiated power stays the same, but Power per unit area falls off as r -2 because the area of the sphere increases as r 2 Not possible! Why does a Radio Signal Weaken? ECE 488 – RF Circuits and Systems – Spring 2009 Swartz 08/4/2 Lecture 29: 4 But starting concept for antenna gain. If an antenna does not radiate isotropically, Then more power goes in some directions, and Less power goes in other directions: Antenna Gain Concepts Beam Width ECE 488 – RF Circuits and Systems – Spring 2009 Swartz 08/4/2 Lecture 29: 5 The more the power is concentrated, The higher the antenna “gain” The narrower the “beam” Transmitter Consider a path between a transmitting antenna and a receiving antenna: How Strong is a Radio Signal? Distance = r Tx ECE 488 – RF Circuits and Systems – Spring 2009 Swartz 08/4/2 Lecture 29: 6 The Poynting flux (power per unit area) at the receiving antenna is H E r G P S t t r × = = 2 4 π Consider the area of the receiving antenna that intercepts the signal: How Strong is a Recieved Radio Signal? Distance ECE 488 – RF Circuits and Systems – Spring 2009 Swartz 08/4/2 Lecture 29: 7 2 4 r A G P A S P effective t t effective r r π = = The Poynting flux intercepted by the area of the antenna is the signal power received: Effective area is related to gain: π λ 4 2 r effective G A = How big is the target? Radar Basics r We need a measure of the “size” of targets ECE 488 – RF Circuits and Systems – Spring 2009 Swartz 08/4/2 Lecture 29: 8 σ π 2 4 r G P P t t scattered = We need a measure of the size of targets. The radar scattering cross section, σ Used to determine the scattered power in terms of the power incident on the target.
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2 How big is the target? Radar Cross Sections r ECE 488 – RF Circuits and Systems – Spring 2009 Swartz 08/4/2 Lecture 29: 9 σ π 2 4 r G P P t t scattered = The cross section, σ Is defined as if the power were scatter equally in all directions (isotropically). σ is measured in m 2 (area), but may not correspond to actual physical area shape does matter How much power gets back? Backscatter Radar Cross Sections r G P t t ECE 488 – RF Circuits and Systems – Spring 2009 Swartz 08/4/2 Lecture 29:10 ( ) σ π 2 2 4 r P scattered back = Power falls off in both directions Falls off going to target Power is scattered Falls off going back to radar antenna How much power is received?
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