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Unformatted text preview: EE161 Electromagnetic Waves Spring, 2010 Prof. Y. Ethan Wang Electrical Engineering Dept. UCLA Lesson 17 Antenna Arrays Arrays with Uniform Excitation Electronic Scanning Effective Area of A Receiving Antenna Effective area is defined as the ratio of the total intercepted power to the incident power density, int P i S i e S P A int = Consider the receiving antenna as a source with certain internal impedance, therefore, in in in s jX R Z Z + = = (the source impedance for a receiving antenna is the load impedance when it is used as a transmitting antenna) The load impedance has to match to the source impedance to obtain the maximum power delivery, thus we must have * in L Z Z = Effective Area of A Receiving Antenna From the transmission line theory, it gives rad in L R R R = in L X X = (ignore the loss) Therefore, the total power delivered to the load is given by, rad oc rad rad oc rad L L R V R R V R I P 8 ~ 2 ~ 2 1 ~ 2 1 2 2 2 = = = is the measured voltage at the antenna terminal when the load is open circuit, oc V ~ When the impedance is matched, rad oc L R V P P 8 ~ 2 int = = 240 2 2 2 i i E E S = = 2 2 int ~ 30 E R V S P A rad oc i e = = For short dipole, l E V i oc = ~ 8 3 2 = e A Notice D=1.5 for short dipole, 4 2 D A e = The incident power density (for any antenna) rad oc L R V I 2 ~ ~ = Friss Transmission Formula int P r S t P For an isotropic lossless antenna, the radiated power density is, 2 4 R P S t iso = Transmitted power to the antenna For a directive antenna, we thus have the radiated power density given, iso t r S G S = gain of the transmitting antenna Friss transmission formula defines the power relationship between the transmitting antenna and the receiving antenna in a wireless link 4 2 D A e = rad P rec P...
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 Spring '08
 HUFFAKER
 Electromagnet

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