This preview has intentionally blurred sections. Sign up to view the full version.
View Full DocumentThis preview has intentionally blurred sections. Sign up to view the full version.
View Full DocumentThis preview has intentionally blurred sections. Sign up to view the full version.
View Full Document
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 = = π η 240 2 2 2 i i E E S = = 2 2 int ~ 30 E R V S P A rad oc i e π = = For short dipole, l E V i oc ⋅ = ~ π λ 8 3 2 = e A Notice D=1.5 for short dipole, π λ 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 π = 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 π λ 4 2 D A e = rad P rec P...
View
Full
Document
This note was uploaded on 06/06/2010 for the course EE 161 taught by Professor Huffaker during the Spring '08 term at UCLA.
 Spring '08
 HUFFAKER
 Electromagnet

Click to edit the document details