ECE 1100: Introduction to Electrical and Computer Engineering David R. Jackson Professor, ECE Dept. Notes 12 Antennas Spring 2008

Antenna An antenna is used to transmit or receive an electromagnetic signal. A “monopole” antenna is a wire antenna that uses ground as the other terminal.

AM tower Monopoles on vehicle Cell phone antenna Examples of Monopole Antennas

Transmitting Antenna: Monopole h “monopole antenna” Monopoles are often used for vertical polarization (e.g., AM) x i(x,t) The current on a monopole is given by: where

Transmitting Antenna (cont.) h For a good antenna, h  / 4 “monopole antenna” x i(x,t) This will maximize the current at the base of the antenna I ( 0 ). This follows from: The sin term is maximum when

Transmitting Antenna (cont.) h “monopole antenna” x i(x,t) x I (x)I (x) top of antenna base of antenna h = / 4

Transmitting Antenna (cont.) h Example Pick f = 1,270 kHz = MHz (KFCC)  = c / f = 3.0  10 8 / (1.27  10 6 ) = meters h  59.0 [m] “monopole antenna” So,

Receive Antenna: Monopole For a good receive antenna, h  / 4 “monopole antenna” h earth ExEx + - v(t)v(t) It may be difficult to make the receive antenna this tall – in this case the taller the better! (The best height for transmitting is also the best height for receiving.)

Dipole Antenna A “dipole antenna” is often used for horizontal polarization (as in FM and TV) h  / 4 L = 2h  / 2 h h L feed (to receiver) Example: Pick f = 99.1 MHz (Sunny 99.1 FM) = c/f = 3.03 m L= 1.51 m