1. ECE 1100: Introduction to Electrical and Computer Engineering Notes 10 Antennas Transmission and Reception of waves Wanda Wosik Associate Professor, ECE Dept. Spring 2011 Slides developed by Dr. Jackson

2. 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.

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

4. 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

5. 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 k

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

7. Transmitting Antenna (cont.) h Example Pick f = 1,270 kHz = 1.270 MHz (KFCC)  = c / f = 3.0  10 8 / (1.27  10 6 ) = 236.06 meters h  59.0 [m] “monopole antenna” So,

8. 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.)

9. 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

10. Transmission and Reception of Waves

11. 11 http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html Signal Modulation

12. Amplitude modulation Frequency modulation http://hyperphysics.phy- astr.gsu.edu/hbase/hframe.html

13. 13 http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html Tuning for Signal Detection

14. Power In Electromagnetic Wave “intrinsic impedance of free space” x z E x [V/m] H y [A/m] power flow y

15. Power In Electromagnetic Wave (cont.) x z ExEx HyHy power flow y

16. Example a) What is the power density 30 [km] away? b)What is the magnitude of the electric field 30 [km] away? c)Describe mathematically the electric field that an observer would see at this distance. 30 [km] The AM radio station KFCC (1270 KHz) transmits 50 [kW] of power equally in all directions above the earth (hemisphere pattern). observer

17. Example (cont.) 30 [km] observer

18. So or Also, please note that we do not have the amplitude value A. This can be calculated from power: Example (cont.)

19. From previous calculation: Hence we have 30 [km] x

20. Receive Voltage h earth + - v(t)v(t) Receive antenna wire is in x direction ExEx HyHy z = z R z Please the units: voltage [V], electric field E[V/m]

21. Receive Voltage (cont.) h earth + - v(t)v(t) For a monopole antenna: z = z R ExEx HyHy

22. Example Assume that the incident electric field for the AM radio station KFCC has a magnitude of Calculate the received voltage if the vertical antenna wire is 3 meters high. so

23. Example (cont.) This is the received voltage at the terminals of the monopole antenna. So we have: (from previous calculation of  and k )