ece 1100: introduction to electrical and computer engineering notes 10 antennas transmission and...

ECE 1100: Introduction toECE 1100: Introduction toElectrical and Computer EngineeringElectrical and Computer Engineering

Notes 10

AntennasTransmission and Reception of waves

Wanda WosikAssociate Professor, ECE Dept.

Spring 2011

Slides developed by Dr. Jackson

AntennaAntenna

An antenna is used to transmit or receive an electromagnetic signal.

Antenna

Ground

Inductor

Variable Capacitor

Diode (1N34A)

High-Impedance Earphone

Antenna

Ground

Inductor

Variable Capacitor

Diode (1N34A)

High-Impedance Earphone

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 AntennasExamples of Monopole Antennas

Transmitting Antenna: MonopoleTransmitting Antenna: Monopole

h

“monopole antenna”

Monopoles are often used for vertical polarization (e.g., AM)

x

i(x,t)

, cosi x t I x t

0= sinI x I k x h

The current on a monopole is given by:

where

Transmitting Antenna (cont.)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).

0= sinI x I k x h

0 0

20 sin sinI I kh I h

This follows from:

The sin term is maximum when

2

2h

4

h

k

Transmitting Antenna (cont.)Transmitting Antenna (cont.)

h

“monopole antenna”

x

i(x,t) x

I (x) top of antenna

base of antenna

h = / 4

, cosi x t I x t

Transmitting Antenna (cont.)Transmitting Antenna (cont.)

h

Example

Pick f = 1,270 kHz = 1.270 MHz (KFCC)

= c / f = 3.0 108 / (1.27 106)

= 236.06 meters

h 59.0 [m]

“monopole antenna”

So,

Receive Antenna: MonopoleReceive Antenna: Monopole

For a good receive antenna, h / 4

“monopole antenna”

h

earth

Ex

+

-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 AntennaDipole Antenna

A “dipole antenna” is often used for horizontal polarization (as in FM and TV)

h / 4

L = 2h / 2

hh

L

feed (to receiver)

Example: Pick f = 99.1 MHz (Sunny 99.1 FM)

= c/f = 3.03 mL= 1.51 m

Transmission and Reception of Waves

h

earth

+-

v(t)

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

Signal ModulationSignal Modulation

Amplitude modulation

Frequency modulation

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

13http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html

Tuning for Signal Detection

Power In Electromagnetic WavePower In Electromagnetic Wave

0x

y

E

H 0

00

376.73

“intrinsic impedance of free space”

x

z

Ex [V/m]

Hy [A/m]power flow

y

Power In Electromagnetic Wave (cont.)Power In Electromagnetic Wave (cont.)

22

0

= Power Density W/m2d

AP

, cosxE t z A t kz x

z

Ex

Hy

power flowy

ExampleExample

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

Example (cont.)Example (cont.)

2

3

22 3

Power= Power Density W/m

Area

50 10 WPower

0.5 4 0.5 4 30 10

dP

r

6 28.84195 10 W/mdP

30 [km]observer

22

0

W/m2d

AP

2

68.84195 102 376.73

A

0.08162 V/mA

So

or

Also, please note that we do not have the amplitude value A. This can be calculated from power:

Example (cont.)Example (cont.)

Example (cont.)Example (cont.)

0.08162 V/mA

, cosxE t z A t kz

60 08162cos 7 979645 10 0 026617 [V/m]xE t,z . . t . z

62 7.979645 10 [rad/s]f 2 / 0.026617 [rad/m]k

From previous calculation:

Hence we have

30 [km] xE z,t

x

Receive VoltageReceive Voltage

h

earth

+-

v(t)

, effx Rv t E t z h

effh effective height of antenna

Receive antenna

wire is in x directionEx

Hy

z = zRz

Please the units: voltage [V], electric field E[V/m]

Receive Voltage (cont.)Receive Voltage (cont.)

h

earth

+-

v(t)

, effx Rv t E t z h

For a monopole antenna:

2

eff hh h

2/ 4effh h h

z = zR

Ex

Hy

ExampleExample

Assume that the incident electric field for the AM radio station KFCC has a magnitude of

0.08162 V/mA

Calculate the received voltage if the vertical antenna wire is 3 meters high.

2

eff hh h

3 m1.5 m

2effh

, , 1.5effx R x Rv t E t z h E t z

so

Example (cont.)Example (cont.)

60 08162cos 7 979645 10 0 026617x R RE t,z . . t . z

60 1224cos 7 979645 10 0 026617 [V]Rv t . . t . z

This is the received voltage at the terminals of the monopole antenna.

So we have:

(from previous calculation of and k)