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