Chapter 6 Antenna

1
Chapter 6 Antenna

• Read Chapter 10 Antennas and Waveguides
• Topics of discussion
• 1. Introduction
• 1. Radiation Pattern,Induction Radiation Field
• 2. Polarization
• 3. Gain
• 4. Types of antenna

2
Introduction

• Basic Concepts ( Read P.371-386)
• Review question 10-1
• Radiation Resistance Antenna Efficiency)
• Review question 10-7
• Isotropic Radiator
• Review question 10-9

3
Basic concepts What is an antenna ?

• An antenna is a metallic conductor system capable
  of radiating and receiving electromagnetic
  waves.
• Ordinary transmission line has low radiation
  efficiency.
• See Figure 10-1 (a), (b)
• Spreading the conductors increases radiation
  efficiency
• See Figure 10-1(c), (d)

4
Role of Antenna

• Role of Antenna
• Coupling energy from a radio transmitter to a
  radio receiver through free space
• It is a passive reciprocal device
• passive no amplification of signal involved
• reciprocal the transmit and receive
  characteristics of an antenna are identical

5
Radiation Resistance

• Radiation resistance Rr
• a fictitious resistance ,when used to replace an
  antenna, would dissipate the same amount of power
  the antenna radiates.
• Mathematically Rr P/ i2
• where P power radiated by antenna
• i antenna feedpoint current
• E.g, If radiated power 10W, antenna current
  0.447A, find the radiation resistance.
• Sol Rr 10/(0.447)2 50 ohms

6
Antenna Efficiency

• Antenna Efficiency(?)
• antenna efficiency(?) is defined as ? Pr/(Pr
  Pd)
• where Pr power radiated by antenna
• Pd power dissipated in antenna
• Alternatively
• ? Rr/( Rr Re )
• where Rr radiation resistance
• Re effective antenna resistance

7
Isotropic Radiator

• Isotropic source/radiator
• An isotropic radiator is one which would radiate
  equally well in all direction
• true isotropic radiator can not be found in
  practical antenna.
• Isotropic radiator ( see Fig.9.3 )
• can be approximated by an omnidirectional antenna
  .

8
Radiation PatternInduction Radiation Field

• Chapter 10 of textbook (Antenna/Radiation
  Pattern)
• Read P.373
• Review questions 10-3.
• Chapter 10 of textbook (Antenna/Near and Far
  Fields)
• Read P.375
• Review questions 10-6.

9
Radiation Pattern

• The radiation pattern of an antenna
• is a polar diagram or graph representing field
  strengths or power density at various angular
  positions relative to an antenna.
• Absolute Radiation Pattern
• is plotted in terms of electric field
  strength/power density.
• see Figure 10-4(a).

10
Relative Radio Pattern

• Relative radiation pattern
• The field strength or power density is plotted
  with respect to a the value at a reference point
• see Fig.10-4(a),(b),(c) of textbook
• For example
• In Fig.10-4(a) the maximum radiation is 5uW/m2 at
  reference point and minimum radiation of 1uW/m2
  is found on position at the opposite side.

11
Induction Radiation Fields

• Induction Field( Near Field )
• is the field pattern close to the antenna
• energy is returned to antenna in the second half
  cycle of excitation similar to inductor that
  stores releases energy
• Radiation Field( Far Fields )
• the field pattern at great distance
• power radiated outward and never returned to
  antenna
• antenna radiation patterns are quoted in
  radiation field

12
Quantification of Near Field

• The near field is defined as the region area
  within a distance R given below
• R D2/?
• where D antenna diameter
• ? wavelength
• note antenna diameter and wavelength are in
  the same dimension unit

13
Polarization Gain

• Chapter 10 of textbook(Antenna Polarization)
• Read P.377-383
• Review question 10-11
• Chapter 10 of textbook(Directive/Power Gain)
• Review question 10-8
• Review problems 10-1 through 10-14.

14
Antenna Polarization

• Polarization
• is referred to the orientation of the electric
  field radiated from an antenna.
• Vertically/Horizontally Polarized Antenna
• the antenna radiates a vertically/horizontally
  polarized electromagnetic wave
• Linearly polarized Antenna
• the antenna lies either in horizontal or vertical
  plane
• see Fig.10-6(a)

15
Elliptical Circular Polarization

• Elliptical Polarization
• the radiated field rotates in elliptical manner
• see fig. 10-6(b) on P.365 of textbook
• Circular Polarization
• the radiated field rotates in circular manner
• see fig. 10-6(c).

16
Gain of Antenna

• Directive Gain (Dirgain)
• is referred to the ratio of the power density
  radiated in a particular direction to the power
  density radiated to the same point by a reference
  antenna(isotropic antenna),assuming both antenna
  radiate the same power level.
• Dirgain P/Pref
• Where P power density at some point with a
  given antenna (W/m2)
• and Pref power density at the same point
  with a reference
• antenna(W/m2)

17
Power Gain ( Pgain)

• Power Gain of an Antenna ( Pgain ) is defined as
• Pgain Dirgain x ?
• where ? antenna efficiency
• In terms of dB, power gain is expressed as
• Pgain 10 log (Dirgain x ?) /Pref
• For example , if the reference is isotropic , the
  directive gain of a dipole is 1.64 or
• Pgain 10 log 1.64 2.15 dBi, assuming ?
  100
• note dBi means the reference is an isotropic
  source

18
Effective Isotropic Radiated Power

• Effective Isotropic Radiated Power(EIRP/ERP)
• is defined as equivalent transmit power
• Mathematically EIRP Pr x At
• where Pr total radiated power
• At transmitted antenna directive
  gain
• Alternatively EIRP Pin x Pgain
• where Pin antenna input power
• Pgain power gain

19
Power density at a given point

• Power density Pden at a given point R
• Pden EIRP/(4pR2)
• where R distance from the radiating source
• Illustrated Example
• An antenna has a directive gain 8 and radiated
  power 10W
• EIRP 8 x 10W 80W
• Power density at a point 20 km away EIRP/(4pR2)
  
• 80 /4p(200002) 1.59 x
  10-8W/m2 0.159 uW/m2
• If the antenna is replaced by an isotropic
  source
• Power density 10/4p(200002) 0.019 uW/m2

20
Types of Antenna

• Antenna types
• Dipole (Review question10-16)
• Grounded Effect on half-wave dipole(Review
  question 10-17)
• Rhombic Antenna (Review question on 10-26)
• Yagi Antenna (Review question on 10-28)
• Log-Periodic Antenna (Review question on 10-29)
• Loop (review question 10-30)
• Parabolic Reflector Antenna (Review questions on
  10-34,10-35)

21
Half Wave Dipole(Hertz antenna)

• A dipole
• is a resonant antenna and multiple of ?/4 long
• standing waves of voltage and current exist along
  a resonant antenna
• see Fig. 10-9 ( voltage current distribution )
  on P.367
• see Fig. 10-10 ( impedance curve ) on P.367
• Simple structure of a horizontal dipole

22
Features of Dipole Antenna

• Voltage maximum current minimum at both ends
• Voltage minimum current maximum at feedpoint
• Feedpoint impedance 73 ohms
• Feedpoint impedance at both ends 2500 ohms
• Directive Gain for isotropic radiator 1.64
• Power Gain 2.15dBi
• Half-wave dipole radiation patterns
• see Fig.10-11(a), 10-11(b),10-11(c) of textbook

23
Grounded effects on a ?/2 dipole

• When a antenna is mounted a number of wavelength
  h above the ground
• the overall effect at any point in space is the
  sum of the direct and ground-effected waves
• see Fig. 10-12 on P.368 of text book
• The ground-reflected wave appears to be radiating
  from an image antenna distance h below earth
  surface

24
Grounded effect on a ?/2 dipole

• When a half-wave dipole is ?/4 above the ground
• the upward field strength is doubled ,
• the lower lobe is vanished ( Fig.10-13(a),(b))

25
Grounded effect on a ?/2 dipole

• When a half-wave dipole is ?/2 above the ground
• The maximum radiation is 30 degree to the
  horizontal and the radiation pattern is in two
  major lobes
• see Fig. 10-13 (c)

26
Rhombic Antenna

• Structure and construction
• 4 wires (each of several wavelength long,e.g ,4?)
  are connected in rhombic shape and terminated by
  a resistor.
• mounted horizontally and placed gt ?/2 above
  ground
• see Fig. 10-22 (a)
• Directional property ( see Fig. 10-22(a),
  Fig.10-22(b) )
• the fields in the X right direction is additive
• fields in other directions are self cancelled
  out.

27
Features of Rhombic Antenna

• Features
• highly directional( see Fig. 10-22(b) radiation
  pattern )
• maximum efficiency 67 and impedance 600800 ohms
• gain of 40 (16dB ) can be achieved
• suitable for the frequency range 3MHz to 30MHz
• wide frequency range e.g if ? is set for 7MHz,
  since each wire has length 4?, operating
  frequency range can be 7MHz to 14MHz
• Disadvantages large site area relatively
  large side lobes

28
Yagi antenna

• Structure ( See Fig. 10-24(a))
• a linear array consisting of a driving elements
  and
• two or more parasite elements one reflector
  one or more directors
• Driving element
• a half-wavelength dipole
• Reflector
• an aluminum rod of 5 longer than the dipole
• Director
• cut 5 shorter than the dipole

29
Features of Yagi antenna

• Directive
• see Fig. 10-24b showing the radiation pattern
• typical 7 dB - 9 dB
• Wide bandwidth can be configured
• the bandwidth can be increased by using more than
  one folded dipole, each cut to a slightly
  different length
• e.g. VHF TV reception band from 54MHz to 216MHz

30
Log-Periodic Antenna(Structure)

• Structure (see Fig.10-26)
• several dipoles of different length and spacing
  are fed from a single driving source
• Adjacent pair of dipoles are driven by current of
  equal amplitude but with phase difference in 180o
• crisscrossing the transmission line between the
  feedpoints of adjacent pairs of dipoles

31
Length of dipoles spacing adjacent dipoles in
Log-Periodic Antenna

• Lengths of dipoles and spacing between adjacent
  dipoles are related to a common ratio a
• See Fig. 10-26 on P.377
• R2/R1R3/R2R4/R3 L2/L2L3/L2L4/L3 1/a
• where R dipole spacing
• L dipole length
• a design ratio which is less than 1
  
• It consists of a basic geometrical pattern that
  repeats except with a different size pattern.

32
Essential features of Log-Periodic

• Major advantage
• independence of radiation resistance and
  radiation pattern to frequency
• Large bandwidth ratio
• highest to the lowest operating frequency of 101
  or greater
• Gain
• unidirectional/bidirectional and low-to-moderate
• higher gain may be achieved by adding more arrays

33
Essential features of Log-Periodic

• Radiation pattern (see fig. 10-26)
• maximum radiation is outward from the small end
• Antenna input impedance( see fig. 10-27)
• varies periodically the log of frequency
• Often used in HF VHF communication

34
Loop Antenna

• Structure (see fig. 10-28)
• a single-turn coil of wire for radius lt ?
• Typical loop antennas A square loop, B
  circular loop
  

35
Features of Loop Antenna

• Radiation resistance Rr for single turn
• Rr (31200 x Area2 )/?
• where A is the area of the loop
• for coil of n turns , the radiation resistance
  n x Rr
• Wide range of operating frequency

36
Loop Antenna application

• Used in radio direction finding (RDF)
• tune the antenna until null in the signal is
  obtained, the transmitting station is at right
  angles to the plane of the loop (See radiation
  pattern below)
• Null Loop Antenna

37
Parabolic Reflector Antenna

• Structure ( see Fig. 10-34(a),(b) Fig. 10-35)
• parabolic reflector and the feed mechanism
• The feed mechanism
• house the the primary antenna(dipole/dipole
  array) which radiates EM waves toward the
  reflector
• The reflector
• reflects the energy into a concentrated, highly
  directional emission eave

38
Features of a parabolic antenna

• Extreme high gain and directivity
• used in µ-wave and satellite link
• Beam width (T )
• T 70?/D
• where ? wavelength
• D antenna mouth diameter
• T beam width of half power point (
  degrees )

39
Power gain of Parabolic Antenna

• Power gain for a transmit parabolic antenna
  (Pgain)
• Pgain ?(pD/?)2
• where Pgain power gain referring to isotropic
  antenna
• D mouth diameter of parabolic
  reflector(meters)
• ? wavelength(meters)
• ? antenna efficiency (typical 55)
• Power gain in terms of dBi (assume ? 55)
• Pgain (db) 20 log f(MHz) 20 log D(m)
  42.2

40
Illustrated Example

• A 2-m diameter parabolic reflector with 5W of
  power radiated by the feed mechanism operating at
  5GHz with transmit efficiency of 55
• Beam Width T
• T 70?/D 70(3 x 108)/(5 x 109)(2) 2.1o
• note how narrow the beam is !
• Transmit power gain
• Pgain(dB) 20 log 5000 20 log 2 - 42.2
  37.78 dBi