Basic Antenna Theory and Concepts

1
Basic Antenna Theory and Concepts

• ICS 620 Communication Technologies
• Class 11

2
Introduction

• An antenna is an electrical conductor or system
  of conductors
• Transmission - radiates electromagnetic energy
  into space
• Reception - collects electromagnetic energy from
  space
• In two-way communication, the same antenna can be
  used for transmission and reception

3
Antenna Definition

• An antenna is a circuit element that provides a
  transition form a guided wave on a transmission
  line to a free space wave and it provides for the
  collection of electromagnetic energy.

Antenna research from Miller Beasley, 2002
4
Antenna Definition-contd

• In transmit systems the RF signal is generated,
  amplified, modulated and applied to the antenna
• In receive systems the antenna collects
  electromagnetic waves that are cutting through
  the antenna and induce alternating currents that
  are used by the receiver

5
Reciprocity

• An antenna ability to transfer energy form the
  atmosphere to its receiver with the same
  efficiency with which it transfers energy from
  the transmitter into the atmosphere
• Antenna characteristics are essentially the same
  regardless of whether an antenna is sending or
  receiving electromagnetic energy

6
Polarization

• Polarization is the direction of the electric
  field and is the same as the physical attitude of
  the antenna
• A vertical antenna will transmit a vertically
  polarized wave
• The receive and transmit antennas need to possess
  the same polarization

7
Types of Antennas

• Isotropic antenna (idealized)
• Radiates power equally in all directions
• Dipole antennas
• Half-wave dipole antenna (or Hertz antenna)
• Quarter-wave vertical antenna (or Marconi
  antenna)
• Parabolic Reflective Antenna

8
(No Transcript)
9
Directional Antenna
beamwidth

A
Max power
Radiated energy is focused in a specific direction
antenna
Power 3dB down from maximum point A
??2 dipole
10
Beamwidth

• Beamwidth is the angular separation of the
  half-power points of the radiated pattern

11
(No Transcript)
12
(No Transcript)
13
(No Transcript)
14
Half-wave Dipole (Hertz) Antenna

• An antenna having a physical length that is
  one-half wavelength of the applied frequency is
  called a Hertz antenna or a half-wave dipole
  antenna. Hertz antennas are not found at
  frequencies below 2MHz because of the physical
  size needed of the antenna to represent a
  half-wave

15
Vertical (Marconi) Antenna

• Vertical Antennas are used for frequencies under
  2 MHz. It uses a conducting path to ground that
  acts as ¼ wavelength portion the antenna above
  the ground. The above ground structure represents
  a ?/4 wavelength

16
Vertical (Marconi) Antenna contd

• Poor grounding conditions of the earth/soil
  surrounding the antenna can result in serious
  signal attenuation. This problem is alleviated by
  installing a counterpoise

17
Counterpoise

• Counterpoise is a grounding grid established
  where the earth grounding cannot satisfy
  electrical requirements for circuit completion.
  It is designed to be non-resonant at the
  operating frequency

18
Counterpoise-contd
radius ¼ ?
antenna
supports
19
Antenna Array

• Antenna array is a group of antennas or antenna
  elements arranged to provide the desired
  directional characteristics. Generally any
  combination of elements can form an array.
  However, equal elements in a regular geometry are
  usually used.

20
Yagi-Uda Antenna

• The Yagi-Uda antenna is a simple form of a
  directional antenna based off of a reflector
  placed ?/4 from the dipole antennas placement.
  Complex analysis to define the radiated patterns
  are experimental rather than theoretical
  calculations

21
Yagi-Uda Antenna-contd
reflector
?/2
?/4
dipole antenna
22
Radiated Directed Signal
antenna
??2 dipole radiated signal without reflector
??2 dipole radiated signal with reflector
23
The Antenna Formula
? ? c?? ? 186,000 mi?sec
frequency of the signal

• c is the speed of light
• is the wavelength of the signal
• use 3 x 108 when dealing in meters for the speed
  of light

24
The Antenna Formula - applied

• If a half-wave dipole antenna needed to be
  constructed for a 60 Hz signal, how large would
  it need to be?

? ? c?? ? 186,000 mi?sec
3100 mi
60
??2 1550 miles!
25
Radiation Induction Fields

• The mechanics launching radio frequencies from an
  antenna are not full understood. The RF fields
  that are created around the antenna have specific
  properties that affect the signals transmission.
  The radiated field field is known as the
  (surprisingly!) radiation field

26
Radiation Induction Fields-contd

• There are two induction fields or areas where
  signals collapse and radiate from the antenna.
  They are known as the near field and far field.
  The distance that antenna inductance has on the
  transmitted signal is directly proportional to
  antenna height and the dimensions of the wave

R ?
2D2
?
27
Radiation Induction Fields-contd
R ?
2D2
?
Where R the distance from the antenna D
dimension of the antenna ? wavelength of the
transmitted signal
28
Radiation Resistance

• Radiation Resistance is the portion of the
  antennas impedance that results in power
  radiated into space (i.e., the effective
  resistance that is related to the power radiated
  by the antenna. Radiation resistance varies with
  antenna length. Resistance increases as the ?
  increases

29
Effective Radiated Power (ERP)

• ERP is the power input value and the gain of the
  antenna multiplied together
• dBi isotropic radiator gain
• dBd dipole antenna gain

30
Radiation Pattern

• Radiation pattern is an indication of radiated
  field strength around the antenna. Power radiated
  from a ?/2 dipole occurs at right angles to the
  antenna with no power emitting from the ends of
  the antenna. Optimum signal strength occurs at
  right angles or 180 from opposite the antenna

31
Radiation Patterns

• Radiation pattern
• Graphical representation of radiation properties
  of an antenna
• Depicted as two-dimensional cross section
• Beam width (or half-power beam width)
• Measure of directivity of antenna
• Reception pattern
• Receiving antennas equivalent to radiation
  pattern

32
Radiation Pattern for Vertical Antennas
?/4
?/2
antenna
33
Antenna Gain

• Antenna gain
• Power output, in a particular direction, compared
  to that produced in any direction by a perfect
  omnidirectional antenna (isotropic antenna)
• Effective area
• Related to physical size and shape of antenna

34
Antenna Gain

• Antenna gain is the measure in dB how much more
  power an antenna will radiate in a certain
  direction with respect to that which would be
  radiated by a reference antenna

35
Antenna Gain

• Relationship between antenna gain and effective
  area
• G antenna gain
• Ae effective area
• f carrier frequency
• c speed of light ( 3 108 m/s)
• ? carrier wavelength

36
(No Transcript)
37
Propagation Modes

• Ground-wave propagation
• Sky-wave propagation
• Line-of-sight propagation

38
Ground Wave Propagation
39
Ground Wave Propagation

• Follows contour of the earth
• Can Propagate considerable distances
• Frequencies up to 2 MHz
• Example
• AM radio

40
Sky Wave Propagation
41
Sky Wave Propagation

• Signal reflected from ionized layer of atmosphere
  back down to earth
• Signal can travel a number of hops, back and
  forth between ionosphere and earths surface
• Reflection effect caused by refraction
• Examples
• Amateur radio
• CB radio

42
Line-of-Sight Propagation
43
Line-of-Sight Propagation

• Transmitting and receiving antennas must be
  within line of sight
• Satellite communication signal above 30 MHz not
  reflected by ionosphere
• Ground communication antennas within effective
  line of site due to refraction
• Refraction bending of microwaves by the
  atmosphere
• Velocity of electromagnetic wave is a function of
  the density of the medium
• When wave changes medium, speed changes
• Wave bends at the boundary between mediums

44
Line-of-Sight Equations

• Optical line of sight
• Effective, or radio, line of sight
• d distance between antenna and horizon (km)
• h antenna height (m)
• K adjustment factor to account for refraction,
  rule of thumb K 4/3

45
Line-of-Sight Equations

• Maximum distance between two antennas for LOS
  propagation
• h1 height of antenna one
• h2 height of antenna two

46
LOS Wireless Transmission Impairments

• Attenuation and attenuation distortion
• Free space loss
• Noise
• Atmospheric absorption
• Multipath
• Refraction
• Thermal noise

47
Thermal Noise

• Thermal noise due to agitation of electrons
• Present in all electronic devices and
  transmission media
• Cannot be eliminated
• Function of temperature
• Particularly significant for satellite
  communication

48
Noise Terminology

• Intermodulation noise occurs if signals with
  different frequencies share the same medium
• Interference caused by a signal produced at a
  frequency that is the sum or difference of
  original frequencies
• Crosstalk unwanted coupling between signal
  paths
• Impulse noise irregular pulses or noise spikes
• Short duration and of relatively high amplitude
• Caused by external electromagnetic disturbances,
  or faults and flaws in the communications system

49
Other Impairments

• Atmospheric absorption water vapor and oxygen
  contribute to attenuation
• Multipath obstacles reflect signals so that
  multiple copies with varying delays are received
• Refraction bending of radio waves as they
  propagate through the atmosphere

50
Multipath Propagation
51
Multipath Propagation

• Reflection - occurs when signal encounters a
  surface that is large relative to the wavelength
  of the signal
• Diffraction - occurs at the edge of an
  impenetrable body that is large compared to
  wavelength of radio wave
• Scattering occurs when incoming signal hits an
  object whose size in the order of the wavelength
  of the signal or less

52
The Effects of Multipath Propagation

• Multiple copies of a signal may arrive at
  different phases
• If phases add destructively, the signal level
  relative to noise declines, making detection more
  difficult
• Intersymbol interference (ISI)
• One or more delayed copies of a pulse may arrive
  at the same time as the primary pulse for a
  subsequent bit

53
Types of Fading

• Fast fading
• Slow fading
• Flat fading
• Selective fading
• Rayleigh fading
• Rician fading

54
Error Compensation Mechanisms

• Forward error correction
• Adaptive equalization
• Diversity techniques

55
Forward Error Correction

• Transmitter adds error-correcting code to data
  block
• Code is a function of the data bits
• Receiver calculates error-correcting code from
  incoming data bits
• If calculated code matches incoming code, no
  error occurred
• If error-correcting codes dont match, receiver
  attempts to determine bits in error and correct

56
Adaptive Equalization

• Can be applied to transmissions that carry analog
  or digital information
• Analog voice or video
• Digital data, digitized voice or video
• Used to combat intersymbol interference
• Involves gathering dispersed symbol energy back
  into its original time interval
• Techniques
• Lumped analog circuits
• Sophisticated digital signal processing algorithms

57
Antenna Height

• Antenna height above the ground is directly
  related to radiation resistance. Ground
  reflections causing out-of-phase signals to be
  radiated to receiving antennas will degrade the
  transmission. Physical length and electrical
  length of most antennas are approximately 95 of
  the physical length. Ideal antenna height is
  usually based on trial and error procedures

58
Smart Antennas
59
Smart Antennas

• smart antennas are base station antennas with a
  pattern that is not fixed, but adapts to the
  current radio conditions
• smart antennas have the possibility for a large
  increase in capacity an increase of three times
  for TDMA systems and five times for CDMA systems
  has been reported.

60
Smart Antennas-contd

• Major drawbacks and cost factors include
  increased transceiver complexity and more complex
  radio resource management

61
Smart Antennas-contd

• The idea of smart antennas is to use base station
  antenna patterns that are not fixed, but adapt to
  the current radio conditions. This can be
  visualized as the antenna directing a beam toward
  the communication partner only

62
(No Transcript)
63
Smart Antennas-contd

• Smart antennas add a new way of separating users,
  namely by space, through SDMA (space division
  multiple access)
• By maximizing the antenna gain in the desired
  direction and simultaneously placing minimal
  radiation pattern in the directions of the
  interferers, the quality of the communication
  link can be significantly improved

64
Elements of a Smart Antenna

• Smart antennas consists of a number of radiating
  elements, a combining/dividing network and a
  control unit

65
Phased Array Antenna

• Phased Array antennas are a combination of
  antennas in which there is a control of the phase
  and power of the signal applied at each antenna
  resulting in a wide variety of possible radiation
  patterns

66
Types of Intelligent Antennas

• Switched lobe (SL) This is also called switched
  beam. It is the simplest technique, and comprises
  only a basic switching function between separate
  directive antennas or predefined beams of an
  array. The setting that gives the best
  performance, usually in terms of received power,
  is chosen

67
Intelligent Antennas-contd

• Dynamically phased array (PA) By including a
  direction of arrival (DoA) algorithm for the
  signal received from the user, continuous
  tracking can be achieved and it can be viewed as
  a generalization of the switched lobe concept

68
Intelligent Antennas-contd

• Adaptive array (AA) In this case, a DoA
  algorithm for determining the direction toward
  interference sources (e.g., other users) is
  added. The radiation pattern can then be adjusted
  to null out the interferers. In addition, by
  using special algorithms and space diversity
  techniques, the radiation pattern can be adapted
  to receive multipath signals which can be
  combined. These techniques will maximize the
  signal to interference ratio (SIR)

69
(No Transcript)
70
SMDA

• Space Division Multiple Access (SDMA) implies
  that more than one user can be allocated to the
  same physical communications channel
  simultaneously in the same cell, only separated
  by angle. In a TDMA system, two users will be
  allocated to the same time slot and carrier
  frequency at the same time and in the same cell

71
SMDA-contd

• In systems providing full SDMA, there will be
  much more intracell handovers than in
  conventional TDMA or CDMA systems, and more
  monitoring by the network is necessary

72
Antenna Installation Considerations

• Safety
• standard operating procedure priority
• Grounding
• lightning strikes
• static charges
• Surge protection
• lightning searches for a second path to ground

73
Antenna Installation Considerations-contd

• Adaptive array antenna placement needs to be
  considered differently than current technologies
  serving the mobile environment. They need to be
  place so they have a greater angular approach to
  the receiving units. Existing tower placement
  with close proximity to roads and highways would
  need to be reconsidered.

74
(No Transcript)
75
Antenna Installation Considerations

• Base, mast, and supporting structure needs
  clearance, serviceability (access), and complies
  with state, federal, and municipal guidelines