Antenna Basics

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Antenna Basics
SPRAWDZIC 4--6!!!!
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Outline

• Reciprocity Theorem
• Point Radiator Concept
• Irradiance, PFD
• Directivity, Gain, Radiation Efficiency
• EIRP
• Power Transfer

3
EM Field EM Forces

• EM Field is a spatial distribution of forces
  which may be exerted on an electric charge
• Force a vector characterized by its intensity,
  direction, orientation
• Classical physics
• Coulomb (1736-1806), Galvani (1737-1798) Volta
  (1745-1827), Ampere (1775-1836), Faraday
  (1791-1867), Maxwell (1831-1879), Hertz
  (1857-1894), Marconi (1874-1937), Popov
  (1874-1937)

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EM Field

• EM forces fill-in the whole space without limits.
  
• They interact with the matter.
• Magnetic forces and electric forces act
  differently, e.g. the magnetic field interact
  with electric charges only when the charges move.
  
• For many years Electric and Magnetic forces were
  considered as being different phenomena and
  different branches of physics. Only in 19 century
  . realized that they both are different faces
  of the same EM phenomenon

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• Abdus Salam, (XX-XX), 1979 Nobel Prize Laureate,
  indicated further that electromagnetism and weak
  interaction known from quantum physics are
  various facets of the same phenomenon.
• Richard Feynmann (1918-1988), 1965 Nobel Prize
  Laureate ( XXX quantum electrodynamics)

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EM forces are stronger than gravity forces, but
how strong they are?

• Imagine 2 persons at 1 m distance. By some magic,
  we decrease the number of protons by 1 in each,
  so that each has more electrons than protons, and
  is no more electrically neutral they repulse
  each other. How strong would be the repulsive
  force?
• Could it be enough to move a sheet of paper? Or
  this table? Or, perhaps, this building?

7

• Feynman calculated that the repulsive force would
  be strong enough to lift the whole Earth!
• EM forces generated in far galaxies can move
  electrons on the Earth Panzias Wilson, Nobel
  Prize Laureates 1978, showed that the EM residual
  noise was generated during the Big Bang

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Maxwell Equations

• Concept of unlimited EM field interacting with
  the matter
• Mathematics 2 coupled vectors E and H (6
  numbers) varying with time and space
• Summary The magnetic and electric components of
  the time-space-variable electromagnetic field and
  the time-variable electric current are mutually
  coupled.

10
EM Field of Linear Antennas

• Summation of all vector components E (or H)
  produced by each antenna element
• In the far-field region, the vector
  componentsare parallel to each other
• Method of moments

O
11
EM Field of Current Element
Er
z
E?
OP
E?
?
r
I, dz
y
?
x
I monochromatic AC ampere dz short element
meter
12
EM Field of Current Element 2
Idz moment of linear current element
Johnson Jasik Antenna Engineering Handbook T.
Dvorak Basics of Radiation Measurements, EMC
Zurich 1991 J. Dunlop, D. Smith
Telecommunications Engineering1995, p. 216
13
EM Field of Current Element 3

• The components of the EM field
• are proportional to the current moment Idz
• are azimuth-independent (axial symmetry)
• decrease with distance as (?r)-1, (?r)-2, or
  (?r)-3 if ?r 1, r ?/(2?), C FF Q
• E? maximal in the equatorial plane
• Er maximal in the direction of current dz
• H? maximal in the equatorial plane

14
EM Field Elementary Current Loop
dm magnetic dipole moment
15
Field Components Intensity
?
C, Q Induction fields
FF Radiation field
16
Field Impedance

• Field impedance
• Z E/H depends
• on the antenna type and on distance

17
Far-Field, Near-Field

• Near-field region
• Angular distribution of energy depends on
  distance from the antenna
• Reactive field components dominate (L, C)
• Far-field region
• Angular distribution of energy is independent on
  distance
• Radiating field component dominates (R)
• The resultant EM field can locally be treated as
  uniform (TEM)

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Source Characteristics 1

• The radiated (far) field in all direction from a
  single monochromatic source in free space is
  completely specified by 4 quantities
• Amplitude of the E? component of the electric
  field as functions of r, ?, and ?
• Amplitude of the E? component of the electric
  field as functions of r, ?, and ?

19
Source Characteristics 2

• Phase lag ? of E? behind E? as a function of ?,
  r, and ?
• Phase lag ? of a field component behind its value
  at a reference point as a function of r, ?, and ?
• Phase characteristics are often disregarded but
  they are important when the fields from 2 or more
  sources are to be added.

20
Reciprocity Theorem

• The proprieties of a receiving antenna are
  identical with the proprieties of the same
  antenna when used for transmitting
• Note This theorem is valid only for linear
  passive antennas (i.e. antennas that do not
  contain nonlinear elements and/or amplifiers)

21
Antenna Functions

• To transform the power of time-dependent
  electrical current into the power of the
  time-and-space-dependent electro-magnetic (EM)
  wave (transmitting antenna)
• To transform the power of the time-and-space-depen
  dent EM wave into the power of the time-dependent
  electrical current (receiving antenna)

22
Intended Unintended Antennas

• Intended antennas
• Radiocommunication antennas
• Measuring antennas, EM sensors, probes
• EM applicators (Industrial, Medical)
• Unintended antennas
• Radiating (any conductor/ installation carrying
  electrical current e.g. electrical installation
  of vehicles)
• Receiving/ Re-radiating (any conducting
  structure/ installation irradiated by EM waves)
• Stationary (e.g. antenna masts or power line
  wires)
• Time-varying (e.g. windmill or helicopter
  propellers)
• Transient (e.g. aeroplanes, missiles)

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Basic Antenna Characteristics

• In terms of field theory (Electromagnetics)
• Gain
• Radiation pattern (Half-power beam width,
  unintended lobes)
• Polarization (Cross-polarization)
• In terms of circuit theory
• Radiation resistance (Impedance)
• VSWR

24
Point Source

• For many purposes, it is sufficient to know the
  direction (angle) variation of the power radiated
  by antenna at large distances.
• For that purpose, any practical antenna,
  regardless of its size and complexity, can be
  represented as a point-source.
• The actual field near the antenna is then
  disregarded.

25
Point Source 2

• The EM field at large distances from an antenna
  can be treated as originated at a point source -
  fictitious volume-less emitter.
• The EM field in a homogenous unlimited medium at
  large distances from an antenna can be
  approximated by an uniform plane TEM wave

26
Power Flow

• The time rate of EM energy flow per unit area in
  free space is the Poynting vector.
• It is the cross-product (vector product,
  right-hand screw direction) of the electric field
  vector (E) and the magnetic field vector (H) P
  E x H.
• For the elementary dipole E? ? H? and only E?xH?
  carry energy into space with the speed of light

27
Power Flow 2

• The Poynting vector gives the irradiance and
  direction of propagation of the electromagnetic
  wave in free space.
• Irradiance radiant power incident per unit area
  upon a surface. It is usually expressed in watts
  per square meter, but may also be expressed in
  joules per square meter.
• Synonyms Power Density, Power Flow Density (PFD).

28
Power Flow 3

• In free space, the radiated energy streams from
  the point source in radial lines, i.e. the
  Poynting vector has only the radial component in
  spherical coordinates.
• A source that radiates uniformly in all
  directions is an isotropic source (radiator,
  antenna). For such a source the radial component
  of the Poynting vector is independent of ? and ?.

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Spherical coordinates for a point source of
radiation in free space
Z
Observation point (r,?,?)
Polar axis
Poynting vector
E?
Point source at origin
?
E ?
r
Y
?
X
Equatorial plane
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Power Flow From Point Source
Z
r sin?
Polar axis
Element of area ds r2 sin? d? d?
r d?
?
r
Y
?
X
Equatorial plane
r sin? d?
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Power Flow - General Case
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Power Flow - Isotropic Source

• Notes
• PFD does not depend on frequency/ wavelength
• Distance increases x 2 ? PFD decreases x 4
• Distance increases x 2 ? E decreases x 2
• Isotropic radiator cannot be physically realized

33
Anisotropic sources

• Every antenna has directional properties
  (radiates more energy in some directions than in
  others).
• Idealized example of directional antenna the
  radiated energy is concentrated in the yellow
  region (cone).
• The power flux density gains it is increased by
  (roughly) the inverse ratio of the yellow area
  and the total surface of the isotropic sphere.

Isotropic sphere
34
Antenna Gain

• The ratio of the power required at the input of a
  loss-free reference antenna to the power supplied
  to the input of the given antenna to produce, in
  a given direction, the same field strength at the
  same distance.

35
Antenna Gain 2
Step 2
Step 1
Actual antenna
Measuring equipment
Reference antenna
Measuring equipment
P Power delivered to the actual antenna
S Power received
Po Power delivered to the reference antenna
S0 Power received
Antenna Gain (P/Po) SS0
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Antenna Gains Gi, Gd, Gr

• Gi Isotropic Power Gain - the reference
  antenna is isotropic
• Gd - the reference antenna is a half-wave dipole
  isolated in space
• Gr - the reference antenna is linear much shorter
  than one quarter of the wavelength, normal to the
  surface of a perfectly conducting plane

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Antenna Gain Comments

• Unless otherwise specified, the gain refers to
  the direction of maximum radiation.
• Gain in the field intensity may also be
  considered - it is equal to the square root of
  the power gain.
• Gain is a dimension-less factor, usually
  expressed in decibels

38
Radiant Intensity

• Radiated power per unit solid angle
  (steradian), ?(?,?), in watts per steradian

z
Observation Point

• A measure of the ability of an antenna to
  concentrate radiated power in a particular
  direction
• Does not depend on distance

?
Transmitting antenna
r
y
?
x
Assumption Distance (r) is very large
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Directivity

• D Has no units
• P0 power radiated

40
Gain, Directivity, Radiation Efficiency

• The radiation intensity, directivity and gain are
  measures of the ability of an antenna to
  concentrate power in a particular direction.
• Directivity relates to the power radiated by
  antenna (P0 )
• Gain relates to the power delivered to antenna
  (PT)

• ? radiation efficiency (0.5 - 0.75)

41
Antenna Gain PFD

• S0 PFD produced by a loss-less isotropic
  radiator

42
Directivity Pattern

• The variation of the field intensity of an
  antenna as an angular function with respect to
  the axis.
• Usually represented graphically for the far-field
  conditions.
• May be considered for a specified polarization
  and/or plane (horizontal, vertical).
• Depends on the polarization and the reference
  plane for which it is defined/measured
• Synonym Radiation pattern.

43
Antenna patterns
1
?
P(?)/Pmax(?)
Relative (normalized) power pattern

• Usually represented in 2 reference planes
  ?const. and ?const.
• E PDF relate to the equivalent uniform plane
  wave
• Note Peak value ?2 x Effective value for
  sinusoidal quantities

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Elements of Radiation Pattern
Main lobe

• Gain
• Beam width
• Nulls (positions)
• Side-lobe levels (envelope)
• Front-to-back ratio

Emax
Sidelobes
Emax /?2
Nulls
0
-180
180
Beamwidth
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Linear Antenna with Sinusoidal Current
Distribution
r(z)
r
?
z
z cos?

• r distance

LinAntLong simulation
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Beam width

• Beamwidth of an antenna pattern the angle
  between the half-power points of the main lobe.
• Defined separately for the horizontal plane and
  for the vertical plane.
• Usually expressed in degrees.

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Antenna Mask (Example 1)

• Typical relative directivity- mask of receiving
  antenna (Yagi ant., TV dcm waves)
• CCIR doc. 11/645, 17-Oct 1989)

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Antenna Mask (Example 2)
0dB
Phi
-3dB

• Reference pattern for co-polar and cross-polar
  components for satellite transmitting antennas in
  Regions 1 and 3 (Broadcasting 12 GHz)

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Typical Gain and Beamwidth
Type of antenna Gi dB BeamW.
Isotropic 0 3600x3600
Half-wave Dipole 2 3600x1200
Helix (10 turn) 14 350x350
Small dish 16 300x300
Large dish 45 10x10
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Gain and Beamwidth

• Gain and beam-width of highly directive antennas
  are inter-related
• G 30000 / (?1?2)
• ?1 and ?2 are the half-power beamwidths in the
  two orthogonal principal planes of antenna
  radiation pattern in degrees.

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Increasing Gain
52
Parabolic Antenna

• For the planar wave front, the times/distances
  FAA FBB CCC
• Extend AA by AA
• Require AA AF
• Locus of points equidistant from F and LL is
  parabola
• Axial symmetry parabolic reflector

53
How to Make Parabolic Reflectors Cheaply
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e.i.r.p.

• Equivalent Isotropically Radiated Power (in a
  given direction)
• The product of the power supplied to the antenna
  and the antenna gain (relative to an isotropic
  antenna) in a given direction

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Antenna Effective Area

• Measure of the effective absorption area
  presented by an antenna to an incident plane
  wave.
• Depends on the antenna gain and wavelength

Aperture efficiency ?a Ae / A A physical area
of antennas aperture, square meters
56
Power Transfer in Free Space

• ? wavelength m
• PR power available at the receiving antenna
• PT power delivered to the transmitting antenna
• GR gain of the transmitting antenna in the
  direction of the receiving antenna
• GT gain of the receiving antenna in the
  direction of the transmitting antenna
• Matched polarizations

57
Linear Polarization

• In a linearly polarized plane wave the direction
  of the E (or H) vector is constant.
• Two linearly polarized waves produce one
  elliptically polarized wave the resultant E
  vector has direction varying in time its tip
  draws an ellipse.

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Elliptical Polarization
LHC
Ex cos (wt) Ey cos (wtpi/4)
Ex cos (wt) Ey cos (wt3pi/4)
Ex cos (wt) Ey cos (wt)
Ex cos (wt) Ey -sin (wt)
RHC
Ex cos (wt) Ey sin (wt)
Ex cos (wt) Ey -cos (wtpi/4)
59
Polarization ellipse
Ex

• The superposition of two plane-wave components
  results in an elliptically polarized wave
• The polarization ellipse is defined by its axial
  ratio N/M (ellipticity), tilt angle ? and sense
  of rotation

M
Ey
?
N
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Polarization states
LHC
(Poincaré sphere)
UPPER HEMISPHERE ELLIPTIC POLARIZATION LEFT_HANDE
D SENSE
LATTITUDE REPRESENTS AXIAL RATIO
EQUATOR LINEAR POLARIZATION
450 LINEAR
LOWER HEMISPHERE ELLIPTIC POLARIZATION
RIGHT_HANDED SENSE
LONGITUDE REPRESENTS TILT ANGLE
RHC
POLES REPRESENT CIRCULAR POLARIZATIONS
61
Comments on Polarization

• At any moment in a chosen reference point in
  space, there is actually a single electric vector
  E (and associated magnetic vector H).
• This is the result of superposition (addition) of
  the instantaneous fields E (and H) produced by
  all radiation sources active at the moment.
• The separation of fields by their wavelength,
  polarization, or direction is the result of
  filtration.

62
Antenna Polarization

• The polarization of an antenna in a specific
  direction is defined to be the polarization of
  the wave produced by the antenna at a great
  distance at this direction

63
Polarization Efficiency (1)

• The power received by an antenna from a
  particular direction is maximal if the
  polarization of the incident wave and the
  polarization of the antenna in the wave arrival
  direction have
• the same axial ratio
• the same sense of polarization
• the same spatial orientation
• .

64
Polarization Efficiency (2)

• When the polarization of the incident wave is
  different from the polarization of the receiving
  antenna, then a loss due to polarization mismatch
  occurs
• Polarization efficiency
• (power actually received) / (power that would
  be received if the polarization of the incident
  wave were matched to the receiving polarization
  of the antenna)

65
Polarization Efficiency (3)
LCH
A POLARIZATION OF RECEIVING ANTENNA W
POLARIZATION OF INCIDENT WAVE
W
2?

• Polarization efficiency cos2?

A
H
450 LINEAR
RCH
66
How to Produce Circularly-Polarized EM Field

• Radio wave of elliptical (circular) polarization
  can be obtained by superposition of 2
  linearly-polarized waves produced by 2 crossed
  dipoles and by controlling the amplitude- ratio
  and phase-difference of their excitations.

y
Iycos(?t?y)
x
Ixcos(?t?x)
67
Reflection Image Theory

• Antenna above perfectly conducting plane surface
• Tangential electrical field component 0
• vertical components the same direction
• horizontal components opposite directions
• The field (above the ground) is the same if the
  ground is replaced by the antenna image

-
68
Polarization Filters
Wall of thin parallel wires (conductors)
E1gt0
E1gt0
E2 0
E2 E2
Vector E ?? wires
Vector E ? wires

• At the surface of ideal conductor the tangential
  electrical field component 0

69
Depolarization
70
e.i.r.p. PFD Example 1

• What is the PFD from a TV broadcast GEO satellite
  at Trieste?
• EIRP 180 kW
• Distance 38'000 km
• Free space

71
e.i.r.p. PFD Example 2

• What is the PFD from a WLAN transmitter?
• EIRP 180 mW
• Distance 3.8 m?
• Free space

In this example, WLAN produces thousand millions
times stronger signal than the satellite!
72
Power Transfer Example 1

• What is the power received from GEO satellite
  (?0.1m, PT 440 W, GT1000) at Trieste
  (distance 38'000 km, GR1)?
• Free space

73
Power Transfer Example 2

• What is the power from a transmitter (?0.1m,
  PT44 mW, GT1) received at distance of 3.8 m
  (GR1)?
• Free space

74
Mismatch Effects
SWR Gain Reduction Gain Reduction
1.0 0 0
1.5 4 0.2 dB
2.0 11 0.5 dB
3.0 25 1.3 dB
5.0 44 2.6 dB
10 67 4.8 dB
75
2 Identical Antennas

• Excitation I1 I I2 Iej?
• Ant1 field-strength E CD(?, ?)
• Ant2 field-strengthE CD(?, ?)ej(??r?)

r
r
r
?r
?
d
1
2

• ?r dcos ?

76
2 Identical Antennas - AAF

• Resultant field-strength E E E
• E E 1ej(??r?) CD(?,
  ?)1ej(??r?) CD(?, ?)F(?, ?) ?
  Pattern multiplication
• AAF(?, ?) F(?, ?) 2 Antenna array
  factor Gain of array

77
2 Antenna Array Factor (1)

• F(?) 1ej(??r?) (??r?) x
• F(?) 1ejx 2(1/2)(e-jx/2 ejx/2)ejx/2
  2cos(x/2)ejx/2
• F(?) 2cos(x/2) 2cos?(d/2)cos?
  ?/2) 2cos(?d/?)cos? ?/2
• F(?)2 ? Antenna Array Factor gain of 2
  isotropic antennas

78
2 Antenna Array Factor (2)

• F(?)2 2cos(?d/?)cos? ?/22
• Gain MaxAAF(?)2 4 (6 dBi)when (?d/?)cos?
  ?/2 0, ?, , k?
• Nulls when (?d/?)cos? ?/2 ?/2, , (k 1)?/2
• Relative gain AAF(?)2 / MaxAAF(?)2
  cos(?d/?)cos? ?/22

Array2ant simulation
79
Isotropic Antenna Over Conducting Plane
2AntOverPlane simulation
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Linear Array of n Antennas

• equally spaced antennas in line
• currents of equal magnitude
• constant phase difference between adjacent
  antennas
• numbered from 0 to (n-1)

• F 1ejxej2xej3xej(N-1)x (1-ejNx) /
  (1-ejx)
• F (1-ejNx) / (1-ejx) sin(Nx/2) /
  sin(x/2) F(?) ? array factor
• x/2 (?d/?)cos? ?/2

Array_Nan simulation
81
Phased Arrays

• Array of N antennas in a linear or spatial
  configuration
• The amplitude and phase excitation of each
  individual antenna controlled electronically
  (software-defined)
• Diode phase shifters
• Ferrite phase shifters
• Inertia-less beam-forming and scanning (?sec)
  with fixed physical structure

82
Antenna Arrays Benefits

• Possibilities to control
• Direction of maximum radiation
• Directions (positions) of nulls
• Beam-width
• Directivity
• Levels of sidelobes
• using standard antennas (or antenna collections)
  independently of their radiation patterns
• Antenna elements can be distributed along
  straight lines, arcs, squares, circles, etc.

83
Beam Steering
Beam direction

• Beam-steering using phase shifters at each
  radiating element

Equi-phase wave front
? (2?/?)d sin?
Radiating elements
Phase shifters
Power distribution
84
4-Bit Phase-Shifter (Example)
Bit 3
Bit 1
Bit 4
Bit 2
Input
Output
00 or 450
00 or 900
00 or 1800
00 or 22.50
Steering/ Beam-forming Circuitry
85
Switched-Line Phase Bit
Delay line
Input
Output
Diode switch

• 2 delay lines and 4 diodes per bit

86
Switching Diode Circuit
PIN diode
PIN diode
Tuning element
Tuning element
b
a

• a RF short-circuited in forward bias
• b RF short-circuited in reverse bias

87
Adaptive (Intelligent)Antennas

• Array of N antennas in a linear or spatial
  configuration
• Used for receiving signals from desired sources
  and suppress incident signals from undesired
  sources
• The amplitude and phase excitation of each
  individual antenna controlled electronically
  (software-defined)
• The weight-determining algorithm uses a-priori
  and/ or measured information
• The weight and summing circuits can operate at
  the RF or at an intermediate frequency

1
w1
?
wN
N
Weight-determining algorithm
88
Direction Separation
RECEIVER
unwanted transmitter
wanted transmitter
U
W
Adaptive antennas
89
Directive Antenna Effectiveness

• An ideal directive antenna receives power coming
  only from within apical angle ?
• It can eliminate (or attenuate) radiation coming
  from a limited number of discrete interferers
  (but cannot eliminate isotropic noise)

90
Directive Antenna Effectiveness
Rec
Rec
Rec
J
J
J
T
T
T
Effective (T within antenna beam J outside)
Limiting case (T and J at edges)
Not effective (T and J within antenna beam)
91
Directive Antenna Effectiveness
Receiver
?
?
R
?
?
?
?
R
R
?
h
?
?
?
T
J
A
92
Directive Antenna Effectiveness
T
Rc
?
T
J
93
Various Antenna Types (Pictures)
94
Antenna Summary

• Antenna substantial element of radio link
• We have just reviewed
• Basic concepts
• Radio wave radiation physics
• Elementary radiators
• Selected issues relevant to antennas

95
Antenna References

• Scoughton TE Antenna Basics Tutorial Microwave
  Journal Jan. 1998, p. 186-191
• Kraus JD Antennas, McGraw-Hill Book Co. 1998
• Stutzman WL, Thiele GA Antenna Theory and Design
  JWiley Sons, 1981
• Johnson RC Antenna Engineering Handbook
  McGraw-Hill Book Co. 1993
• Pozar D. Antenna Design Using Personal
  Computers
• Li et al., Microcomputer Tools for Communication
  Engineering
• Software
• http//www.feko.co.za/apl_ant_pla.htm
• www.gsl.net/wb6tpu /swindex.html (NEC Archives)