Introduction to Antennas

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Introduction to Antennas

• A. The antenna as a wave launcher
• 1. Waves Two basic experiments and a wise man
  showed the principles of
• a. Action at a distance b.
  Electromagnetism c. Current without wires

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• Faraday Electrical current produced in a wire
  from varying
  magnetism.
• OR voltage produced across open loop
  contacts.
• Electricity From Magnetism.

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• Ampere Magnetism can be produced from Electric
  Current in a wire
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  Magnetism from electricity

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• NOTE
• The effect of the magnetism is felt at a
  distance away from the magnet and similarly the
  effect of the electric current is felt at a
  distance away from the wire. Action At A
  Distance! (with appropriate time
  delay)
• Electricity from magnetism and magnetism from
  electricityWhat about the magnetism which came
  from the electricity? Aha!...
• It can produce more electricity at a distance
  AND the more electricity can produce still
  more magnetism at a distance even farther away
  AND

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• ElectromagnetismAnd all you need is
  current to start it off exceptA problem! What
  is electricity without wires for current to flow
  in? Do we need wires to make magnetism if no
  magnets are available to jiggle?MAXWELL to the
  rescue!Current flows through space, too.
  (wires not necessary!)

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• Experiment
• How does the current Get Thru the capacitor?

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Displacement Current Flows thru the capacitor
(space) Conduction Current Flows thru the
circuit (wires)
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• Dont Need Wires For Current To Flow!
• That means that every Waving Magnet has
  Displacement Current flowing in space around it
  andevery time you start Electromagnetism off it
  goes on and on by itself. ( A Wave)

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• 2. A Simple Dipole Antenna is a Circuit Turned
  Inside Out

Space Current
Generator
More Plate Spacing
Capacitor
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A Dipole Antenna!
Very Tiny Capacitor Plates
Space Current (Displacement Current)
Smaller Space Currents (With Magnetism Around
Them)
Lot of Spacing
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• 3. Nature of Displacement (Space) current
• Remember Charges Piled up on the capacitor
  plates. The Magic Stuff between the charges
  turns out to be electric field which varies with
  time. Thats All Displacement Current Really
  is? since displacement current makes magnetism
  too, that means varying Electric Fields make
  Magnetism varying Magnetic Fields (Magnetism)
  make Electric Fields in space. You cant have
  one without the other. In fact, there arent two
  things There is only one thing!

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• ELECTROMAGNETISM (We like to talk about two
  Effects Of Electromagnetism) There is only one
  Source Of Electromagnetism).CURRENT
  (time-varying)(We like to talk about current
  confined to two types of environment-conductors
  space).

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• 4. Antennas As Transitions Between Transmission
  Line Wave Space Waves.
• Typical Transmission System

Traveling Wave Captured
Z
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Farther out, the field lines will detach
themselves from the wire will form closed
lines. If antenna is short (compared to ? ),
the lines are from a small part of a period of
the sine wave do not show reversal of direction
from the center to the ends (at a fixed time).
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At real high frequencies, we use hollow pipes
(Waveguides) to pipe waves either you opening
up a wave pipe gives a Horn.
energy flow outward
If you cut holes in wavepipes, the holes leak
energy out. Slot Antennas!
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B. Classes Of Antennas. 1. Wire antennas are
used as extensions of ordinary circuits are
most often found in Lower frequency
applications. They can operate with two
terminals in a Balanced configuration like the
dipole or with an Unbalanced configuration using
a Ground Plane for the other half of the
structure.
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Wire antennas are characterized by a line along
which current flows producing the electromagnetic
wave. Short wire antennas have all points on the
wire ltlt ?/4 away (the structure is only a few
degrees-of-A-wave long). Longer wires can
sustain phase changes (hence field direction
strength current direction strength changes)
on them.
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Half Wave Dipole
Current is Almost ½ of a Sinusoid
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2. Aperture antennas radiate from an opening or
from a surface rather than a line and are found
at Higher frequencies where wavelengths are
Shorter. Aperture antennas often have handfuls
of Sq. Wavelengths of area are very seldom
fractions of a wavelength. Ex parabolic
reflector antenna.
Ex. Parabolic Reflector Antenna
Small Feed Antenna
Big Parabolic Dish Focuses Feed Radiation
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Example Lens Antenna
Small Horn Feed Antenna
Plastic Lens for Focusing Feed Radiation
3. Arrays are collections of linear (Wire)
antennas or aperture antennas arranged to squirt
more Juice in a desired direction than in any
other directions by causing addition
subtraction of waves from all of the individual
Element antennas.
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Since arrays count on Phase Difference (i.e.
distances which are of about wavelength size).
They are never Small (compared to ?).
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So if both antennas are identical, we expect
their individual omni directional waves to add up
to a wave that radiates with directional
preference like this
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By choosing Spacing, Phasing (starting time of
feed generators), and Number of Elements almost
any kind of shape of radiation can be had. (of
course you cant beat mother nature must obey
the laws of the universe, the most important one
being the Law of Conservation of Difficulty or
you dont ever get something for nothing.)
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If the elemental antennas in an array are able
to, individually, squirt energy better in one
direction than in others, their Pattern Shape
will help control the Pattern Shape of an array
of them. (Our previous example considered a
fictitious omni directional pattern for the
elemental antennas) to get the pattern shape of
an array of hi class elemental antennas, you
can Multiply the Pattern of the array if it were
made up of omni directional elements times the
Pattern of the actual Element Antennas.
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4.Traveling wave standing wave describes the
basic distinction between two types of currents
found on (in) antennas. Traveling wave currents
are smooth shaped distributions associated with
transmission lines (or wave pipes) which are
properly terminated by a load which gobbles up
the current that gets to the end of the line
(antenna). Standing Wave Currents are
sinusoidal (or almost) shaped distributions
associated with transmission lines (or wave
pipes) which are not properly terminated by a
load which reflects or rejects the current that
gets to the end of the line (antenna) sends it
whistling back toward from whence it cometh.
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In Real-Life we always have some combination of
the two types of currents on any antenna but one
(usually standing) dominates.The reason we care
about this is that the pattern shapes are
Different for each type. 5. Vertically
Horizontally Polarized refers to the orientation
of the current on the antenna, hence the
orientation of the electric part of the
electromagnetic field. We choose the
polarization base on application (radio wave
propagation) or on physical surroundings.
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C. Performance Parameters of Antennas
1.Radiation Pattern (or reception pattern) is a
plot of the strength of the electric ( or
magnetic) part of the electromagnetic field in
all directions around an antenna. We usually
consider a huge imaginary spherical surface which
surrounds the antenna make a graph of the Field
Strength along a N-S or E-W path of the sphere.
A Power Pattern is a plot of the density of power
flowing thru the surface of the sphere and is
obviously proportional to the square of the field
pattern. Typical pattern shapes are shown on the
next slide along with coordinates of the sphere.

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Idealized Point Radiator
(a) Isotropic
Vertical Dipole
(b) Omni directional
Radar Dish
(c) The Pencil
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Typical Coordinate System Typical
Coordinate System
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Polar Plot
Same Pattern
Rectangular Plot
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Patterns Showing Field Power Density
Field Intensity
Power Density
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2. Gain is used to compare the ability of
different antennas to squirt out signals. Hi gain
implies narrow beams a better job of focusing
energy in a given direction. Usually compared to
isotropic, uniform radiator. 3. Impedance
represents the kind of load the antenna presents
to its transmission line. Since antennas cant
radiate if they dont receive absorb power from
their sources, impedance is very important. Also
the manner in which it varies affects its
bandwidth. The part of the impedance associated
with power accepted then radiated is called the
Radiation Resistance and is easily calculated.
Any other resistance an antenna presents usually
represents losses.
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4. Effective Aperture Effective Length signify
the reception capability of an antenna. If an
antenna is able to extract a certain amount of
power from a passing wave of known power density,
an effective area or aperture can be calculated
and associated with it. Likewise, if a linear
(wire) antenna develops a certain voltage across
its terminals when it is excited by a known field
intensity from a passing wave, an effective
length or height (for vertical wires) can be
determines. 5. Coupling between nearby antennas
is a measure of the power delivered or received
by one when the other transmits a given power.
Coupling depends on antenna gain, orientation,
relative location physical surroundings. It is
of prime importance on crowded platforms when
considering interference.
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• Near Field The radiation pattern was a Far Field
  parameter because we considered a Huge sphere.
  What we actually meant by huge was that we were
  so far away that the wave coming from the antenna
  towards us appeared to emanate from a far
  distance point. When we need to consider the Near
  Zone of an antenna, it no longer appears as a
  tiny point but rather we are aware that the
  energy coming toward us comes from a collection
  of locations. The reason we distinguish between
  near far is that it is much more difficult to
  determine by calculation what the near-in fields
  are.
• (We have just taken a very simplistic look at
  some antenna concepts. No math was used. The
  ideas were presented for the benefit of those not
  actively engaged in working with antennas.)

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D. Examples Pictures Of Some Antennas and
Their Patterns
(c) A folded dipole (d) A folded unipole
(a) A Dipole antenna and (b) A Loop Antenna
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Marconis first transatlantic transmitting
antenna (1901)
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