antenna - UTH e

Wireless signal propagation
(antenna patterns)
Thanasis Korakis
University of Thessaly
[email protected]
Dipole, monopole and isotropic antenna
● The simplest antennas, monopole and dipole
antennas, consist of one or two straight
metal rods along a common axis
● Isotropic antenna is a hypothetical antenna
that radiates equally in all directions and is
used as a reference to calculate antenna
gain
Dipole or monopole antennas
Dipole radiation
pattern
● The radiation pattern of a
vertical 1/2λ (half-wave) dipole
antenna
● The antenna is at the center of the "donut," or torus.
● Radial distance from the center represents the power
radiated in that direction
● The power radiated is maximum in horizontal
directions, dropping to zero directly above and below
the antenna
Monopole radiation
pattern
● Radiation pattern of a
typical monopole
● Monopole antennas up to 1/4λ long have a single
"lobe", with field strength declining monotonically from
a maximum in the horizontal direction
● Longer monopoles have more complicated patterns
with several conical "lobes" (radiation maxima)
directed at angles into the sky
Dipole and monopole antennas
Monopole antenna used at high frequencies.
The three conductors projecting downward are
the ground plane
"Rabbit-ears" television dipole antenna
Omnidirectional antenna
● EM wave power is radiated
uniformly in all directions in
one plane
● Radiated power is decreased
with elevation angle above or
below the plane, dropping to
group of monopole antennas
zero on the antenna's axis Aforming
an omnidirectional antenna
Directional (or beam) antenna
● Radiates greater power in one or more
directions allowing for increased
performance on transmit and receive and
reduced interference from unwanted sources
● Residential TV antennas are usually:
○ Yagi-Uda directional antennas
○ Log-periodic directional antennas
○ Corner reflector directional antennas
Directional antennas
Yagi-Uda array consists of a driven
element (typically a dipole) and additional
parasitic elements (usually a so-called
reflector and one or more directors)
Log-periodic array consists of a series
of dipoles positioned along the antenna
axis, spaced at following a logarithmic
function of the frequency
Lobes or beams
● The main lobe, or main beam, of an
antenna radiation pattern is the lobe
containing the maximum power
● The radiation pattern of most antennas
shows:
○ "lobes" at various angles, directions where the
radiated signal strength reaches a maximum,
○ separated by "nulls", angles at which the radiation
falls to zero
Main lobe, sidelobes and backlobe
● In a directional antenna one lobe is designed
to be bigger than the others; this is the main
lobe
● The other lobes are called "sidelobes", and
usually represent unwanted radiation in
undesired directions
● The sidelobe in the opposite direction from
the main lobe is called the "backlobe"
Main lobe, sidelobes and backlobe
How do we estimate the
antenna gain?
Radiation pattern (another view)
● Imagine an elastic sphere that is filled with an
incompressible medium
● A dot at the center represents an isotropic
antenna and the sphere surface is its radiation
pattern
● The radius of the sphere is proportional to the
power radiated by the isotropic antenna
Radiation pattern (another view)
● ...Now, let the sphere be deformed to create a
new shape (or a new radiation pattern)
● The volume remains unchanged regardless of
the change in shape
● The ratio of the distance from the center to any
point on the surface of the new shape, to the
radius of the initial sphere is the directive gain
● The maximum directive gain is its directivity
Beamwidth
The half-power beamwidth is the angle (in the
horizontal plane) between:
● the two directions in which the directive gain of
the major lobe is one half the directivity
or equivalently
● the half-power (-3 dB) points of the main lobe,
when referenced to the peak effective radiated
power of the main lobe
Beamwidth 30
o
* ( 0.7072 = 0.5 )
* ( -3dB = 10log10(0.5) )
NYU Poly WiMax antenna
120o beamwidth
Why we use antenna gains?
● Propagation models assume isotropic antennas
● Other antennas feature gains comparing to the
isotropic antenna:
○ As a transmitting antenna, the gain describes how
well the antenna converts input power into radio
waves headed in a specified direction.
○ As a receiving antenna, the figure describes how
well the antenna converts radio waves arriving from
a specified direction into electrical power
Antenna gain
● When no direction is specified, gain is the
peak value of the gain (under direction)
● Due to reciprocity, the gain of any antenna
when receiving is equal to its gain when
transmitting
● Antenna gain is usually defined as the ratio of
the peak power produced by this antenna to
the power produced by an isotropic antenna
Antenna gain units
● This ratio is expressed in decibels and the
related units are decibels-isotropic (dBi)
● Half-wave dipole antenna has gain 2.15 dBi
● Alternatively, we compare the antenna to the
power received by a lossless half-wave
dipole antenna, writing the units as dBd
● So, gain in dBd = gain in dBi - 2.15
How we estimate antenna gains?
● The gain is inversely proportional to
beamwidth; as the beamwidth decreases,
the gain increases
G = 10log10(4π2 / bh * bv)
(bh is the beamwidth in the horizontal plane)
(bv is the beamwidth in the vertical plane)
References
1. Andrea Goldsmith, Wireless
Communications, Cambridge University
Press, 2005.
2. T. S. Rappaport, Wireless Communications:
Principle and Practice, 2nd ed. Singapore:
Pearson Education, Inc., 2002.
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