Chapter 5
Antennas
Antennas for
for
Wireless
Wireless Systems
Systems
Dipole
Isotropic
Typical Wireless
Omni Antenna
May 27, 1997
RF Engineering 102 v1.0 (c)1997 Scott Baxter
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Chapter 5 Section A
Introduction
Introduction to
to
Antennas
Antennas for
for Wireless
Wireless
May 27, 1997
RF Engineering 102 v1.0 (c)1997 Scott Baxter
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Understanding Antenna Radiation
The Principle Of Current Moments
■ An antenna is just a passive
conductor carrying RF current
Zero current
at each end
each tiny
imaginary “slice”
of the antenna
does its share
of radiating
TX
RX
Maximum current
at the middle
Current induced in
receiving antenna
is vector sum of
contribution of every
tiny “slice” of
radiating antenna
Width of band
denotes current
magnitude
May 27, 1997
• RF power causes the current
flow
• Current flowing radiates
electromagnetic fields
• Electromagnetic fields cause
current in receiving antennas
■ The effect of the total antenna is the
sum of what every tiny “slice” of the
antenna is doing
• Radiation of a tiny “slice” is
proportional to its length times
the magnitude of the current in
it, at the phase of the current
RF Engineering 102 v1.0 (c)1997 Scott Baxter
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Different Radiation In Different Directions
■ Each “slice” of the antenna produces
a definite amount of radiation at a
specific phase angle
■ Strength of signal received varies,
depending on direction of departure
from radiating antenna
Minimum
Radiation:
contributions
out of phase,
cancel
Maximum
Radiation:
TX
contributions
in phase,
reinforce
Minimum
Radiation:
contributions
out of phase,
cancel
May 27, 1997
• In some directions, the
components add up in phase
to a strong signal level
• In other directions, due to the
different distances the various
components must travel to
reach the receiver, they are
out of phase and cancel,
leaving a much weaker signal
■ An antenna’s directivity is the same
for transmission & reception
RF Engineering 102 v1.0 (c)1997 Scott Baxter
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Antenna Polarization
Antenna 1
Vertically
Polarized
Electromagnetic
Field
Antenna 2
Horizontally
Polarized
TX
current
RX
almost
no
current
RF current in a conductor
causes electromagnetic fields
that seek to induce current
flowing in the same direction
in other conductors.
The orientation of the antenna is
called its polarization.
■ To intercept significant energy, a receiving antenna must be oriented
parallel to the transmitting antenna
• A receiving antenna oriented at right angles to the transmitting
antenna is “cross-polarized”; will have very little current induced
• Vertical polarization is the default convention in wireless telephony
• In the cluttered urban environment, energy becomes scattered and
“de-polarized” during propagation, so polarization is not as critical
• Handset users hold the antennas at seemingly random angles…..
May 27, 1997
RF Engineering 102 v1.0 (c)1997 Scott Baxter
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Antenna Gain
■ Antennas are passive devices: they do not produce
power
• Can only receive power in one form and pass
it on in another, minus incidental losses
• Cannot generate power or “amplify”
Omni-directional
Antenna
■ However, an antenna can appear to have “gain”
compared against another antenna or condition. This
gain can be expressed in dB or as a power ratio. It
applies both to radiating and receiving
■ A directional antenna, in its direction of maximum
radiation, appears to have “gain” compared against a
non-directional antenna
■ Gain in one direction comes at the expense of less
radiation in other directions
■ Antenna Gain is RELATIVE, not ABSOLUTE
• When describing antenna “gain”, the
comparison condition must be stated or
implied
May 27, 1997
RF Engineering 102 v1.0 (c)1997 Scott Baxter
Directional
Antenna
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Effective Radiated Power
Reference
Antenna
■ An antenna radiates all power fed to it from the
transmitter, minus any incidental losses.
Every direction gets some amount of power
■ Effective Radiated Power (ERP) is the apparent
power in a particular direction
A
100 W
• Equal to actual transmitter power times
antenna gain in that direction
■ Effective Radiated Power is expressed in
comparison to a standard radiator
• ERP: compared with dipole antenna
• EIRP: compared with Isotropic antenna
Example: Antennas A and B each radiate 100 watts from
their own transmitters. Antenna A is our reference.
Antenna B is directional. In its maximum direction, its
signal seems 2.75 stronger than the signal from antenna
A. Antenna B’s ERP in this case is 275 watts.
May 27, 1997
RF Engineering 102 v1.0 (c)1997 Scott Baxter
TX
B
Directional
Antenna
TX
100 W
ERP B A (ref)
A
B
275w
100w
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Reference Antennas
Defining Gain And Effective Radiated Power
■ Isotropic Radiator
• Truly non-directional -- in 3 dimensions
• Difficult to build or approximate physically,
but mathematically very simple to describe
• A popular reference: 1000 MHz and above
Isotropic
Antenna
– PCS, microwave, etc.
■ Dipole Antenna
• Non-directional in 2-dimensional plane only
• Can be easily constructed, physically
practical
• A popular reference: below 1000 MHz
– 800 MHz. cellular, land mobile, TV & FM
Quantity
Units
Gain above Isotropic radiator
dBi
Gain above Dipole reference
dBd
Effective Radiated Power Vs. Isotropic
(watts or dBm) EIRP
Effective Radiated Power Vs. Dipole
(watts or dBm) ERP
May 27, 1997
RF Engineering 102 v1.0 (c)1997 Scott Baxter
Dipole Antenna
Notice that a dipole
has 2.15 dB gain
compared to an
isotropic antenna.
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Antenna Gain And ERP
Examples
■ Many wireless systems at 1900 & 800 MHz use omni
antennas like the one shown in this figure
■ These patterns are drawn to scale in E-field radiation
units, based on equal power to each antenna
■ Notice the typical wireless omni antenna concentrates
most of its radiation toward the horizon, where users
are, at the expense of sending less radiation sharply
upward or downward
■ The wireless antenna’s maximum radiation is 12.1 dB
stronger than the isotropic (thus 12.1 dBi gain), and
10 dB stronger than the dipole (so 10 dBd gain).
Gain Comparison
12.1 dBi
Isotropic
10dBd
Dipole
Isotropic
Dipole
Typical Wireless
Omni Antenna
Gain 12.1 dBi or 10 dBd
Omni
May 27, 1997
RF Engineering 102 v1.0 (c)1997 Scott Baxter
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Radiation Patterns
Key Features And Terminology
An antenna’s directivity is
expressed as a series of patterns
■ The Horizontal Plane Pattern graphs
the radiation as a function of azimuth
(i.e..,direction N-E-S-W)
■ The Vertical Plane Pattern graphs the
radiation as a function of elevation (i.e..,
up, down, horizontal)
■ Antennas are often compared by noting
specific landmark points on their
patterns:
• -3 dB (“HPBW”), -6 dB, -10 dB
points
• Front-to-back ratio
• Angles of nulls, minor lobes, etc.
Typical Example
Horizontal Plane Pattern
Notice -3 dB points
0 (N)
0
-10
-20
-30 dB
270
(W)
10 dB
points
Main
Lobe
nulls or
a Minor
minim
Lobe
Front-to-back Ratio
180 (S)
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RF Engineering 102 v1.0 (c)1997 Scott Baxter
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90
(E)
How Antennas Achieve Their Gain
Quasi-Optical Techniques (reflection, focusing)
• Reflectors can be used to concentrate
radiation
– technique works best at microwave frequencies,
where reflectors are small
• Examples:
– corner reflector used at cellular or higher
frequencies
– parabolic reflector used at microwave
frequencies
– grid or single pipe reflector for cellular
Array techniques (discrete elements)
• Power is fed or coupled to multiple
antenna elements; each element radiates
• Elements’ radiation in phase in some
directions
• In other directions, a phase delay for each
element creates pattern lobes and nulls
May 27, 1997
RF Engineering 102 v1.0 (c)1997 Scott Baxter
In phase
Out of
phase
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Types Of Arrays
■ Collinear vertical arrays
• Essentially omnidirectional in
horizontal plane
• Power gain approximately
equal to the number of
elements
• Nulls exist in vertical pattern,
unless deliberately filled
■ Arrays in horizontal plane
• Directional in horizontal
plane: useful for sectorization
• Yagi
RF
power
– one driven element, parasitic
coupling to others
• Log-periodic
– all elements driven
– wide bandwidth
RF
power
■ All of these types of antennas are
used in wireless
May 27, 1997
RF Engineering 102 v1.0 (c)1997 Scott Baxter
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Omni Antennas
Collinear Vertical Arrays
The family of omni-directional wireless
antennas:
■ Number of elements determines
Typical Collinear Arrays
Number of
Elements
1
2
3
4
5
6
7
8
9
10
11
12
13
14
• Physical size
• Gain
• Beamwidth, first null angle
■ Models with many elements have
very narrow beamwidths
• Require stable mounting and
careful alignment
• Watch out: be sure nulls do
not fall in important coverage
areas
■ Rod and grid reflectors are
sometimes added for mild directivity
Examples: 800 MHz.: dB803, PD10017,
BCR-10O, Kathrein 740-198
1900 MHz.: dB-910, ASPP2933
May 27, 1997
Power
Gain
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Gain,
dB
0.00
3.01
4.77
6.02
6.99
7.78
8.45
9.03
9.54
10.00
10.41
10.79
11.14
11.46
Angle
θ
n/a
26.57°
18.43°
14.04°
11.31°
9.46°
8.13°
7.13°
6.34°
5.71°
5.19°
4.76°
4.40°
4.09°
Vertical Plane Pattern
beamwidth
-3
d
B
RF Engineering 102 v1.0 (c)1997 Scott Baxter
θ
Angle
of
first
null
5A - 13
Sector Antennas
Reflectors And Vertical Arrays
■ Typical commercial sector
antennas are vertical combinations
of dipoles, yagis, or log-periodic
elements with reflector (panel or
grid) backing
• Vertical plane pattern is
determined by number of
vertically-separated
elements
– varies from 1 to 8, affecting
mainly gain and vertical plane
beamwidth
• Horizontal plane pattern is
determined by:
– number of horizontally-spaced
elements
– shape of reflectors (is reflector
folded?)
May 27, 1997
RF Engineering 102 v1.0 (c)1997 Scott Baxter
Vertical Plane Pattern
Up
Down
Horizontal Plane Pattern
N
W
E
S
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Example Of Antenna Catalog Specifications
Electrical Data
ASPP2933
1850-1990
3/5.1
<1.5:1
32°
Vertical
400
50
Direct Ground
N-Female
Order Sep.
ASPP2936
1850-1990
6/8.1
<1.5:1
15°
Vertical
400
50
Direct Ground
N-Female
Order Sep.
dB910C-M
1850-1970
10/12.1
<1.5:1
5°
Vertical
400
50
Direct Ground
N-Female
Order Sep.
Mechanical Data
Antenna Model
ASPP2933
Overall length - in (mm)
24 (610)
Radome OD - in (mm)
1.1 (25.4)
Wind area - ft2 (m2)
.17 (.0155)
Wind load @ 125 mph/201 kph lb-f (n)
4 (17)
Maximum wind speed - mph (kph)
140 (225)
ASPP2936
36 (915)
1.0 (25.4)
.25 (.0233)
6 (26)
140 (225)
dB910C-M
77 (1955)
1.5 (38)
.54 (.05)
14 (61)
125 (201)
6 (2.7)
13 (5.9)
ASPA320
5.2 (2.4)
9 (4.1)
Integral
Antenna Model
Frequency Range, MHz.
Gain - dBd/dBi
VSWR
Beamwidth (3 dB from maximum)
Polarization
Maximum power input - Watts
Input Impedance - Ohms
Lightning Protection
Termination - Standard
Jumper Cable
Weight - lbs (kg)
Shipping Weight - lbs (kg)
Clamps (steel)
May 27, 1997
4 (1.8)
11 (4.9)
ASPA320
RF Engineering 102 v1.0 (c)1997 Scott Baxter
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Example Of Antenna Catalog Radiation Pattern
■ Vertical Plane Pattern
• E-Plane (elevation plane)
• Gain: 10 dBd
• Dipole pattern is superimposed at
scale for comparison (not often
shown in commercial catalogs)
• Frequency is shown
• Pattern values shown in dBd
• Note 1-degree indices through
region of main lobe for most
accurate reading
• Notice minor lobe and null detail!
May 27, 1997
RF Engineering 102 v1.0 (c)1997 Scott Baxter
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