Antenna Design For
Wireless Products
Kerry Greer
Vice President of Engineering
SkyCross Inc.
February 25, 2002
©2002 SkyCross. All Rights Reserved.
Topics
• Antenna Evolution
• Changing Wireless Marketplace
• Issues Driving Antenna Design
• Antenna Physics
• Antenna Performance Specifications
• External Antennas
• Embedded Antennas
• Advanced Wireless Device Antenna Concepts
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Antenna Evolution
•
•
Antennas Have Always Been the Part That Makes a
Wireless Device Wireless
Have Traditionally Been External, Connectorized
Components
- Misunderstood, considered “black magic”
- Gangly, obtrusive
- Added on at the end of the design
•
Antennas for Mobile Devices Have Evolved Since
Their Introduction
- Whips  Retractables  Stubbies  Embedded
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Changing Market
•
•
Antennas are Slowly Becoming More and More Integral,
as More and More Devices are
Adding Wireless Capability
OEMs/Consumers Are Demanding
More from Their Wireless Devices
-
•
Smaller
More functionality
More power
Improved performance
FCC Mandate That All Cell Phones
Have E-911 Capability
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Wired/Wireless Networks of Today
RCA
Real-Time
Video
GPS
DVD/TV/VCR
Auto
Cable
Ethernet
PCS
Mobile
Network
Mobile Data
Devices
PC / Server
USB
PCS
Device
Printer
Mobile
Devices
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DSL
Broadband
Internet
Service
Digital
Camera
Wired
Devices
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Future of the Wired/Wireless World
Wireless Legend
Black: Multi-Mode
Orange: Single Mode
Auto
GPS
3G
WWN
DVD /
HDTV
802.11
WLAN
BT
WPAN
Mobile Data
Devices
3G Mobile
Devices
PC / Server
OC3
WAN
Broadband
Digital Data
(Fiber/FWA)
SODA
Consumer
POS
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Digital
Camera
Printer
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Real-Time
Video
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Issues Facing Antenna Design
•
•
•
Traditional Cost vs. Performance Tradeoffs
Three Different Groups Have Three Different
Sets of Priorities (OEMs  Service
Providers  Consumers)
Antenna Considerations (OEM Priority)
1)
2)
3)
4)
Cost
Size
Performance
Multiple operation modes
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How Much is Better Performance Worth?
Churning
Total Subscribers
Churn in the US exceeds 200% of new subscribers
187,679
20 million new
subscribers
200000
180000
166,158
144,804
160000
123,740
140000
103,418
120000
83,924
100000
76,948
80000
64,801
60000
40000
25,177
34,127
43,309
53,577
20000
0
1999
2000
2001
2002
Cost to sign-up new customer $350 - $400
2003
2004
Source: Cahners in-stat
43% of Subscribers Change Carrier Because of Coverage Problems*
Poor Coverage is worth: (43,309)*($350)*(0.43) = $6.5M / year
* Source: Yankee Group
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1. Cost
•
Antennas are Viewed as Components, with
Traditional Price Points:
- External whip/monopole assemblies: $1 - $2
- Internal/embedded antennas: ~$0.50
•
OEMs Very Reluctant to Consider Higher-priced
Antennas for Existing Applications
- Regardless of Performance Gains
• Alternative: Provide Multiple Functionality
- Example: 3 antennas in 1 (Cell/PCS/ISM)
- $3 for 1 multi-band antenna is better overall choice
than 3 separate antennas for $1 each
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2. Reduced Size
•
•
With the Evolution of Wireless Devices the Antenna
has Been Forced to Reduce in Size
Some Size Reduction Has Been Natural Result of
Physics as Frequencies Increase
- Example: commercial radio  broadcast television 
analog cellular  digital PCS  wireless data
•
But Further Reduction in Size Introduces New
Complexities
- Must continue to increase performance
- But, must maintain a minimum certain size in order to
meet bandwidth and energy requirements
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3. Performance
•
•
Better Performance is Usually Achieved
by Increased S/N in the Wireless Link
- Performance improvements can be realized
by higher gain antenna (if beam is properly
focused)
• Example: Want horizontal beam for cell
phone, zenith beam for GPS
Increased Gain Can be Used in Different
Ways
- Better cell coverage area
• Increase cell size / range
• Given all mobiles at max power, then
less dropouts
- Less battery power
• Given strong signal area, then reduced
Tx Battery
• Especially critical in CDMA networks
- Some combination of above
R1
R2
Increase Cell Coverage
R1
Reduce Dropouts
Reduce Battery Size
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4. Multiple Operating Frequencies/
Modes
•
•
RF Spectrum is Allocated by Governments
Worldwide
Operating Modes and Frequencies Are Not the
Same Thing
- Example: US “Tri-mode” cell phone is really a
dual-mode/dual-frequency phone
• AMPS at 800, digital at 800, and digital at 1900 MHz
Dig TV
470
Cellular/GSM
806
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960
GPS
1550
PCS/IMT-2000
1610 1710
Audio ISM
1990 21102200 2335 2400
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12
2500
UNII
5150
ISM2
53505725
5875
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Antenna Physics
•
Antenna is Fundamentally a Transmission Line
- Electrical energy is converted to Electromagnetic
•
•
Radiation
f = c/l
- as frequency goes up, wavelength gets shorter
Loosely Speaking, Radiation Occurs Anywhere
There is a Change in an Electric Current’s
Velocity (Speed and/or Direction)
- Consequently, antennas come in all forms of
curved, bent and folded metal shapes designed to
alter current velocity or density
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Antenna Performance Specifications
•
•
Antenna is Traditionally Evaluated for
Performance Under These Criteria
A.
B.
C.
D.
E.
Frequency
Gain / directivity
Return Loss / VSWR
Bandwidth
Impedance
Today Additional Antenna Performance
Parameters Must be Evaluated
F. Efficiency
G. Volumetric size
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A. Frequency of Operation
•
•
•
Electromagnetic Spectrum Is Measured in Terms
of Frequency
Most Antennas Transceive Over a Narrow
Frequency Range Which Is Usually ~10% of the
Center Frequency
For Antennas, This Includes Both the Uplink and
Downlink Frequencies (if duplex)
- Example: (US PCS Tx and Rx) = 1850 MHz to 1990
MHz
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B. Gain & Directivity
•
•
Combined parameter that
characterizes the actual
performance achievable in a “real”
antenna
Antenna gain is expressed as the
ratio of the power transmitted by an
antenna in a given direction and the
power that would be transmitted in
that direction by a perfectly efficient
isotropic radiator (spherical) in that
direction
- Isotropic is uniform 0dBi, by
definition
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Perfectly Efficient
Isotropic Radiator
(3D Sphere)
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Units of Antenna Gain - dBi
•
•
•
Ratios of power are often expressed in decibels (dB)
and are a comparison to some other known reference
value. For instance, power referred to 1 watt is
indicated as dBw. In the case of antennas, the unit is
dBi since the reference measurement is
relative to an isotropic radiator (i for isotropic)
dBi = 10 x log(Pa/Pi), where Pa is the power
transmitted by the antenna in a given direction,
and Pi is the power that would be transmitted
by a perfectly efficient isotropic (spherical)
radiator in that same direction
Gain values >0dBi indicate that the antenna emits
more power in that direction than the theoretically
perfect isotropic radiator. Values < 0 dBi indicate that
the antenna emits less power in the given direction
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> 0 dBi
<0 dBi
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Peak Gain
• Peak gain is the value of the gain in the
•
•
direction in which the most power per
unit area (flux) at a fixed distance or the
most power per unit of solid angle is
radiated by the antenna
A narrow beam antenna will have a peak
gain much greater than 0dBi, assuming
it is a highly efficient radiator
A broad beam or omni-directional
antenna will have a peak gain around 0
dBi, because its power is spread out
over a large area of the radiation sphere
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Peak
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Average Gain
•
•
•
As with peak gain, average gain for an antenna is also
expressed in dBi, because it is referenced to a perfectly
efficient isotropic radiator
The highest possible average gain for an antenna is 0
dBi, because the total power emitted by an antenna can
never be greater than the total power emitted by a
perfectly efficient radiator. This assumes the averaging
is done over the entire 4p steradian volume—the entire
sphere
Often average gain is expressed only over a portion of
the sphere, such as a 2D planar cut in azimuth
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C. Return Loss / VSWR
•
•
•
Difference between the power input to and
the power reflected back from a discontinuity
in a transmission circuit
In a perfectly matched transmission system,
there are no standing waves and the Voltage
Standing Wave Ratio (a ratiometric measure
of the crest to null of the voltage standing on
the line) is 1:1
Antennas having VSWR less than 3:1 are
acceptable for receive applications and low
power transmission,with 2:1 being very good
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D. Bandwidth of Operation
•
Amount of Spectrum Needed for a Particular
Communications Channel or Group of Channels
- Defined in units of frequency and is computed as the
•
•
difference between an upper and lower band edge limit
- Example: (PCS)
• 1990 – 1850 = 140 MHz BW
• 140 / 1920 (center) = 7.3 % BW
- Channel Bandwidth is usually much less than total allocated
bandwidth
Narrowband Antennas
- Operate only on the band of frequencies for which the device
was intended
Broadband Antennas
- Tend to perform less effectively than narrowband antennas but
provide multiple frequency integration
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E. Input Impedance
•
Antenna Input Impedance Is Traditionally
Specified at 50 Ohms for Most Antenna Devices
- Many not necessarily be optimal, but provides
easy test / debug via standard coax test
equipment
- OEMs have begun to consider lower impedances,
primarily driven to better match the output of the
transmitting power amplifier (which is where most
of the battery power is consumed)
•
75 Ohms Typical for Video Equipment
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F. Efficiency
•
Direct Measure of How Well an Antenna
Transforms Onboard Electrical Energy Into
Transmitted Signal Energy
- 100% efficient antenna would theoretically convert
all input power into radiated power, with no loss to
resistive or dielectric elements
- Most all antennas in use exhibit at least 50%
efficiency, with 70%-80% being very good designs
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G. Volumetric size
•
Volumetric Size of the Best-designed Antenna
Is Ultimately Limited by Theoretical
Considerations That Depend on the Maximum
Bandwidth Over Which the Antenna Must
Operate
- Some antennas exhibit a smaller occupied
volume for a given degree of performance
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Wireless Device Antenna Choices
Loop
PIFA
Patch
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Monopole
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MLA
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Monopole Type Patterns
• Monopole Pattern Is a
Torus-shape
• PIFA and Loop Patterns
Are Similar, Only Beam
Is Directed in Different
Ways
This Antenna Has Gain of About 1
dBi Gain From 75 to 120 Degrees of
Elevation (Referenced From Zenith
Above)
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Patch Pattern
• Patch Pattern Is
Uni-directional
• Projects a Cone Upward
From the Surface of the
Patch
This Antenna Has Gain of About 4
dBi Peak Gain Focused in One
Direction
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MLA and Other Patterns
•
Meander Line Antenna (MLA)
- Has both monopole and patch radiating modes,
depending on operating frequency
•
Monopole, Loop, PIFA and MLA May All Have
Similar Beam Patterns
- Depending on the size of the antenna relative to
the wavelength of the signal, loop
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Embedded Antenna Considerations
•
•
Include Antenna Designer as Part of the Design Team
from Beginning
Critical Considerations
- Identify all frequency bands (GPS later ?)
- Identify multiple-frequency RF front-end architecture
-
-
• Number of antenna feed points
• Diplexers, Tx/Rx paths, filtering needed
Identify orientation
• Desired beam pattern under consumer usage scenario
• Problem: PDA is used as both handheld and desktop
Plan and allocate for internal volume needed to achieve
desired level of performance
Identify upfront the grounding scheme and adjacent metal
surfaces
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Multi-band or Wideband?
•
•
•
How Many Antennas are Needed
for a 4-Band Device?
- Ex: 800, 1575, 1900,
2450 MHz
Multiple Band Antennas
- A 2-band is feasible, 3-band
or more is difficult to achieve
adequate BW in all needed
bands
Wideband Antennas
- More aesthetic
- Easier to manufacture
- Needs detailed systems
work to determine filtering,
interference and isolation
issues
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Wideband Antenna VSWR
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Advanced Concepts in Antenna
Design
•
•
•
Higher Data Rate Wireless Links
Diversity Combining
Multiple Input Multiple Output (MIMO) Systems
2G
GSM
2.5G
GPRS
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N-CDMA
EDGE
TDMA
CDMA
3G
1xRTT
HDR
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CDMA 2000
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Higher and Higher Datarates
•
•
•
The Convergence of Wireless Access and
Broadband Internet Surfing Has Created a
Demand for Low BER, High Datarate Wireless
Systems
Interference Dominates System Capacity
Implying That Increasing Radiated Power at the
Source Is Not the Solution
The Electromagnetic Spectrum Is Limited and
Efficient Usage Is Required From a Economic
Perspective
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The Road Ahead
• Improved Spectral Efficiency Is Achievable in
Environments of Interest for Commercial Wireless
Services
- Multipath characteristics of the channel determine the
improvement in spectral efficiency
- Published test results indicate that very small separation
provides measurable improvements at mobile terminals
• As a Result, Multiple Antenna Diversity Is Being
Introduced Into the Standards for Mobile 3G
Systems (UMTS, EDGE) As Well As Indoor Systems
(802.11 WLAN)
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Diversity Methods
•
To Mitigate These Problems, Diversity Techniques are Being
Developed
- Spatial Diversity is a widespread technique based on the use of an
array of sufficiently spaced antennas at either transmitter or receiver
- Polarization Diversity takes advantage of existence of statistical
independence of different polarization states as a wave is scattered
in the environment
- Pattern Diversity uses the antenna itself to select angularly diverse
components of the scattered wave to mitigate fading and increase
SNR
- Temporal Diversity exploits the multidimensionality of the channel to
improve SNR
- MIMO uses antenna arrays at both transmitter and receiver—
multipath propagation can actually be exploited to establish multiple
parallel channels
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Diversity Improvements
•
There is Strong Evidence That One or More Forms
of Diversity can Improve the Channel Performance
of a Radio by Several dB
- Cho, et al. 7 to 8 dB gain due to polarization
• A combination of height (spatial) and polarization
diversity provide a robust scheme
- Braun, et al. 9 dB with a two antenna configuration
- Dietrich, Stutzman,et al. Also 5 to 7 dB
- Many other references in
• Bibliography at GLOMO Project VPI
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Directional (Pattern)
• To Date, WLAN OEMs Have Not Exploited Directional
- Traditional WLAN systems have relied on spatial only
- Why not use 2 different WLAN antennas with higher peak
gains focused in different directions?
• Example: 1 antenna omni-directional, 1 antenna patch diversity
combines to better total hemi-spherical coverage
• “Smart Antennas”
•
- Discriminate multi-path components
- Process them separately
Butler Matrix
- If the signals from different directions can be processed
separately, co-channel interference can be suppressed
- Improve transmission quality and/or capacity
- Can reduce delay spread
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Wireless Device Considerations for MIMO
•
Multiple-Input, Multiple-Output (MIMO)
•
Size Limited, Two Antennas Is About the Maximum
Allowable in a Handheld Device
Can Use Embedded Antenna as the Second One
Can Exploit Polarization Diversity
•
•
•
•
- Diversity Transmit and Diversity Receive
- Second polarization for the whip antenna
- Dual polarization antenna for the hidden antenna
Treat Polarization Terms as Spatial Term
Beam-formed Elements Again Treat Them as
Spatial Terms and Process the Same Way
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