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Low-Power RF
Texas Instruments Incorporated
Selecting antennas for low-power
wireless applications
By Audun Andersen
Field Application Engineer, Low-Power Wireless
Table 1. Pros and cons for different antenna solutions
ANTENNA TYPES
PROS
CONS
Introduction
The antenna is a key component in an RF system and can
have a major impact on performance. High performance,
small size, and low cost are common requirements for
many RF applications. To meet these requirements, it is
important to implement a proper antenna and to charac-
terize its performance. This article describes typical
antenna types and covers important parameters to
consider when choosing an antenna.
Antenna types
Antenna size, cost, and performance are the most impor-
tant factors to consider when choosing an antenna. The
three most common antenna types for short-range devices
are PCB antennas, chip antennas, and whip antennas.
Their pros and cons are shown in Table 1.
PCB antennas
Designing a PCB antenna is not straightforward and usually
requires a simulation tool to obtain an acceptable solution.
In addition to deriving an optimum design, configuring
such a tool to perform accurate simulations can be diffi-
cult and time consuming.
PCB Antenna
• Low cost
Good performance is
possible
Small size is possible
at high frequencies
Difficult to design
small and efficient
antennas
Potentially large size
at low frequencies
Chip Antenna
Small size
Medium performance
Medium cost
Whip Antenna
Good performance
High cost
Difficult to fit in many
applications
Chip antennas
If the board space for the antenna is limited, a chip antenna
could be a good solution. This antenna type supports a
small solution size even for frequencies below 1 GHz. The
trade-off compared to PCB antennas is that this solution
will add materials and mounting cost. The typical cost of a
chip antenna is between $0.10 and $1.00. Even if chip-
antenna manufacturers state that the antenna is matched
to 50
Ω
for a certain frequency band, additional matching
components are often required to obtain proper
performance.
Figure 1. Typical antenna solutions
(b) Whip antenna
(a) PCB antenna
(c) Chip antenna
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Analog Applications Journal
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2Q 2008
Texas Instruments Incorporated
Low-Power RF
Figure 2. Radiation pattern
Whip antennas
If good performance is the most
important factor, and size and cost
are not critical, an external antenna
with a connector could be a good
solution. These antennas are often
monopoles and have an omni-direc-
tional radiation pattern. This means
that the antenna has approximately
the same performance for all direc-
tions in one plane. The whip anten-
na should be mounted normally on
the ground plane to obtain best
performance. For maximum econo-
my, a quarter-wavelength piece of
wire can provide an effective solu-
tion.
Antenna parameters
Some of the most important things
to consider when choosing an
antenna are: the radiation pattern,
antenna efficiency, and antenna
bandwidth.
Radiation pattern and gain
Figure 2 shows how the radiation
pattern from a PCB antenna varies
in different directions in the plane
of the PCB. Several parameters are
important to know when interpret-
ing such a plot. Some of these parameters are stated in
the lower left portion of Figure 2.
In addition to the plot information, it is important to
relate the radiation pattern to the positioning of the
antenna. Radiation pattern is typically measured in three
orthogonal planes, XY, XZ and YZ. It is possible to perform
full 3D pattern measurements, but it is usually not done
because it is time consuming and requires expensive
equipment. Another way of defining these three planes is
by using a spherical coordinate system. The planes will
then typically be defined by
θ
= 90°,
ϕ
= 0° and
ϕ
= 90°.
Figure 3 shows how to relate the spherical notation to the
three planes. If no information is given on how to relate
the directions on the radiation pattern plot to the position-
ing of the antenna, 0° is the X direction and angles
increase towards Y for the XY plane. For the XZ plane, 0°
is in the Z direction and angles increase towards X. For
the YZ plane, 0° is in the Z direction and angles increase
towards Y.
The gain, or reference level, usually refers to an isotropic
radiating antenna, which is an ideal antenna with uniform
radiation in all directions. When an isotropic antenna is
used as a reference, the gain is given in dBi or specified as
the effective isotropic radiated power (EIRP). The outer
circle in Figure 2 corresponds to 5.6 dBi and the 4-dB/div
label in the lower left means that for each progressively
0°
345°
15°
330°
30°
315°
45°
300°
60°
285°
75°
270°
90°
255°
105°
240°
120°
135°
225°
Gain = 5.6 dBi
Scale = 4 dB/div
Frequency = 2.44 GHz
Horizontal polarization
150°
210°
165°
195°
180°
Figure 3. Spherical coordinate system
Z
XY
= 90°
≥θ
≥
≥
≥θ
≥
≥
XZ
ϕ
ϕ
ϕ
ϕ
= 0°
YZ
= 90°
θ
Y
ϕ
X
smaller circle, the emission level is reduced by 4 dB.
Compared to an isotropic antenna, the PCB antenna will
have a 5.6-dB higher level of radiation in the 0° direction.
21
Analog Applications Journal
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High-Performance Analog Products
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Texas Instruments Incorporated
As shown by Equation 1, antenna gain, G, is defined as
the ratio of maximum-to-average radiation intensity multi-
plied by the efficiency of the antenna.
Bandwidth and impedance matching
Two common methods to determine antenna bandwidth
are: 1) measuring the radiated power while stepping a
carrier across the frequency band of interest, and 2) mea-
suring the reflection at the antenna feed point with a net-
work analyzer. Figure 4 shows the first method which is
measurement of radiated power from a 2.4-GHz antenna
that has approximately 2-dB variation in output power
across the 2.4-GHz frequency band and has maximum
radiation near the center of this band. This measurement
was done by stepping a continuous-wave signal from 2.3
GHz to 2.8 GHz. Such measurements should be performed
in an anechoic chamber to obtain a correct absolute level.
However, this measurement can be very useful even if an
anechoic chamber is not available.
Measurement in an ordinary lab environment can give a
relative result that shows if the antenna has optimum per-
formance in the middle of the desired frequency band.
The performance characteristics of the receiving antenna
being used to conduct the measurement will affect results.
Therefore, it is important that this antenna has approxi-
mately the same performance across the measured fre-
quency band. This precaution will help ensure that the
observed relative change in performance across the
measured frequency band is valid.
The second method to characterize antenna bandwidth
is to measure the reflected power at the antenna feed
GeD
P
P
P
P
U
U
rad
in
rad
in
max
,
=
×
=
×
D
=
×
(1)
avg
where U
max
is the maximum radiation intensity, U
avg
is the
average intensity, and the ratio of these two values is
known as directivity, D. Ohmic losses in the antenna ele-
ment and reflections at the antenna feed point determine
the efficiency, e, which is simply the radiated power, P
rad
,
divided by the input power, P
in
. High gain does not auto-
matically mean that the antenna has good performance.
Typically, mobile systems require an omnidirectional radia-
tion pattern so the performance will be about the same for
any antenna orientation. For an application where both
the receiver and the transmitter have fixed positions,
higher performance can be achieved when the antennas
are positioned to direct their high-gain lobes toward each
other.
To accurately measure an antenna radiation pattern, it
is important to measure only the direct wave from the
device under test and avoid reflecting waves that could
affect the result. To minimize picking up reflected energy,
measurements are often performed in an anechoic cham-
ber or at an antenna range. Another requirement is that
the measured signal must be a plane wave in the far field
of the antenna. The far field distance, R
f
, is determined by
the wavelength,
λ
, and the largest antenna dimension,
DIM, as shown by Equation 2. Since the size of anechoic
chambers is limited, it is common to test large, low-
frequency antennas in outdoor ranges.
Figure 4. Bandwidth of a 2.4-GHz antenna
2DIM
2
λ
10
(2)
R
f
=
–0.45 dBm
at 2.442 GHz
5
Polarization
Polarization describes the direction of the electric field. All
electromagnetic waves propagating in free space have
electric and magnetic fields perpendicular to the direction
of propagation. When considering polarization, the electric-
field vector is usually described and the magnetic field is
ignored because it is perpendicular and proportional to
the electric field. To obtain optimum performance, the
receiving and transmitting antenna should have the same
polarization. In practice, most antennas in short-range
applications will produce a field with polarization in more
than one direction. Reflections change the polarization of
en electromagnetic wave. Since indoor equipment experi-
ences a lot of reflections, polarization is not as critical as it
is with equipment operating outside with line-of-sight
limitations.
0
–5
–10
2.4-GHz
Band
–15
–20
–25
–30
–35
–40
2.2
2.34
2.48
2.62
2.76
2.9
Frequency (GHz)
22
Analog Applications Journal
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Texas Instruments Incorporated
Low-Power RF
point. Disconnecting the antenna and connecting a net-
work analyzer with a coax cable to the antenna allows
such a measurement. The bandwidth of an antenna is typi-
cally defined as the frequency range for which the reflec-
tion is lower than –10 dB or the VSWR is less than 2. This
is equivalent to the frequency range where less than 10%
of the available power is reflected by the antenna. More
information about reflection measurements can be found
in Reference 1.
Size, cost and performance
The ideal antenna is infinitely small, has zero cost and has
excellent performance. In real life, however, a compromise
between parameters is necessary. For example, decreasing
the operating frequency by a factor of two can double the
RF range. Thus, one of the reasons to operate at a lower
frequency is often to achieve longer range. The down side
is that most antennas need to be larger at lower frequen-
cies to achieve good performance. In some cases where
the board space is limited, a small, efficient high-frequency
antenna may provide equal or greater range performance
than a small, inefficient low-frequency antenna. A chip
antenna is good alternative when seeking a small antenna
solution. This is particulary true with frequencies below
1 GHz because the chip antenna will allow a much smaller
solution than the traditional PCB antenna. The main draw-
backs with chip antennas are the increased cost and typi-
cally narrow-band performance.
Antenna reference designs
Texas Instruments (TI) offers a wide range of RF products
that are design to operate in license-free frequency bands.
The newest products consist of the CC11xx, CC24xx, and
CC25xx families. TI also offers several antenna reference
designs. Each reference design includes documentation of
the antenna dimensions and the measured performance.
Since the size and shape of the ground plane affects
antenna performance, implementing the reference designs
on a PCB with different ground-plane shapes and sizes
may produce slightly different results. It is important to
carefully copy the exact dimensions of the antenna to
obtain optimum performance. No ground plane or traces
should be placed beneath the antenna. Reference 2 gives
an overview of available antenna reference designs and
provides links to the relevant documentation.
References
For more information related to this article, you can down-
load an Acrobat Reader file at www.ti.com/litv/pdf/
litnumber
and replace
“litnumber”
with the
TI Lit. #
for
the materials listed below.
Document Title
TI Lit. #
1. DN001, “Antenna measurement with network
analyzer” . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
.swra096
2. AN058, “Antenna Selection Guide” . . . . . . . . .
.swra161
Related Web sites
RF/IF and ZigBee
®
Solutions:
www.ti.com/lpw
Low-Power RF Selection Guide:
www-s.ti.com/sc/techlit/slab052
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