TECHNICAL WHITE PAPER
TECHNICAL
WHITE PAPER
Antenna design for end
products operating on
the SIGFOX network
NOTICE: The contents of this document are proprietary of SIGFOX
and shall not be disclosed, disseminated, copied, or used except for
purposes expressly authorized in writing by SIGFOX.
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TECHNICAL WHITE PAPER - Antenna design for end products operating on the SIGFOX network
ABSTRACT
I
n this document, several aspects of antenna design and integration are developed. The
document focuses on SIGFOX IOT application and small form factor devices.
The importance of implementing a high-performance antenna in a device is demonstrated by
analyzing the SIGFOX radio link budget.
Theories are then developed to explain why antennas can be extremely sensitive to their
environment and may therefore have varying performance (e.g. when held compared to standing
alone). Using the same arguments, it is shown that the same antenna can have different
behavior when implemented in two distinct device architectures. This leads to an initial finding
that for each device development, a specific antenna design, or at least antenna tuning, must
be carried out.
Then, the most important physical parameters that impact antenna characteristics and behavior
are presented. Several typical antenna topologies, miniaturization techniques, and manufacturing
techniques applicable to SIGFOX IOT applications and small form factor devices are listed.
Advice regarding off-the-shelf antennas is given to explain why antenna performance depicted
in datasheets may differ from “real-life implementation” performance.
A selection tool will help the reader to select the best antenna topology for the device, based
on several parameters such as expected performance, integration or miniaturization level, and
design complexity.
To close the document, a simple method that roughly estimates devices radiated performance
is given. This method does not involve very expensive equipment, such as anechoic chambers,
and is relatively easy to set up.
DISCLAIMER
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such individual is acting) (“You” and “Your”) by SIGFOX SA, on behalf of itself and its affiliates
(“SIGFOX”) for informational purposes only.
• You are responsible of making Your own assessments concerning the Information and are
advised to verify all representations, statements and Information contained in this White Paper
before using or relying upon any of the Information.
• SIGFOX is providing the Information to you “AS IS”.
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arising from Your use of or reliance upon the Information.
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merchantability, fitness for a particular purpose, satisfactory quality, or non-infringment.
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be liable for any direct, indirect, incidental, special or consequential damages, howsoever
caused and whether or not such losses are foreseeable, even if that party has been advised
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or other intangible losses, resulting from the use of the white paper
© Copyright SIGFOX. All rights reserved
3 / 40
TABLE OF CONTENTS
Purpose of the document
6
Acronyms
6
Reference documents
6
1. Introduction
7
2. Antenna: a critical component for a device operating on the SIGFOX network 8
• 2.1 SIGFOX Radio link budget
8
• 2.2 Link between SIGFOX classes and antenna performance
8
3. Antenna: a very sensible component
9
• 3.1 A little bit of theory
• 3.2 Sensitivity to the environment
10
• 3.3 Sensitivity to the design integration
10
4. Antenna design technical considerations
12
• 4.1 Antenna parameters that matter
12
• 4.2 The earlier the better
13
• 4.3 Some antenna topologies
13
• 4.4 Miniaturization technics
18
• 4.5 Antenna manufacturing technologies
20
• 4.6 Advices on off-the-shelf internal antenna integration
22
5. Antenna topology selection Tool
9
24
• 5.1 External antennas
24
• 5.2 Integral antennas
25
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TECHNICAL WHITE PAPER - Antenna design for end products operating on the SIGFOX network
6. Conclusions
27
7. Appendix A: Antenna parameters definitions
28
• 7.1 VSWR and Return Loss
28
• 7.2 Efficiency
29
• 7.3 Directivity
29
• 7.4 Gain
29
• 7.5 EIRP and ERP
30
• 7.6 Antenna bandwidth
31
• 7.7 Polarization 32
• 7.8 Radiation pattern
33
8. A ppendix B: A method to assess your antenna design without using
expensive equipment
34
• 8.1 Measurement procedure
35
• 8.2 Measurement post-processing
37
9. Appendix C: RF Anechoic chamber
37
© Copyright SIGFOX. All rights reserved
5 / 40
PURPOSE OF THE DOCUMENT
The purpose of this white paper is to help device manufacturers with antenna design and
integration. The document is targeting those new to RF/antennas, and will give them some
insights in antenna design.
This document will mainly highlight good practices in antenna design and help the reader
understand and avoid the usual mistakes in integrated antenna design.
ACRONYMS
Below are the definitions of acronyms used in this document.
ACRONYM
DEFINITION
ERP
Effective Radiated Power
EIRP
Effective Isotropic Radiated Power
DUT
Device Under Test
RF
Radio Frequency
ETSI
European Telecommunications Standards Institute
FCC
Federal Communications Commission
TX
Transmission (RF)
RX
Reception (RF)
IOT
Internet Of Things
PCB
Printed Circuit Board
ABS
Acrylonitrile Butadiene Styrene
PC
Poly Carbonate
PIFA
Planar Inverted F Antenna
CW
Continuous Wave
VSWR
Voltage Standing Wave Ration
RL
Return Loss
RCZ
Radio Configuration Zone
CATR
Compact Antenna Test Range
REFERENCE DOCUMENTS
REF. NUMBER
DOCUMENT REFERENCE
DOCUMENT NAME
[REF1]
EM Device classification – Class of devices under SIGFOX network
[REF2]
Chu, L. J. “Physical limitations of omni-directional antennas”, Journal
Applied Physics 19: 1163–1175. doi:10.1063/1.1715038, Dec.1948
[REF3]
Antenna Theory, Analysis and Design, second edition, C. A. Balanis.
[REF4]
Metamaterial-Inspired Efficient Electrically Small Antennas A. Erentok, R. W. Ziolkowski.
[REF5]
CTIA Test Plan for Wireless Device Over-the-Air Performance V3.5.2
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TECHNICAL WHITE PAPER - Antenna design for end products operating on the SIGFOX network
1 - INTRODUCTION
T
he SIGFOX network (Figure 1) is dimensioned to cover a whole territory with a good
quality of service for outdoor and light indoor applications. In the case of devices
placed in a difficult environment or operating at the edges of the SIGFOX coverage
area, devices’ radiated performance is critical to ensure good connectivity to the SIGFOX
network. In any case, devices presenting high radiated performance will derive full benefit
from the SIGFOX network. The antenna is therefore a key element in an end product design,
since it is the component that will convert conducted signals into electromagnetic waves.
Public
internet
Cloud
Base-stations
Devices
Figure 1 – The SIGFOX network topology
This document is meant to help device makers in their antenna design in order for them
to design best-in-class devices with the highest possible radiated performance. Several
aspects of antenna design and integration are presented in a simple way, starting from the
understanding of antenna parameters and descriptions of several antenna implementations,
to a device radiation performance evaluation method.
Antenna implementation can be fairly straightforward in the case of an external antenna,
while it can become very tricky in the case of highly integrated and small antenna designs.
Indeed, unlike regular electronic components, antennas are impacted by their close
environment; thus, their implementation can be difficult.
This document will mainly focus on the case of small devices requiring small and/or integrated
antennas and will only present the useful related antenna theory.
© Copyright SIGFOX. All rights reserved
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2 - ANTENNA: A CRITICAL COMPONENT FOR A DEVICE
OPERATING ON THE SIGFOX NETWORK
2.1 SIGFOX Radio link budget
The SIGFOX network is dimensioned based on a given radio link budget, assuming a certain
radiated power level from the devices as described in [REF1]. It means that the SIGFOX
network is dimensioned for devices with good radiated performance.
Devices with low radiated performance may be able to operate under the SIGFOX network,
but will have some limitations due to weaker coverage.
A SIGFOX classification [REF1] is used to let the end product makers know:
• What class they should target according to their application and coverage needs
• What class their device belongs to and thus what coverage they can expect
2.2 Link between SIGFOX classes and antenna performance.
As part of the SIGFOX Ready TM certification for end products, the maximum ERP (Effective
Radiated Power) of the Device Under Test (DUT) is measured to define the quality of service
it will restore while operating the SIGFOX deployed network. As shown in the equation
below, the result of this test is mainly impacted by the antenna design quality, since all
SIGFOX Ready TM RF chipsets provide same limited RF conducted power depending on the
operation zone:
ERPdBm= Conducted_RF_Power dBm + Antenna_Gain dBm
Equation 1
Based on this ERP measurement, the class of device is determined as presented in Table 1.
This table summarizes the link between SIGFOX classes of the end products and their
antenna gain. The antenna gain computations assume an available RF power of 14dBm in
ETSI (RCZ 1) regions and 22dBm in FCC (RCZ2) regions at the antenna port. This table is
to be updated with other RCZ when available.
Table 1 - Uplink SIGFOX device classes definition for RCZ1 and RCZ2
RCZ 1
RCZ 2
ERP ETSI
(dBm)
EIRP ETSI
(dBm)
Antenna gain
(dBi)
ERP FCC
(dBm)
EIRP FCC
(dBm)
Antenna gain
(dBi)
Class 0u
>12
>14.15
>0.15
>20
>22.15
>0.15
Class 1u
>7
>9.15
>-4.85
>15
>17.15
>-4.85
Class 2u
>0
>2.15
>-11.85
>5
>7.15
>-15.85
Class 3u
<0
<2.15
<-11.85
<5
>7.15
<-15.85
Note: It is possible to increase the conducted RF power at the antenna port to reach the 0u class with a nonoptimized antenna. However, this only works for devices using SIGFOX uplink connectivity. For devices using
both uplink and downlink connectivity, this action will lead to an unbalanced link budget, and will be noticed
by the end user, who will be able to send messages in areas where he will not be able to receive messages.
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TECHNICAL WHITE PAPER - Antenna design for end products operating on the SIGFOX network
It must be noted that SIGFOX Ready TM certification for end products cannot replace ETSI
or FCC certification, which are regulatory and mandatory certifications. Both SIGFOX and
regulatory certifications should be completed.
3 - ANTENNA: A VERY SENSITIVE COMPONENT
In this section, an explanation of why antennas are sensitive to their surroundings, and why
they should be treated differently from other electronic components, is given.
3.1 A little bit of theory
■ What is an antenna?
A conductive structure in which an alternating electric current flows (RF signal) generates
electric and magnetic fields. If the generated fields are strong enough, they will manage
to “escape” from the structure into free space and become radiated fields. The strength of
those fields depends, of course, on the electric current magnitude, but also on the structure’s
dimensions and characteristics.
An antenna is such a structure, dimensioned so that most of the generated fields escape
the close surrounding area of the antenna, called reactive area, and become radiated
fields. The optimal dimensions of an antenna can vary depending on its type (radiation
mechanism). For instance, in the case of a dipole, the antenna size will be optimal when it
reaches half a wavelength at operating frequency.
An antenna is a reciprocal device, which means that it has the same properties when
transmitting as when receiving. Assuming an antenna in TX mode can generate a known
electromagnetic power density for a given electrical current, then, the same electromagnetic
power density will generate the same amount of electric current in this antenna used in Rx
mode.
■ Antenna reactive and far field regions
In order to understand why an antenna is a sensitive component, it is important to know that
the space around an antenna can be divided in two distinct areas (Figure 2):
• The near field area, where the reactive fields (looping back to the antenna) predominate,
and where an object will disturb the antenna’s behavior and characteristics. In this area,
the electric and magnetic fields’ orientation and intensity depend on the antenna type. If
those fields are disturbed, the antenna’s behavior and properties are impacted.
• The far field area, where radiating fields (not going back to the antenna) predominate,
and where an object will not disturb the antenna. It will only disturb the propagation of
the electromagnetic waves coming from the antenna, not the antenna’s properties. The
2D^2
far field area starts at a distance of λ from the antenna, where D is the antenna’s
maximum dimension, and λ the wavelength at operating frequency.
An intermediate area called the radiating near field area is also described in the literature.
© Copyright SIGFOX. All rights reserved
9 / 40
This is the area where both near and far fields coexist.
Nearfield
FarField
Transmiter
Boundary
Figure 2 - Near field and far field area illustration
3.2 Sensitivity to the environment
As previously explained, an object will disturb an antenna when placed in its near field area.
This is the reason why a wireless device will have a distinct radiated performance depending
on its environment. For instance, a device placed against a wall will not behave as if it were
in free space or held by a user. Therefore, it is important to consider the different use cases
when performing the integration of an antenna.
It should be noted that each antenna topology has a different level of sensitivity to its
environment. In fact, depending on their radiation mechanism, they can be more or less
sensitive to their nearby environment.
For instance, small loop antennas are usually less sensitive to the body than more classic
monopole-type antennas. However, small loop antennas are also much less efficient than
monopole-type antennas. In the case of a hand-held device, the choice of antenna topology
will be a tradeoff between pure efficiency and sensitivity to the hand user.
3.3 Sensitivity to the design integration
■ Sensitivity due to the antenna’s surroundings
For the same reason that an antenna is sensitive to a nearby object, it will be also sensitive
to the other device components (e.g. batteries, screws, RF shielding) as well as the device
casing. This is a key point, because it means that the antenna must be designed and
integrated in consideration of every component of the end product. In fact, an antenna
designed with no regard for the device’s mechanics will not show optimized behavior once
placed in its end environment.
This is also one of the reasons why off-the-shelf antennas often need some additional
matching components in order to be re-tuned to their end environment, and may not perform
as described in the datasheet.
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TECHNICAL WHITE PAPER - Antenna design for end products operating on the SIGFOX network
■ Sensitivity due to the antenna topologies
In the case of IOT (Internet Of Things), small form factor devices predominate, which means
that antennas need to be as small as possible to allow their integration inside small devices.
In that case, bulky half wavelength antennas such as dipoles are avoided, and quarter
wavelength or smaller antennas are better options. The problem with this second antenna
type is that, unlike half wavelength antennas, they need and use the other metallic parts of
the device, such as the PCB ground plane, to radiate. This unfortunately implies that the
whole device somehow becomes the antenna and that the size of the ground plane will
impact the device’s radiated performance and antenna characteristics.
In Figure 3, the case of a canonical monopole mounted over a large ground plane is
illustrated. The figure shows that the currents are flowing through the whole ground plane
even far from the antenna. Those currents participate in the antenna’s radiation mechanism.
Thus, if the ground plane is reduced, the current distribution will be modified, as will the
antenna’s radiation and electrical properties.
This is true for many antenna topologies using a ground plane in their radiation mechanism.
Figure 3 - Example of a monopole surface current distribution
Figure 4 shows the impact of reducing the size of the PCB on a folded monopole antenna
performance. This is a typical antenna implementation for a small SIGFOX device. A small peak
efficiency drop is noticeable, as well as significant operating frequency bandwidth reduction.
0
0,7
0,8
0,9
1
1,1
80mm
Total efficiency (dB)
-5
-10
Small PCB
Large PCB
-15
20mm
-20
Folded
monopole on a
large PCB
-25
Freq (GHz)
Folded
monopole on a
small PCB
Figure 4 - PCB size impact on antenna performance
© Copyright SIGFOX. All rights reserved
11 / 40
In the case of a small device, the ground plane size and shape are often dictated by the
device’s form factor and size, and are not optimized for the selected antenna. Several
antenna parameters are impacted by a non-optimal ground plane, such as:
• Frequency Bandwidth (reduced)
• Antenna radiation efficiency (lowered)
• Radiation pattern (modified – usually less directivity with a small ground plane)
Consequently, an antenna must be designed or at least tuned specifically for each
design.
4 - ANTENNA DESIGN: TECHNICAL CONSIDERATIONS
In this section, several technical points regarding antenna design and integration are
developed, such as basic antenna topologies, miniaturization techniques, and manufacturing
technologies.
4.1 Antenna parameters that matter
Here is a short list of important parameters to consider during antenna design:
• Antenna volume: The volume of an antenna directly limits its bandwidth and radiation
efficiency (limits defined by Chu and Harrington [REF2]). Thus, enough volume should be
reserved for the antenna. The antenna is actually a component that often takes up a large
percentage of a wireless device total volume.
• Antenna location: The location of an antenna is critical and will have an impact on almost
all antenna parameters. The best location will depend on the antenna’s topology. There is
no universal law for this parameter.
• Casing material: Materials used to manufacture the device casing can be critical for
the antenna’s performance. This is even more critical for small antennas. If the dielectric
material properties are available, it is better to choose a material with low permittivity
and dielectric losses that are as low as possible. Plastic such as ABS and PC are very
common and are appropriate materials for casing.
• Metal casing: In the case of integral antennas, partially metal casing can be an option,
though the antenna’s integration will become more challenging. In most cases, a metal
casing will have to be connected to the antenna’s ground plane to avoid unintentional
resonances, which may lead to a decline in performance.
Full metal casings should be avoided in the case of integral antennas.
•C
omponents around the antenna: The best way to disturb an antenna is to place a
metallic part next to it. Antenna performance can be impacted by a metallic structure lying
next to it. Therefore, it is best to avoid placing large electronic components (e.g. batteries,
cameras, speakers) close to the antenna, or to take them into account while designing or
tuning the antenna. The device’s mechanical layout should be frozen before the antenna’s
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TECHNICAL WHITE PAPER - Antenna design for end products operating on the SIGFOX network
final tuning (any late changes can impact the antenna’s tuning).
At some point during the design and test phases of your device’s antenna, you will need
to build a prototype. It is important to have a prototype that is made of the same materials
and uses the same manufacturing process as the end product. A casing built using a 3D
printer, for instance, may have different electrical properties than the final version using a
mold (even if the same material is used due to different plastic density). It may be a good
idea to build a first prototype with a 3D printer, to evaluate the achievable performance, but
remember to check the first production devices and retune the antenna if needed.
4.2 The earlier the better
As soon as a wireless device design starts, the antenna should be considered,
especially if it is the SIGFOX highest device class with integral antenna. Indeed, the earlier
the antenna design is taken into account, the easier it will be to find the antenna topology
that best fits your device in terms of implementation and performance.
Antenna performance should also be estimated as soon as possible if very high performance
is expected.
If the antenna design starts too late, three things can happen:
• The antenna works well in your device design without modifying it (unlikely to happen).
• The antenna does not work well, but you can make some design changes to achieve target
performance, meaning there will be some delays in your schedule.
• The antenna does not radiate enough, and you cannot modify your device design, thus,
you will not achieve the expected radiated performance.
A hardware update of an already existing product can sometimes be a good opportunity
to add some functionalities such as SIGFOX connectivity. In that case, a specific antenna
implementation study must be carried out to find a good antenna topology that fits the
device design and evaluate the achievable radiated performance.
4.3 Some antenna topologies
In this section, a few basic antenna topologies are presented, on which most modern
integrated antenna topologies are based.
A detailed description of these antenna topologies can be found in [REF3].
■ Dipole antenna
Often consisting of two straight conductive rods or wires, with a total length of half the
radiated/received wavelength (thus called half wavelength antenna) (Figure 5), the dipole
antenna is probably the simplest and most widely used type of antenna. This kind of antenna
is omnidirectional, i.e. it has a non-directional radiation pattern (circular pattern) in a given
plane (the plane that is actually orthogonal to the rods).
© Copyright SIGFOX. All rights reserved
13 / 40
As shown above in Figure 6, the dipole antenna radiates (and receives) equally well in all
horizontal directions (H plane), while its directivity drops down to zero on the antenna axis.
Neglecting electrical inefficiency (e.g ohmic losses for a dipole antenna), a dipole has a
typical gain of 2.15 dBi.
λ/2
l
λ/4
λ/4
V
Transmission line (75Ω)
Figure 5 - Illustration of a dipole antenna
E-Plane
H-Plane
0
0
45
315
45
315
90 270
270
135
225
180
90
135
225
180
Figure 6 - Dipole radiation pattern in E and H plane
Dipole are excellent candidates in the case of external antennas if volume is not a constraint.
They are almost independent from the rest of the device integration since they don’t use the
device structure to radiate (e.g. ground plane).
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TECHNICAL WHITE PAPER - Antenna design for end products operating on the SIGFOX network
■ Monopole antenna
Basically, a quarter-wavelength monopole antenna is half of a dipole antenna placed over a
ground plane which acts as a reflector (image current theory) as shown in the picture below (7).
The set “quarter-wave plus image” forms a half-wave dipole that radiates only in the upper half
of space. As a consequence, a quarter-wave monopole will have a typical gain of 5.15 dBi.
Monopole
I
λ
4
ground
I
Virtual monopole image
Figure 7 - Monopole antenna mechanism
In practice, monopoles are often placed on one edge of the device PCB (which acts as
ground plane) instead of over it, and will therefore have different behavior than a classic
monopole. For instance, in a typical implementation such as the one in Figure 7, the radiation
pattern will be similar to a dipole if the PCB size is large enough, since its radiation will not
be limited to half the space. Such an implementation will have similar directivity to a dipole.
Meandered monopole
Ground plane
Figure 8 - Example of a folded monopole implementation
The size and shape of the PCB will impact the radiation pattern as well as the bandwidth of
the antenna. Indeed, if the PCB is too small, the bandwidth will be limited.
© Copyright SIGFOX. All rights reserved
15 / 40
■ Slot antenna
Slot antennas are quite simple structures, as they only consist of a large metal surface with
a slot cut out. The shape and size of the slot determine the antenna’s resonant frequency.
They can be one-half wavelength long (Figure 9a) if they are terminated slots, and can be
one-quarter wavelength long (Figure 9b) if the slot is placed at the edge of the metal surface
and left open.
(a)
(b)
Metallic ground
plane
λg/2
λg/4
Slot
50Ώ line
feed
λg is the equivalent wavelength in the dielectric substrate
Figure 9 - Terminated (a) and opened slot antennas (b)
Slot antennas can be a simple straight cut or have a more complex shape to reduce their
size or to better integrate them on a PCB (Figure 10).
Metallic ground
plane
λg/4
Slot
50Ώ line
feed
Figure 10 - Example of complex shaped slot antenna
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TECHNICAL WHITE PAPER - Antenna design for end products operating on the SIGFOX network
■ Patch antenna
The microstrip patch antenna is one of the most popular types of antenna, due to its
numerous advantages, such as low profile, easy manufacture, low cost, and conformability
to curved surfaces. It consists of a metallic patch etched on the top of a dielectric substrate
having thickness h and relative permittivity ε; this dielectric substrate is backed by a metallic
ground plane. Patch antennas come in a multitude of forms. However, the most commonly
used is the rectangular patch, which looks like a section of microstrip transmission line.
Rectangular patch antennas are one-half wavelength long, but most of the time they are
loaded with a dielectric material as a substrate, which results in the decrease in the physical
length of the antenna (the length actually decreases as the relative dielectric constant of
the substrate increases). The typical gain range of a patch antenna is between 6 and 10
dBi, while the impedance bandwidth range is between 1% and 5% and depends on both the
dielectric permittivity and the thickness of the substrate.
Patch antennas (Figure 11) have a directional radiation pattern with a main direction of
propagation. In the case of a device operating on the SIGFOX network, it might be an
adequate solution for a device placed against a wall, whereas it might not be a good option
for a mobile device.
Microstrip Feed
Patch
Ground plane
Substrate
Figure 11 - Patch antenna description and radiation patterns
Based on this antenna topology, a lot of antenna types have been created, such as PIFAs
and Microstrip Wire patch antennas. In fact, this topology is one of the most widely used
and declined topologies for small device applications.
© Copyright SIGFOX. All rights reserved
17 / 40
4.4 Miniaturization techniques
The size of an antenna is defined by the wavelength at its operating frequency. For a device
operating on the SIGFOX network in the RCZ1 at 868 MHz, the wavelength in free space is
34.5cm, which means that a dipole or patch would measure 17.3 cm, and a monopole 8.6 cm.
Of course, in the case of a small device, these dimensions are too large. Fortunately,
several techniques exist to miniaturize antennas, but they come at a cost. In fact, as the
size of an antenna is limiting its bandwidth and its radiation efficiency, thus, miniaturizing
an antenna will decrease the efficiency or the bandwidth, or both.
In this section, some common miniaturization techniques are presented.
Antenna loading
• Add a capacitive load at the open end
of the antenna (a) or an inductive load
at the feed point (b).
•L
oading can be done by shaping the
antenna or adding lumped components
on the antenna structure.
(a)
(b)
Antenna loading using materials
•E
lectrical characteristics of surrounding
antenna materials (ε r,μ r) can be used
to reduce the antenna size.
Dielectric
Air
h=
λ0
4
h=
λ0
4 εrμr
•H
igh permittivity material such as
ceramic material can be used at
SIGFOX operating frequencies
Example: The size of a monopole being
•M
agnetic materials only work for low
frequencies and are not appropriate at h= λ0
, (where ε r is the permittivity
SIGFOX frequencies.
4 εrμr
and μ r the permeability) increasing ε r and μ r helps
reduce the size of the monopole.
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TECHNICAL WHITE PAPER - Antenna design for end products operating on the SIGFOX network
Use short circuits
• It is possible to reduce the size of a
patch antenna by a factor of 2, placing
a short circuit at the middle of the
patch (where the E-field is null).
•P
IFA and patch antennas have slightly
different radiation properties due to
miniaturization.
λg
α~
=
2
λg
α~
=
4
Patch antenna
PIFA
Patch antenna
Increase current path length
•P
lanar antennas: slit and notches are
inserted in the structure to increase
the surface current path
λg
α~
=
2
•W
ire antennas: meandered lines or
coils are used to increase the surface
current path
α<
λg
2
Monopole antenna
λg
α~
=
4
α<
λg
4
A metamterial inspired antenna design (REF4)
Exotic structures
n top of classic miniaturization
O
techniques, several exotic antenna
structures have been developed
allowing strong antenna miniaturization.
Example:
antennas
Metamaterial
inspired
• Extremely small antenna
• Naturally matched to 50 ohms
• Usually highly efficient but very narrow
band antennas
© Copyright SIGFOX. All rights reserved
19 / 40
4.5 Antenna manufacturing technologies
There are many different ways to manufacture an antenna, and this is an important aspect
to consider when you design your device, since it will have an impact on:
•
•
•
•
The
The
The
The
antenna’s design freedom and possibilities
antenna’s price
antenna’s performance
mechanical assembly of your device
Here is a non-exhaustive list of common antenna manufacturing technologies adapted to
wireless devices:
Metal wire antenna
•
•
•
•
Wire antenna
Very cheap solution
Mechanical reliability
Easy to solder to a PCB
Relatively low design freedom
Stamped metal antenna
• Cheap solution
• Easy to affix to the PCB
•
Off-the-shelf antennas often based
on this technology.
PCB printed antenna
• Very cheap and easy-to-build solution.
• No interconnection issues.
•
Only planar antenna (low design
freedom).
•
FR4 substrate are lossy and will
impact antenna efficiency.
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Antenna
PCB
Antenna trace
Electronic
TECHNICAL WHITE PAPER - Antenna design for end products operating on the SIGFOX network
Ceramic antenna
• High dielectric properties of ceramic
material used for miniaturization.
• Often quite narrow band antennas.
•
Off-the-shelf antennas often based
on this technology.
• Difficult to make a custom design.
Ceramic material
Metallization
LDS antenna (Laser Direct Structuring)
•
Fully 3D shaped antenna (only few
technology limitations)  highest
antenna design freedom  best
performance.
• Antenna trace directly “printed” on the
antenna carrier.
• Highest level of integration possible.
• Only few manufacturers support this
technology.
• Plastic antenna carrier is needed.
Flex antenna
• Antenna printed on a thin polyimide
substrate and glued to a plastic
antenna carrier.
• Design flexibility
• Good tolerances
• Plastic antenna carrier is needed
Antenna
copper trace
Antenna
plastic carrier
Antenna Flex
PCB
Antenna carrier
Stand-alone
flex antenna
Repeatability should be taken into account in the manufacturing technology selection. In
fact, it is crucial to make sure that every device will have the same radiated performance
and thus that antenna design is reliable and repeatable.
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4.6 Advice on off-the-shelf internal antenna integration
Using an off-the-shelf antenna can be an excellent idea, since they are relatively cheap and
only require little development effort. However, some precautions need to be taken. In this
section, some advice is given to help you succeed in implementing such an antenna in your
device.
■ Differences between antenna performance presented in datasheets and real-life
performance
Antenna performance announced in datasheets usually applies to the reference design used
to characterize the antenna. Thus, if your integration differs from this reference design, the
performance will most likely be different. Since the reference design is probably the best
configuration for the tested antenna, you should expect lower performance using a different
implementation.
Here is a list of parameters that will impact the antenna’s performance and behavior when
changed compared to the initial datasheet:
•P
CB size is quite important, since it is part of the antenna’s radiation mechanism. The
same antenna placed on different PCBs will not provide the same performance.
•A
ntenna location on the PCB is also an important parameter. For instance, a side board
antenna cannot be implemented in the middle of a PCB unless specified. In fact, several
antenna locations are often evaluated in the datasheets. Select one of them and the
antenna should work properly. If you have to place the antenna in another location, a
study should be performed to validate the implementation.
•G
round clearance areas defined in the datasheets must be respected. If not, the impact
might be:
- different antenna loading configuration, leading to lower radiated performance and
antenna frequency detuning.
- complete change of the antenna radiation mechanism, leading to extremely poor
antenna efficiency.
•Y
our device will most likely have a casing enclosing all the electronics, including the
antenna, which is probably not the case with the reference design. This casing will have an
impact on the antenna frequency tuning and efficiency, depending on the kind of material,
its thickness, and the distance between the antenna and the casing. Ideally, the material
should be as lossless, as thin, and as far away as possible from the antenna. Of course,
this is not often possible, so here is a list of more realistic advice:
- Plastics with dielectric properties similar to ABS are acceptable. Typically, ABS plastic
dielectric properties are ε r≈3 and tan δ ≈0.01; however, they can vary depending on
the material supplier.
- A minimum distance of 1-2 mm should be kept between the antenna and the casing.
- A maximum plastic thickness of 2 mm should be respected.
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These recommendations are not universal, but they should allow you to integrate an offthe-shelf antenna in your device with only a minimum of matching re-work to retune the
antenna, and without any substantial loss in performanc.
Antenna manufacturers can often help you to tune the selected antenna to your specific
design. They can also give you some advice for selecting the best antenna solution for your
device.
■ How to read an antenna datasheet
Antenna datasheets can substantially differ depending on the antenna manufacturer, which
sometimes make them difficult to read and compare.
In fact, antenna manufacturers do not always present the same parameters (see Appendix
A for parameter definitions) in their datasheets, even for the same kind of antenna:
•G
ain vs efficiency: Most datasheets will only provide one of those two parameters. It
can be confusing when comparing two antennas, since they do not compare the same
quantity. Furthermore, it is not always stated whether the datasheet refers to radiation or
total efficiency, and to total or IEEE gain.
•V
SWR vs Return Loss: Most datasheets will only provide one of those two parameters.
They give the same information, and you can easily do the conversion from one to the
other.
•F
requency bandwidth: The frequency bandwidth is not always defined using the same
criteria. For small device antennas, even though most datasheets use the Return Loss to
determine the bandwidth, the limit level used to compute the bandwidth can differ.
The units used in datasheets can also cause some confusion. For instance, efficiency can
be presented in either dB scale or linear scale. It is easy to make the conversion using
equations 2 and 3.
EffdB = 10xLog 10 (Efflin)
Equation 2
EffdB
10
Equation 3
Efflin = 10
Another confusing piece of information is the antenna size or volume defined in antenna
datasheet. Indeed, the antenna dimensions presented in datasheets do not always include
PCB ground clearance or component-free areas (keep-away distance). In fact, a small
antenna that requires a large ground free area could actually occupy a larger volume than
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a large antenna that does not require any ground free area. It is important to consider
the antenna volume including the ground free and component free area when comparing
antennas.
Chip antenna
antenna clearance
Figure 12 - Example of off-the-shelf antenna requiring a ground free area
Figure 12 shows an example of a small antenna that requires a large ground free area
compared to its size. The antenna is three times smaller than the area it requires to operate
properly (10x3.2x0.5mm3 for a ground free area of 11x10.4mm²).
5 - ANTENNA TOPOLOGY SELECTION TOOL
This section will help SIGFOX device manufacturers select a type of antenna based on
technical requirements and antenna design capacity.
5.1 External antennas
If there is no integration constraint and/or you are looking for the best possible performance,
external antennas are definitely the best option. External antennas are easy to implement
and provide excellent radiated performance. Depending on the type of antenna you select,
some “light” antenna skills might be needed, but implementation of this kind of antenna will
still be relatively straightforward.
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Use the chart below to select the external antenna type according to your requirements:
External Antenna
Half Wave: dipole
quarter wave: monopole
miniature: helical antenna
@868MHz ~ 17cm
@868MHz ~ 8.5cm
@868MHz < 8.5cm
Performance
10
10
8
Design simplicity
10
8
7
Integration level
0
1
3
Plug-and-play solution
Little work on antenna
location
Requires a ground plane
to work
Some matching circuitry
may be required
Requires a ground plane
to work
For best performance, external antennas should not be placed along the ground plane. The
antenna should be kept away from the ground plane (Figure 13).
Antenna
X
Figure 13 - Example of bad and good external antenna integration
5.2 Integral antennas
If an integral antenna is required in your device, it is important to evaluate which option is
best, depending on the integration level needed and the targeted performance. The effort
you want to put into the antenna design is also an important parameter to take into account.
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In fact, it is often possible to find a much better solution than an off-the-shelf solution, but it
can require a relatively high effort in terms of antenna design.
Use the chart below to select the integral antenna type according to your requirements:
Integral antenna
Low integration level
Off-the-shelf
antenna
Generic antenna
design*
Specific antenna
design**
(non optimized solution)
(optimized solution)
Performance
8
8
9
Design simplicity
5
4
3
Integration level
5
5
7
Matching required
Matching required
Matching required
High integration level
Off-the-shelf
antenna
Generic antenna
design*
Specific antenna
design**
(non optimized solution)
(optimized solution)
Performance
3
3
8
Design simplicity
5
4
1
Integration level
5
6
7
Not always possible
depending on the
device integration
Matching required
Not always possible
depending on the
device integration
Matching required
Skilled antenna
designer required
long development
time
* Generic antenna design refers to classic antenna topology, such as monopole antennas which have been copied
in the device design without any size or performance optimization.
** Specific antenna design refers to a complex antenna design specifically designed and optimized for the device.
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6 - CONCLUSIONS
Several aspects of antenna design and implementation have been presented in this
document to help device makers without a strong background in antennas select a good
antenna solution for their device operating on the SIGFOX network.
The document also explains why antennas are so sensitive to their environment and why
poor implementation can lead to extremely low radiated performance.
It presents the most common antenna topologies and miniaturization techniques and gives
some suggestions for preventing confusion when reading an antenna datasheet.
This document will not replace a skilled antenna designer; however, it can help a device
maker avoid some common mistakes, for instance, in implementing an off-the-shelf antenna.
© Copyright SIGFOX. All rights reserved
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7 - APPENDIX A: ANTENNA PARAMETER DEFINITIONS
In this section, some important antenna parameter definitions are given.
7.1 VSWR and Return Loss
VSWR (Voltage Standing Wave Ratio) and Return Loss are two parameters used to
characterize the matching between the transmission line providing the RF signal and the
antenna.
At the antenna port, a fraction of the incident signal is reflected, and the other fraction is
accepted by the antenna. In the case of a perfectly matched antenna, the incident signal
is completely accepted, and there is no reflected signal. In reality, there is always a small
fraction of the signal reflected.
VSWR is a ratio between voltage levels, while Return Loss is a ratio between power levels.
IV maxI
VSWR = IV minI
Pt
RL dB = 10xLog 10 Pr
Antenna
interface
Incident
power
Accepted
power
RF
Module
Reflected
power
Figure 14 - Antenna interface illustration
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These two parameters are often used in antenna datasheets.
Using the return loss or the VSWR, it is possible to determine the mismatch loss at the
antenna port, as described in the table below:
RL (dB)
-15
-10
-6
-4
VSWR
1.4
2
3
4.4
Mismatch loss (dB)
0.14
0.46
1.25
2.2
In the case of a device operating on the SIGFOX network, a minimum return loss of -10 dB
over the whole operating frequency band is a good target.
If too much signal is reflected, on top of increasing the mismatch losses, this reflected
signal will go back to the RF module and possibly make it generate higher harmonics.
7.2 Efficiency
Efficiency defines the capability of an antenna to transform a guided RF signal into a radiated
wave. For integrated antennas, efficiency is a very important criterion that can, by itself,
characterize the quality and potential of an antenna’s design. In the literature, two kinds of
antenna efficiency can be found:
•R
adiation efficiency, which is the ratio between the power accepted by the antenna and
the radiated power.
•T
otal efficiency, which is the ratio between the incident power at the antenna port and the
radiated power.
Total efficiency takes into account the insertion loss (due to mismatch between the RF line
and the antenna input), while radiation efficiency does not.
TotEffdB = RadEff dB - Mismatch_losses dB
Equation 4
These parameters are often used in antenna datasheets.
7.3 Directivity
An antenna never radiates uniformly over all directions. Such an antenna would be the
isotropic antenna which does not exist. Based on this observation, antenna directivity was
defined and describes its capacity to concentrate its radiation into one direction. It is the
ratio between the radiation intensity in one direction and the average radiation intensity.
It is important to note that directivity does not take into account any kind of losses. For
instance, an antenna can have relatively high directivity, while having a low gain if its
efficiency is poor.
Another relevant observation to note about directivity is that when directivity increases in
© Copyright SIGFOX. All rights reserved
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one direction, it automatically decreases in other directions.
7.4 Gain
Antenna gain is very similar to directivity, but takes losses into account. In fact, antenna
gain combines antenna directivity and antenna efficiency.
Two definitions of gain can be found in the literature. The total gain, defined in Equation 7,
refers to total efficiency, while IEEE gain, defined in Equation 8, refers to radiation efficiency.
TotGaindBi = TotEffdB + Directivity dBi
Equation 5
IEEE GaindBi = RadEff dB + Directivity dBi
Equation 6
Two units can be used to express the gain:
• A gain expressed in dBi is normalized compared to the isotropic antenna
• A gain expressed in dBd is normalized compared to a dipole antenna exhibiting a maximum
gain of 2.15 dBi.
GaindBd = Gain dBi - 2.15
Equation 7
Gain is often a parameter presented in antenna datasheets.
7.5 EIRP and ERP
EIRP (Effective Isotropic Radiated Power) and ERP (Effective Radiated Power) are more
system criteria than antenna parameters, since they depend on the available power at the
antenna port. They describe how much power a device radiates in a given direction.
EIRP is the amount of power an isotropic antenna would need to radiate the same amount
of power in a given direction as the measured antenna.
EIRPdBm = RF_Power dBm+ Gain dBi
Equation 8
While EIRP refers to the isotropic antenna, ERP refers to a perfect dipole antenna with a
gain of 2.15dBi. Therefore, the relation between ERP and EIRP values is simply an offset
of 2.15dB, as described in the equation below.
ERPdBm = RF_Power dBm+ Gain dBd
Equation 9
ERP dBm = EIRPdBm - 2.15
Equation 10
ERP is the parameter used in the SIGFOX device classification certification.
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Example:
Arecibo Planetary Radar
RF power @ 2380MHz = 60dBW (10 6 W)
Antenna Gain = 73.3dBi
EIRP = 133dBW = 163dBm = 20TW!!!
7.6 Antenna bandwidth
Antenna bandwidth defines the operating frequency bandwidth of an antenna.
This parameter can be calculated in several manners:
• It can be defined by the frequency bandwidth in which the antenna is considered wellmatched to the transmission line providing the RF signal. In this case, a level of return loss
must be defined according to the type of application. In the case of a device operating on
the SIGFOX network, the bandwidth can be defined with a -10dB Return Loss criterion
(Figure 15 a).
• The antenna bandwidth can also be defined by its gain. In this case, the gain defines the
frequency bandwidth in which the antenna gain remains within a certain range. A -3dB
gain bandwidth is usually defined in this case (Figure 15 b). This criterion is often used for
directive antennas such as parabolic dishes, but is not well-suited to a device operating
on the SIGFOX network.
Return Loss (dB)
0,7
0,8
0,85
0,9
Return Loss (dB)
0,95
1
1,1
3
-2
2
-4
1
-6
-8
-10
Bandwidth
Max Gain (dBi)
Return Loss (dB)
0
0,75
0
-14
-4
(a)
0,9
1
1,1
Bandwidth
-2
-3
Frequency (GHz)
0,8
-1
-12
-16
0,7
-5
Frequency (GHz)
(b)
Figure 15 - Return Loss defined bandwidth (a) ; Gain defined bandwidth (b)
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31 / 40
In the case of small antennas, the return loss is often used to determine the antenna
bandwidth, since the gain is not a critical parameter.
7.7 Polarization
Antenna polarization refers to the polarization of the electrical component of the
electromagnetic wave generated by the antenna. It can be linear, elliptical, or circular.
Figure 16- Antenna polarization
In the case of the SIGFOX network, linear vertical polarization is used at base station level.
However, the polarization of a traveling wave may change due to several phenomena such
as diffraction, reflection, etc. Consequently, transmitting a vertically polarized EM wave does
not necessarily mean that the EM wave will still be vertically polarized at the reception side.
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7.8 Radiation pattern
The antenna radiation pattern is a measure of its far field power or radiation distribution with
respect to a particular type of coordinates.
It gives an image of how an antenna radiates in free space.
It is important to note that:
• Every single antenna topology has a specific radiation pattern.
• The same antenna implemented in two different devices can exhibit different radiation patterns.
• Two identical devices can have two different radiation patterns depending on their environment.
Farfield Directivity Abs (phi = 90)
Phi = 90
30
0
0
60
0
90
-30
-10
0
120
90
120
150
180
150
Figure 17 - Example of a dipole antenna’s 3D and 2D radiation patterns
In the case of a device operating in the SIGFOX network, omnidirectional or dipole-like
radiation patterns (Figure 17) are preferred due to the star topology of the network
(spatial redundancy).
© Copyright SIGFOX. All rights reserved
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8 - APPENDIX B: A METHOD FOR ASSESSING YOUR ANTENNA
DESIGN WITHOUT USING EXPENSIVE EQUIPMENT
In this section, a streamlined outdoor ERP measurement procedure, compared to the lab
measurement performed following the regulation, is presented. It will help the device maker
to roughly evaluate the radiated performance of a device operating on the SIGFOX
network without involving expensive equipment such as anechoic chambers.
This measurement method only requires a limited setup:
• 1 spectrum analyzer
• 1 receiving antenna
• 1 reference device with known radiated performance (ERP or EIRP)
The method is also quite easy to perform and does not require in-depth knowledge of
antenna measurements. Only the use of a spectrum analyzer is needed.
Disclosure: This method is not accurate enough to provide reliable results (due to
possible reflection and an uncontrolled environment). The results you will get from
this method are only an indication of the performance level you can expect from your
device. Therefore, additional testing in an anechoic chamber should be considered
for accurate final ERP results.
Polarization is not considered in this measurement method. It is better to select a linearly
polarized antenna as reception antenna.
Streamlined ERP measurement setup description:
The proposed light ERP measurement method needs to be performed in an open field
environment. Typically a rugby field will work. The spectrum analyzer is positioned on a
mast, and a reception antenna is connected to it. The DUT holder in placed in front of the
receiving antenna so that both the reception antenna and the DUT are at the same height.
For mobile phone measurements, a minimum distance of about 1.20m is recommended
between the measurement antenna and the DUT ([REF5]). In the case of a device as
small as those generally operating on the SIGFOX network (maximum dimension < λ/2), a
minimum distance of 1.05m is recommended between the measurement antenna and the
DUT. To avoid ground reflections, the DUT and the measurement antenna should be placed
high enough from the ground, and not too far to minimize the ground reflection impact on
the measurement.
In Figure 18, we recommend placing the DUT 1.5 m away from the reception antenna and
1.5 m away from the ground.
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d1 = 1.5m
Spectrum analyzer
DUT
Measurement
antenna
Mast
d 2 = 1.5m
d 2 = 1.5m
Mast
Figure 18 - Example of the proposed measurement setup
Note: According to the CTIA Test Plan for Wireless Device Over-the-Air Performance [REF5],
the minimum distance to respect between the measurement antenna and the DUT should be:
MAX
[
2D 2 , 3D, 3 λ
λ
]
Where D is the maximum dimension of the DUT.
Where λ is the wavelength at operating frequency.
8.1 Measurement procedure
An easy-to-implement measurement procedure is proposed in this section. This procedure
allows the operator to perform the measurement without disturbing it by his presence.
In classic anechoic chamber measurement, instruments are often outside the room. Therefore,
the operator can easily read the measured level without disturbing the measurement. In
the proposed method, the operator must be near the instrument to read the measured level,
and therefore will disturb the measurement. An easy solution involving a simple SW test
mode can be used to tackle this issue, as explained below.
Measurement procedure description:
[1] Clear spectrum analyzer
[2] Spectrum analyzer mode “max hold” ON
[3] Launch specific device test mode
[4] The operator leaves the measurement setup (~10-15 meters away)
© Copyright SIGFOX. All rights reserved
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[5] Wait until the device SW test mode cycle is over
[6] Read the measured level on the spectrum analyzer
The following SW test mode should be implemented in the DUT and used in measurement
procedure step [3]:
[1] Wait x seconds (the time the operator needs to leave the measurement setup)
[2] Mode CW ON
[3] Wait y seconds (time for the spectrum analyzer to measure the signal)
[4] Mode CW OFF
[5] Optional: generate an alert (can be useful to let the operator know the cycle is over)
This measurement procedure should be performed once with the reference device positioned
at its highest ERP position (known position).
Then, the reference device is removed from the setup and the DUT is put in its place. The
DUT is then measured following the same procedure.
To get a 2D planar radiation pattern, rotate the device along the Z axis (Figure 19) and
perform the measurement for each angle you want to measure.
Spectrum analyzer
DUT
Figure 19 - Measurement setup for a 2D radiation pattern
The setup should not be modified during the measurement procedure, and specifically, the
measurement antenna should not be moved at all during the measurement procedure.
Only the DUT should be moved. If you need to change something in the setup during the
measurement (e.g. d 1 or d2 distance), a measurement of the reference device should be
performed again and used as reference for the rest of the experiment.
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8.2 Measurement post-processing
The presented method being a relative measurement, it requires knowing the radiated
performance of the reference device (in this case, ERP or EIRP). Also, the measurement of
both reference device and DUT should be performed following the measurement procedure
described above.
From these three values, it is quite easy to extract the DUT ERP using the following equation:
ERPDUT = Meas DUT+ ERPREF - Meas REFEquation 11
If the reference device was evaluated in EIRP, then, the following equation should be used.
EIRPDUT = Meas DUT + EIRPREF - Meas REFEquation 12
The proposed method does not require long-term repeatability, because each time you
measure a DUT, you compare it to a reference device that is equivalent to calibrate your
system each time you do a measurement.
Performing the measurement in an open field avoids reflection from trees or buildings,
which would make this method completely unreliable. However, the closer to the ground
the measurement is performed, the more reflection from the ground there will be. This is
the weak point of this method. These reflections will bring some inaccuracy and error in the
measurement. To minimize this problem, the measurement should be performed away from
the ground and at a controlled distance between the DUT and the receiving antenna.
Example:
The reference Design has an ERP of 14dBm (previously measured in a calibrated setup).
The reference device measured level on the spectrum analyser is -20dBm.
The DUT measured level is -24dBm.
The computed DUT ERP level is then -24+14+20 = 10dBm.
9 - APPENDIX C: RF ANECHOIC CHAMBER
Anechoic chambers are very popular tools used to carry out measurements of antenna
radiation properties. They have several characteristics that allow accurate and repeatable
antenna measurements, which would be very difficult to achieve in an outdoor test range.
An anechoic chamber is a faraday cage with high insulation between the inside and the
outside. It is very important not to disturb this property with external noise or signal during
the DUT measurement. This property is essential in sensitivity measurements.
Radio-absorbing materials are placed on the walls, floor, and ceiling of the anechoic
chamber to avoid signal reflections and simulate a free space environment. The absorbers
are dimensioned and placed so that, within a certain volume called a “quiet zone,” the
reflection level becomes low enough to be neglected within the anechoic chamber frequency
range. The DUT should be placed inside the quiet zone during the measurement.
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37 / 40
Different kinds of anechoic chambers exist. The three most widely used are described below:
• Far field anechoic chambers: where the DUT is placed far enough from the measurement
antenna to perform far-field measurement.
• Compact antenna test range (CATR): where one or several reflectors are placed between
the DUT and the measurement antenna. This type of anechoic chamber is often used
in the case of high gain antennas (satellite antennas), where classic far field anechoic
chambers would need to have unreasonable dimensions.
• Near field anechoic chambers: where the antenna measurement consists of near field
scanning. Complex post-processing is necessary to extract the DUT far field properties
from the near field scanning. This method allows relatively small anechoic chambers
compared to far field anechoic chambers.
To characterize the radiated performance of a SIGFOX device, far field or near field anechoic
chambers are the most appropriate.
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TECHNICAL WHITE PAPER - Antenna design for end products operating on the SIGFOX network
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Bâtiment E-volution ∙ 425, rue Jean Rostand
31670 Labège ∙ FRANCE ∙ Tel: +33 (0)5 34 31 03 16
[email protected] ∙ www.sigfox.com
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