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ic.2007.1332

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15 YEARS OF TERMINAL ANTENNA DESIGN AT LEMA :
AN OVERVIEW
A.K. Skrivervik, J.F. Zürcher
Laboratoire d'Electromagnétisme et d'Acoustique, Ecole Polytechnique Fédérale de Lausanne, Station 11,
CH-1015 Lausanne, Switzerland
anja.skrivervik@epfl.ch
Keywords: terminal antenna, multi-frequency antennas.
Abstract
The enlarged demand for electrically small antennas in the
900MHz – 5GHz range started in the early nineties, with the
boom in mobile phones launched by the second generation of
mobile phones. The laboratory of Electromagnetics and
Acoustics of EPFL was involved in the design of dedicated
antennas for mobile services since 1990, and has been
involved in this topic since. More specifically, we have been
interested in electrically very small antennas, multiband
antennas, optimization, design rules and measurement. In this
paper some of the key aspects encountered during these years
of work on terminal antennas will be evoked, and illustrated
by practical examples.
1 Introduction
The boom of wireless services, which started in the early
nineties with the democratization of the mobile phone has
lead to the revival of a class of antenna which had been
somewhat forgotten since world war II and the apparition of
microwaves, namely the electrically small antenna.
Indeed, even if the wavelength corresponding to these new
wireless services (roughly 20 cm to 6 cm) remains very
reasonable, the fact that the purpose of these services is to
connect mobile users, a new paradigm has changed the
viewpoint of antenna engineers : volume, weight and cost
have become key issues for the antenna design. This trend is
moreover increasing rapidly with time, as more and more
services share the same mobile terminal.
Thus, the nature of these new services, as well as the
competition for the available frequency spectrum have
broadened the range of requirements that are made for the
antennas which are used on the mobile terminals. These
requirements can be summarized as follows :
•
•
•
•
•
•
•
small dimensions
low weight
low induced SAR
low cost
high efficiency
capability to handle multiple frequency bands
broadband
•
•
robust to changes in the environment
optimize the use of the available channel capacity
The first four requirements are typically "user defined
requirements", the others being defined by the service
provider or he network.
Of course, depending on the considered service, the relative
importance of all these requirements varies a lot. The size is
for instance far less critical for WLAN system located in a
laptop than for a DCS phone. The bandwidth and capacity
however will be far more critical in the former example.
Considering this, the design of terminal antennas is more than
ever an art of defining the right compromise between all the
requirement for a specific application. The Laboratoire
d'Electromagétisme et d'Acoustique of EPFL has been active
in the field of terminal antennas since the early nineties. Our
first designs were related to the Swiss watch industry, which
was very interested to check the feasibility of integrating
various communication services like cordless phones, cell
phones, pagers, GPS in a wrist watch (it works !), and some
of these designs will be presented in section II. The following
section, will be dedicated to multifrequency terminal
antennas, and section IV to some critical aspects of antenna
measurements. We will then conclude with new trends and
perspectives.
2 Examples of very small antennas : integration
in a wrist watch
Our laboratory had a fruitful collaboration with the Swiss
watch industry, as several mobile communication devices
where planned to be incorporated in wristwatches. Some
examples are shown below.
2.1 A miniature GPS antenna
The main challenge in this design was to obtain a circular
polarization is the very small space allotted to the antenna in
this application. Indeed, the antenna was to be place at the top
of the wrist watch, just beneath the hands, and the maximum
available volume was of 30mm in diameter (meaning a radius
of 0.075λo) and a height of 1.5 mm (0.0075 λo). The selected
design was circular patch antenna with a slight notch on one
axe to obtain circular polarization. In order to reach the
specified dimensions, a dielectric substrate with a high
permittivity was used (εr =10.5). This value of the dielectric
constant was however not enough to obtain a resonance at
GPS frequencies within the allotted space, so slots were
etched in both axes of the patch in order to further reduce the
latter. This option was preferred to selecting a higher
permittivity, as the latter would have reduced the achievable
bandwidth too much. The Antenna is depicted in figure 1, and
its axial ratio in figure 2. The impedance bandwidth at -10dB
of this antenna is rather small (0.6%), as was expected due
too the very small size, but it is sufficient if a temperature
stabilized dielectric material (like Rogers TMM for instance)
is used. The axial ration bandwidth is lightly smaller (Figure
5), and the gain of the antenna was measured at – 5dBi .
29mm
Figure 1 : GPS antenna for a wristwatch
but the performances where nevertheless acceptable of the
application in mind.
2.2 Miniature DECT and Bluetooth antennas
A second design performed again, for an service to be
integrated in a wrist watch was an antenna for a DECT phone
[4, 5]. The overall dimension of the watch had to be
comprised in a cylinder of 35 mm of diameter and 8mm
height, this volume giving also space for the phone and the
elements of the watch, including batteries. The antenna had to
be in the watch itself, not in the bracelet, so there were only
two possible locations : On the top (under the hands of the
watch) or on the circumference of the cylinder. In order to
achieve more easily the bandwidth required by DECT (1.881.9 GHz), we decided for the second option, thus to conform
our antenna around the cylinder. The antenna type selected is
a PIFA, as it has good bandwidth characteristics with respect
to dimensions. The PIFA was first conformed around the
cylinder, than integrated into it in order to avoid
protuberances, as is sketched in figure 3. We named the final
design SMILA (Small Monobloc Integrated L Antenna)
Figure 3 : PIFA integrated in a wrist watch
A realized prototype of the antenna is presented in Figure 4
2.5
MHz
2.6 MHz
Figure 2 : axial ratio of the GPS antenna of fig. 1
Figure 4 : Prototype of the SMILA
This performance can be considered as being quite
satisfactory when compared to the fundamental limits for
bandwidth and gain, as introduced by Chu [1], McLean [2]
and Harrington [3], which are of @@% for the theoretical
maximal achievable bandwidth and 1 dBi for the gain. We are
of course quite far from these limits, due to the unfavourable
aspect ratio of imposed by the specifications to the antenna,
The measured performances of this antenna are presented in
Figure 5. The – 10dB impedance bandwidth reached is of 4
%, and the measured gain of 0.5 dB, for a theoretical
maximum of the gain after Harrington [3] of 2.5. This result
is thus very satisfactory, taking into account the severe
volume requirements, which shows us again all the potential
of the PIFA for a compact antenna.
Figure 5 : return loss of the SMILA antenna
Figure 7 : PIFA integrated on the top of the watch
The same antenna was designed for a Bluetooth application to
be integrated in a wristwatch of same size. In this case, we
measured also the effect of the users arm on the behaviour of
the antenna [6].
Unfortunately, the gain performances of the antenna
deteriorate very drastically when the watch is worn on the
wrist, as is illustrated by the measurements of figure 6. These
measurements were done by placing the watch cylinder on a
human arm phantom, with different spacing between the
phantom and the watch. We see that when the antenna is too
close to the arm, as is the case in practical situations, the
performance of the antennas are degraded.
Figure 8 : current distribution of the structure described in
figure 7.
3. Multifrequency antennas
Figure 6 : Gain versus frequency for the antenna at different
distances of the arm
We decided thus finally to modify our approach and to put the
antenna on the top of the watch, keeping the integrated PIFA
concept. The resulting design is illustrated in figure 7. The
dimensions of the cylinder are the same as before. The
measured gain a frequency of 2.42 GHz is 1.6 dB , which is
an excellent result. Figure 8 shows the surface current
distribution on the structure.
The venue of new generations of voice services (DCS and
UMTS to GSM for instance), and the offer of new services
incorporated in phone terminals (like Bluetooth, GPS) require
antennas which provides multiband possibilities. Indeed, a
multiband antenna solution is often smaller and less costly
than a solution with a distinct antenna for each frequency
band. Several scenarii can occur, which are all illustrated on
the well known PIFA antenna :
•
Multi bands but single feed [7-10]. The big advantage of
having a common feed point for all the bands is that we
do not need to care about mutual coupling problems. The
drawback is that the radio front end has to discriminate
the signal belonging to different services. This is the
usual choice for dual band mobile phone handset
antennas. An example of this kind of antenna is shown in
figure 8.
4. On the Characterization of electrically small
antennas
Figure 8 : dual band PIFA antenna (GSM/DCS).
•
Multi bands with one feed per frequency band [11]. The
drawback is that the mutual coupling between the feed
point can be high, which can degrade the overall
performances in certain circumstances. However, each
service is decoupled already at the antenna stage. This
scenario is usually chosen when the two offered services
are uncorrelated (voice and GSM for instance). This can
also be a good choice for frequency bands which are far
apart. An example of a double PIFA antenna is shown in
figure 9.
Figure 9 : dual PIFA antenna (GSM/DCS)
•
Multi bands with multi-feeds, however less feed than
bands [12]. This can be considered as a mixture of two
former scenarios, and is often used to cope with the
evolution of one type of service. For mobile voice in
Europe for instance, one could use a multi band antenna
with two feed points, one for the GSM/DCS bands and a
second for UMTS. An example covering GSM/DCS on
one port and UMTS on the other is shown in figure 3.
During the middle nineties, the need for miniature antennas
for mobile communication terminal became larger and larger,
and in the euphoria of the fast miniaturization of chips and
other electronically components due to the improvement of
technologies, it was often forgotten that the fundamental
limits linking antenna size and performances were not
technological limits but physical limits. It was thus quite
common to find, in specialized journals, press release on
extremely small antennas boasting excellent (but unphysical)
characteristics. Most of these adds were done in good faith by
there authors, so what had happened ? The problem came
from an incorrect measurement of the antennas.
The difficulty in measuring electrically small antennas comes
from the fact that they are often neither a symmetrical nor an
asymmetrical structure. This is easily understood looking at
the case of a microstrip patch antenna : on an infinite ground
plane, the structure is clearly asymmetric and should be fed
by an unbalanced transmission line. In the limiting case of an
electrically small microstrip patch the ground plane had to be
cut to the similar size than the patch, and the structure is
symmetric, needing thus a balanced transmission line. In most
practical cases an electrically small antenna will have a
structure somewhere in between. As a result, there does not
exist a transmission line which is able to feed these structures
having no spurious current flowing on it. The consequence of
this is that measuring an electrically small antenna using
connecting cables on a conventional way, can lead to results
as erroneous as when a dipole is measured connected to the
equipment by a coaxial cable without balun.
The errors introduced by the cable are particularly important
when radiation characteristics of the antenna like the gain,
pattern and efficiency are measured and can completely mask
the true behaviour of the antenna [6]. On the other hand,
impedance measurements are less affected and often are still
usable.
In order to correctly measure the radiation characteristics of
an electrically small antenna, following rules should be
followed :
• avoid the use of cable
• perform system measurements, which means for the
antenna to measure it in its final environment, taking
into the casing, ground planes, batteries, etc.
There has been much work on defining correct measurement
procedures for electrically small antennas [5,13-17], and the
topic is well understood today. Moreover, these procedures
are evolving in order to take into account new terminal
antenna features, like diversity gain, MIMO, etc. [18].
Figure 10 : double PIFA for GSM/DCS (feed point to the left)
and UMTS bands
At EPFL-LEMA we also set up a measurement procedure
dedicated to electrically small antennas [6]. It is based on the
principle described in figure 11, and is placed in an anechoic
chamber.
Figure 11 : Measurement principle
The system, consists of a random positioner allowing to move
the antenna and its casing in any direction (azimuth, elevation
and polarization). The antenna is fed by a VCO inside the
casing, and is thus transmitting. The receiving antenna is
connected to a spectrum analyzer which records the
maximum power received (for peak gain measurement) or the
mean power received (for efficiency measurements). The
antenna under test is then replaced by a known antenna (a
dipole for instance) fed by known power, and the maximum
gain or the efficiency are obtained by comparison. Note that
the random positioner is made using only dielectric materials,
the only metallic part being an electrical motor located at the
bottom end of its foot, as depicted in figure 12.To obtain the
maximum gain of the antenna under test, the random
positioner turns the antenna until the maximum received
power is detected by the spectrum analyzer. The Antenna
under test is then replaced by a reference antenna which is
powered so that the same level is reached on the spectrum
analyzer. The maximum gain of the antenna under test is then
given by :
Pto _ dip + GainDip − Pow e rVCO = Gain AUT
where GAUT is the maximum gain of the antenna under test.
PVCO the known power delivered by the VCO, Pref the known
power delivered to the reference antenna and Gref the gain of
the reference antenna. The accuracy of the measurement is of
±0.5 dB, and depends mainly on the stability of the VCO and
the precision of the reference antenna. The efficiency of the
antenna can be obtained by measuring the average gain of the
antenna instead of its peak gain.
5. Conclusions
The demand for new mobile communication devices and
systems will certainly continue for several years, increasing
the market pressure for efficient electrically small antenna.
These radiating devices should be optimized for each
application, in order to get the best compromise between
antenna volume, gain and bandwidth. In addition, more and
more services will have to share the same terminal ; for this
antenna, this means that a multifrequency or very broad band
behaviour will be requested. Performances like some diversity
or MIMO will also be introduced in order to improve the
overall system gain. All this means that there remains a
tremendous challenge for the antenna engineer to meet all
these often antinomous specifications with one design.
References
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