Design of a multimode MIMO antenna using characteristic modes

Abstract

In this communication, the design procedure of a multimode multiple-input multiple-output (MIMO) antenna is presented. The antenna consists of a metallic ring antenna operating with different orthogonal modes, whose performance in a MIMO system is similar to traditional antenna arrays. Thus, a compact MIMO antenna is obtained, which is very suitable for mobile terminals. A modal analysis of the antenna is carried out first by means of the theory of characteristic modes, in order to identify the different radiating modes of the antenna. Then a set of feeding configurations is proposed so as to excite these modes. As the modes must operate in the same frequency band, a loading technique is used in the antenna in order to shift the resonance frequency of the modes to the proper band.

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Design of a Multimode MIMO Antenna Using
Characteristic Modes
E. Antonino-Daviu#1, M. Cabedo-Fabres#1, M. Gallo#2, M. Ferrando-Bataller#1, and M. Bozzetti#2
#1 Instituto de Telecomunicaciones y Aplicaciones Multimedia (iTEAM),
Universidad Politécnica de Valencia, Edificio 8G, Camino de Vera, s/n 46022 Valencia (Spain)
1evanda@upvnet.upv.es
* Politecnico di Bari, via Orabona 4, 70125 Bari, Italy
3gallo@deemail.poliba.it

Abstract— In this communication, the design procedure of a
multimode Multiple-Input Multiple-Output (MIMO) antenna is
presented. The antenna consists of a metallic ring antenna
operating with different orthogonal modes, whose performance
in a MIMO system is similar to traditional antenna arrays. Thus,
a compact MIMO antenna is obtained, which is very suitable for
mobile terminals. A modal analysis of the antenna is carried out
first by means of the Theory of Characteristic Modes, in order to
identify the different radiating modes of the antenna. Then a set
of feeding configurations is proposed so as to excite these modes.
As the modes must operate in the same frequency band, a
loading technique is used in the antenna in order to shift the
resonance frequency of the modes to the proper band.
I. INTRODUCTION
The use of Multiple-Input Multiple-Output (MIMO)
systems has emerged as a very interesting strategy to increase
the throughput and capacity of wireless systems in rich
scattering environments [1]. Moreover, MIMO systems have
recently proven to be an attractive option for on-body sensor
networks, as they can counteract the signal fading produced
by the presence of the human body [2].
Traditionally, MIMO systems employ multiple antennas
spaced half wavelength or more to send or receive signals.
The integration of multiple antennas in mobile handsets or
sensors is not easy, due to the usually limited available space.
Thus, in order to achieve high performances, good isolation
and decoupling between antennas are required. To suppress
mutual coupling, several methods have been proposed in the
available literature [3]-[5].
Nevertheless, a more attractive option, that allows to
compact even more the size of the MIMO antenna consists of
using the different radiation patterns produced by different
modes of an antenna. Therefore, using a single antenna
different radiation characteristics can be obtained, which
exhibit a similar behaviour to those MIMO systems based on
multiple antennas or arrays [6].
Some multimode antennas for MIMO systems can be found
in the literature. In [6], a biconical antenna is proposed for
MIMO applications, while multimode spiral antennas are
presented in [7].
In this paper, a new multimode MIMO antenna is proposed,
which consists of a simple metallic ring excited at four points
with specific phase configurations.
The objective of this paper is to present a design procedure
of multimode MIMO antennas, based of the use of the Theory
of Characteristic Modes (TCM), that allows to identify the
different radiating modes of any antenna. By properly
choosing the excitation mechanism for each mode, a simple
compact antenna can be designed for MIMO applications. The
design procedure can be applied to other types of antennas,
whose current modes can also be computed with the TCM.
II. MODAL ANALYSIS OF A RING ANTENNA
A. Theory of Characteristic Modes
The TCM, first developed by Garbacz [8] and later refined
by Harrington and Mautz in the seventies [9], can be used to
obtain the radiating modes of any arbitrarily-shaped metallic
structure. This Theory has been already used for the design of
diverse wire and planar antennas, obtaining excellent results
[10].
Characteristic modes (Jn) can be defined as a set of
orthogonal real surface currents associated to any conducting
object, which depend of its shape and size, and are
independent of any excitation source. By definition, modes are
related to the power that can be radiated by the conducting
body [9].
Associated to characteristic currents, a set of characteristic
fields can be computed (En). Therefore, the field radiated by
the antenna can be expressed as a superposition of these
characteristic fields or modal fields. Moreover, since
characteristic far-fields are orthogonal, they provide
orthogonal radiation patterns, what is very interesting for their
use in MIMO systems.
A set of eigenvalues (λ λn) is also associated to characteristic
modes. Each eigenvalue determines the weight of each mode
in the total surface current [9]. The variation of eigenvalues
with frequency allows to determine the resonance frequency
and radiating bandwidth of the different modes, as it will be
shown in next section.
B. Characteristic Modes of a Metallic Ring
The geometry of the proposed MIMO antenna is shown in
Fig. 1. As observed, the antenna consists of a metallic ring in
free space, with 31 mm outer radius and 21 mm inner radius.
The antenna has also four slits located at φ=0º, 90º, 180º and
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270º, that accounts for the points where the excitation ports
will be placed.
YY
XX
rin
rin
rout
rout
φφ

Fig. 1 Geometry of the proposed ring antenna (rin=21 mm; rout=31 mm)

Mode J0
Mode J0

Mode J1
Mode J1

Mode J1’Mode J1’

Mode J2
Mode J2

Mode J3
Mode J3

Fig. 2 Normalized current distribution at 2.4 GHz of the first five
characteristic modes (Jn) of the ring antenna shown in Fig. 1

Next, the TCM has been used to compute the characteristic
modes of this structure. Fig. 2 shows the normalized current
distribution (Jn) at 2.4 GHz, associated to the first five
characteristic modes of this antenna. Arrows have also been
included in the figure, so as to facilitate the visualization of
the current flow.
As observed, mode J0 presents currents forming a close
loop around the ring, and as it will be demonstrated later, it is
a special non-resonant mode with inductive contribution at all
frequencies. Because of the symmetry of the ring, the first two
resonating modes, J1 and J1’, are degenerated modes that
present exactly the same current distribution, but with 90º
phase difference. Mode J1 presents two current nulls at
φ=±90º, while current nulls in mode J1’ are at φ=0º and 180º.
Finally, modes J2 and J3 are higher order modes with four
current nulls. Nulls of mode J2 are at φ=±45º and ±135º,
whereas nulls of mode J3 are at φ=0º, 180º and ±90º.
The resonance frequency of the current modes already
described can be determined using the information provided
by its associated characteristic angles (αn). Characteristic
angles can be defined as
180º tan
n
α=
where λn are the eigenvalues associated to each characteristic
mode [9]. From a physical point of view, the characteristic
angle models the phase angle between a characteristic current
Jn and the associated characteristic field En. Hence, a mode is
at resonance when its characteristic angle αn is 180º. The
closer the characteristic angle is to 180º, the better radiating
behavior presents the mode.
Fig. 3 illustrates the variation with frequency for the
characteristic angles associated to the first five characteristic
modes of the ring antenna. As shown, degenerated modes J1
and J1’ resonate at 2.1 GHz, mode J2 resonates at 3.5 GHz,
and mode J3 at 4 GHz. Note that since the ring does not
present rotational symmetry, modes J2 and J3 are not
degenerated. It can also be observed that characteristic angles
associated to mode J0 remain below 180º at all frequencies.
This means that mode J0 does not resonate and presents
inductive behaviour in the range of frequencies considered.

()
1
n
λ
−
−

Frequency (GHz)
1234567
Characteristic Angle αn (º)
90
120
150
180
210
240
270
α0
α1
α1'
α2
α3

Fig. 3 Variation of the characteristic angle with frequency, for the first five
characteristic modes of the ring antenna shown in Fig. 1


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III. EXCITATION OF MULTIPLE MODES IN THE ANTENNA
Once the modes have been extracted, next step is to find
optimum feeding schemes to excite the different modes
existing on the antenna. In this case, multimode operation can
be achieved by using four feeding ports symmetrically
distributed along the structure, as shown in Fig. 4. Table I
shows the different distribution of phases that can be
employed at the ports in order to excite modes J0, J1, J1’ and
J2. Due to the orthogonality properties of characteristic modes
over both the surface of the body and the enclosing sphere at
infinity, these modes radiate power independently of one
another.
YY
XX
++
--
--
++
++
--
--
++
P1 P1P3 P3
P2P2
P4 P4

Fig. 4 Geometry of the antenna fed with four sources

TABLE I
FEEDING CONFIGURATIONS (AMPLITUDE AND PHASE) FOR THE EXCITATION
OF DIFFERENT MODES IN THE ANTENNA

P1
1?
P2
1?
P3
1?
P4
1?
Excited modes
1st Conf.
0º 0º0º0º0 J
2nd Conf.
180º
1?

180º
1?

0º
1?
0º
1?
1J ,
1'J
3rd Conf.

Fig. 5 shows the current distributions obtained at 2.4 GHz
with the electromagnetic simulator Zeland IE3D when using
the three feeding configurations described in Table I. As
observed, the use of the first feeding configuration results in
the excitation of the non-resonant mode J0, whereas the
second configuration excites degenerated modes J1 and J1’
simultaneously, and the third configuration excites the higher
order mode J2. Due to the location of the sources, mode J3 is
not excited with any of the feeding configurations.
Considering these three feeding configurations, return loss
computed at each port of the antenna is depicted in Fig. 6.
Because of the symmetry of the structure, the return loss
obtained at every port is exactly the same. As observed, the
first configuration only provides good matching for
frequencies higher than 7 GHz. This is because mode J0,
which dominates at lowest frequencies, presents high
inductive nature and it is not well matched at lower
frequencies. In contrast, the second and third feeding
0º
1?
180º
1?

0º
1?
180º
1?

2 J
configurations yield broad matching bands at 2 GHz and 4
GHz, respectively.
Nevertheless, it is observed that each mode operates in a
different frequency band, which is centred near the resonance
frequency of each mode. In a MIMO system, modes must
operate in the same range of frequencies, being then necessary
to modify the geometry of the antenna for that purpose.

(a) 1stConf.: Excitation of
mode J0
mode J0

(a) 1stConf.: Excitation of

(b) 2ndConf.: Excitation
of modes J1 +J1’.
of modes J1 +J1’.
(b) 2ndConf.: Excitation

(c) 3thd Conf: Excitation
of mode J2
of mode J2
(c) 3thd Conf: Excitation

Fig. 5 Simulated current distribution at 2.4 GHz using the feeding
configurations shown in Table I

Frequency (GHz)
12345678
S11(dB)
-30
-25
-20
-15
-10
-5
0
1st Conf
2nd Conf
3rd Conf

Fig.6 Return loss computed for each of the feeding configurations shown in
Table I
IV. RING ANTENNA WITH CAPACITIVE LOADING
A possible solution to obtain multimode operation by
combination of four orthogonal modes working at the same
frequency band would consist of inserting capacitive loading
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in the antenna. The use of inductive or capacitive loading
allows to control the resonance frequency of each mode, and
hence the frequency band of operation [11].
Due to the inductive behavior of mode J0, the insertion of
four capacitive loads has been considered. This loading has
been implemented by inserting four slots in the antenna at
φ=±45º y ±135º. Fig. 7 shows the new geometry of the ring
antenna with four capacitive loads. From the current
distribution associated to the modes of the original antenna
(see Fig. 2), it is inferred that the location decided for these
loads will change the resonance frequency of modes J0, J1 and
J1’. In contrast, mode J2 would not be really influenced by the
presence of the capacitive loading since it presents very small
current amplitude at the loading allocation.

YY
XX

Fig. 7 Geometry of the ring antenna with capacitive loading

The current distributions associated to the characteristic
modes of the antenna with capacitive loading are very similar
to those shown previously in Fig. 2. However, characteristic
angles associated to the modes of this new antenna change as
expected. Fig. 8 represents the variation of the characteristic
angle versus frequency for the ring antenna with capacitive
loading. As it can be observed, if we compare these new
characteristic angles with those depicted in Fig. 3 for the
antenna without reactive loading, it can be seen that
resonance frequency of lower modes has been shifted up.
Thus, it can be observed that characteristic angle associated to
mode J2 does not change, whereas for the degenerated modes
J1 and J1’ the curve of the characteristic angle has shifted
towards higher frequencies. The resonance frequency of
modes J1, J1’ and J2 is now very close. Moreover, the
insertion of capacitive loads has compensated the inductive
effect exhibited by mode J0, making this mode to become a
resonant mode. Resonance frequency of mode J0 is around
2.1 GHz, which is also very close to the resonance
frequencies of the other three modes.
Then, in order to excite the four different modes in the
loaded antenna, the same feeding configurations shown in
Table I have been used. Fig. 9 shows the return loss obtained
at each input port in this case, for each one of the feeding
configurations.

Frequency (GHz)
1234567
Characteristic Angle αn (º)
90
120
150
180
210
240
270
α0
α1
α1'
α2
α3

Fig. 8 Variation of the characteristic angle with frequency, for the first five
characteristic modes of the ring antenna with capacitive loading

Frequency (GHz)Frequency (GHz)
1122334455667788
S11 (dB)
-26-26
-24 -24
-22-22
-20-20
-18-18
-16-16
-14 -14
-12-12
-10 -10
-8 -8
-6 -6
-4-4
-2-2
00
1st Conf
2nd Conf
3rd Conf3rd Conf
BWBW
S11 (dB)
1st Conf
2nd Conf

Fig. 9 Return loss obtained for the loaded antenna for the three feeding
configurations
1st. Configuration1st. Configuration
2nd. Configuration2nd. Configuration

3rd. Configuration3rd. Configuration

Fig. 10 Simulated radiation patterns obtained for the loaded antenna with the
three feeding configurations
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As observed, by adjusting the capacitance value of the
load, modal resonances have moved closer in frequency. As it
can be seen in the figure, a bandwidth (BW) of 43.55% is
obtained with the proposed design, if we consider a reference
value of -6 dB for the return loss. As observed, the central
frequency of the operating band is 4.1 GHz, which can be
shifted to other frequencies by simply scaling the antenna
dimensions.
Finally, Fig. 10 shows the radiation pattern exhibited by
the antenna for each of the three feeding configurations, at 4.1
GHz. As expected, the pattern associated to the mode excited
by each of the feeding configurations is obtained.
V. CONCLUSIONS
In the present paper the design procedure of a multimode
MIMO antenna has been presented. The design of the antenna
starts from the modal analysis of a metallic structure with a
ring shape, in which the modes computed exhibit orthogonal
radiation patterns. After analyzing the current distribution of
each mode, the optimum feeding mechanism to excite these
modes is chosen. Thus, three different feeding configurations
have been considered to excite four orthogonal modes in the
antenna. In order to make these modes to operate
simultaneously in the same frequency band, the antenna has
been capacitively loaded, obtaining a new geometry.
Therefore, a multimode antenna has been finally obtained,
which is suitable to operate with four orthogonal modes in a
MIMO system.
ACKNOWLEDGMENT
This work has been supported by Spanish Ministry of
Education and Science under project TEC2007-66698-C04-
03/TCM.

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