# Multiband Antennas for SDR Applications

Authors: E. Surducan; V. Surducan; D. Iancu; J. Glossner
Journal: International Journal of Digital Multimedia Broadcasting
 (2009)

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## Abstract

We present multiband antennas configurations for SDR applications. Using a composite folded dipole structure as starting point, we derived more complex antenna configurations to support multiple communication protocols for mobile application with linear and circular polarizations. Prototypes as single antenna with circular polarization, tunable single antenna with PIN diode and MIMO systems with three and four antennas, all derivatives of the same basic structure, were produced in an iterative fashion until the desired parameters were achieved. These antennas are suitable for microstrip circuit realizations and can be included in the printed circuit board (PCB) of the device, or used as stand alone. The shapes and measurement results are presented throughout the paper. From the illustrated graphs it can be seen that the stand-alone antennas exhibit positive gain for all the frequency bands of interest while the separation between antennas, for the multiple-input multiple-output (MIMO) case, is better than 15 dB.


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## Body

## 1. Introduction

To comply with the
requirements of portable multiple-communication protocol enabled hand-held
devices, a multiband antenna solution is highly desired. Most of the existing antennas
used in current hand-held devices have negative gain of several dBs  while base-band processing struggles to
recover the last tenth of dB gain through more complex algorithms. However,
considering antenna performance requirements and the tradeoffs associated with
operation over multiple disparate frequency bands, constraints on size and
limitations of commercial low-cost materials, typically required in handset
designs, this task becomes very
challenging and highly difficult. The multiband antennas configurations
presented in this paper are part of the SDR platform developed by Sandbridge
Technologies [1].Ultra-wideband antennas
and diversity techniques were intensively investigated and presented in the
literature with not only the goal of optimizing the gain and bandwidth [2], but
also by optimizing the number and position of antennas in stationary as well as
in mobile devices. Multiple-input multiple-output (MIMO) antenna systems and
diversity techniques are used successfully in today’s wireless communications
as part of the effort of increasing the overall data throughput and link
reliability [3, 4].  However, for space
diversity, one requirement is to place the antennas at least one half
wavelength apart. This restriction raises significant challenges when two or
more antennas are to be placed on today’s hand-held devices. Released and future release standards such as high-speed
packed access (HSPA), long-term evolution (LTE), and worldwide interoperability
microwave access (WiMAX), all specify open and/or close loop MIMO systems.In our approach,
we started the antenna design with a modified printed dipole antenna as
miniature dipoles with split arms, targeting wireless local area network (WLAN)
at 2.45 and 5 GHz frequencies bands [5–7]. A dual mode microstrip antenna [8]
using a switch for selecting one of the two antennas is described in [9]. To
avoid the extra components cost and the associated power consumption, in our
solution, we choose two dipoles working on different frequencies and fed by
microstrip lines at the same location. This configuration  looks like miniature
microstrip dipoles with split arms. We applied this technique first to the digital
video broadcasting terrestrial/hand-held (DVB-T/H) frequency bands by using a
composite structure, miniature directive antenna with high gain as shown in
[10, 11], similar to metamaterial dielectric support antennas [12, 13] as a
solution to enlarge the frequencies bands. As a second approach, similar to a Yagi antenna applied to microstrip
structures [14], we used a selective reflector placed behind the folded dipole
on multilayer dielectric substrate in order to enlarge de frequency bands. We
have redesigned the composite antenna for multiple communication standard hand-held
devices, supporting frequency bands specified by DVB-H, GSM, and WCDMA and we
incorporated the antenna to the printed circuit board (PCB) of the device [15].
Early efforts to implement MIMO antenna systems are described in automotive
[2], and hand-held devices [10, 11, 16].
Our form factor WLAN and WiMAX antenna design, with two receive and one
transmit antennas, is described in [17].A common solution
to tune the multiband antenna is the PIN diode switching technique [18, 19].
Our first effort in adding a PIN diode to our antenna is presented in [20]. The
antenna polarization can also be controlled by PIN diodes, as shown later the
paper, servicing this way the GPS L2 band.The paper is structured as follows. Section2 describes the approach we took for the antennas design, the measurement
approach is shown in Section 3 while Section 4 presents practical realization
and measurement results for different antennas. Section 5 concludes the paper.


## 2. Antenna Design

The new antenna
configurations are based on our previous work on multiband composite antennas
supporting DVB and WLAN frequency bands. The basic idea is to start with the
composite antenna and add a selective reflector in the near field location of
the active antenna (active in the sense of main radiator not in the sense of
active devices on the layer). Constructively, it is an antenna with two conductive
layers of similar printed patterns, top and bottom, and a composite dielectric
substrate of one or more dielectric layers with different permittivity [10, 11]. On
the top side is placed the matching circuit to the microwave line. The equivalent
schematic of the matching circuit, and the top and bottom side PCB of the antenna
(with related dimensions) are illustrated in Figure 1. L1 and C1 are distributed values due to the PCB, L2, C2, and ZSB are lumped surface mount components.  The
line impedance Z0 and the dipole impedance Zdip  are 50
and 200 Ohms, respectively.Figure 1
Schematic of the basic
antenna configurations.L1 and C1 are distributed values due to the PCB and L2, C2, and ZSB are surface mount components; Z0  is the line impedance
(50 Ohm) and Zdip ~ 200 Ohm. The di)  and wi)
dimensions figured on the PCB top and bottom side are specific to the particular
antenna design.In fact, the
antenna is an inductive multifolded dipole structure [5], exhibiting multiple
resonant frequencies F1<F2<F3, and having a common feed point located at zero voltage and
maximum current. In Figure 1, the
feed point on the top side is FPT and the related feed point on the
bottom side is FPB. The load impedance at the feed point on the
reflector side FPB may be short or
open circuit or, an impedance load, ZSB. The position (d10) of the feed on the
top side FPT is crucial for
the multiresonance frequencies of the antenna. The electrical properties of the
composite dielectric between the two conductive layers are directly related to
the gain as well as the width of the frequency bands covered by the antenna.
With this antenna configuration it is possible to cover all frequency bands
required by a specific application, with little change of the basic pattern. A
large variety of antennas  have been designed with dimensions ranging from 10 to
25 mm width, 20 to 50 mm length, and PCB thickness from 0.8 to 2.54 mm.  The dielectric permittivity εr was also varied from 2.8 to 10. In the following we present only the most
representatives of the produced antennas. The range of the frequency bands for
different communication standards is illustrated in Figure 2.Figure 2
Frequency bands covered
with composite antennas.


## 3.  Measurements

The most important
parameters characteristic to antennas are the return loss
(S11)
or the standing
wave ration (SWR), gain and directivity. All measurements were performed using
the Agilent N5230A (PNA-L)
VectorNetwork
Analyzer
(VNA). The S11 (or SWR) was measured directly with the VNA. Between S11
and SWR there is the following well known conversion formula [8]: (1)SWR=1+|S11|1−|S11|. For a reasonably good antenna the
SWR must be less than 3.For the gain
calculation, the Friis  transmission equation
was considered with the far-field condition as
described in [21, 22]:
(2)PR=(λ4πR)2G2PT,withR≥2d2λ.In the above equation,PR is the power measured at the receive antenna output
port, PT is the
power measured at the transmit antenna input port, G is the gain for both transmit and receive antennas, considered
identical, λ  is the wavelength, R  is the separation between
antennas, and d is the largest physical dimension of the antenna. From (2),
knowing that S12=S21, through simple algebraic manipulation,
the gain follows:(3)G=−3.779+10log(RF)+0.5(S21)+2.15, where G  is measured
in dBi, the transmission parameter S12 in dB, R in cm, and F in GHz.The directivityD  is the measure of the
directional properties of the antenna related to the isotropic antenna. The
directivity gain G, related to the isotropic dipole is also defined in [8] as (4)G=eD,with0<e<1,  where e is the radiation efficiency of the
antenna.The directive gain
was calculated with relation (3). The S21 parameter was measured
using two identical antennas or an antennas system. To obtain the directive
gain diagram one antenna was rotated around OZ direction as shown in Figure 3. In
the case of the antennas system the isolation between antennas was measured directly
with the VNA through the transmission parameter the between two antennas considering
that  Sij=Sji.Figure 3
TheXOY plan where the
directive gain was measured for the experimental antenna.


## 4. Antennas

### 4.1. GPS Antenna

The GPS
satellites transmit right-hand circularly polarized (RHCP)L-band signal known
as L1 at 1575.42 MHz. The minimum signal power level is −160  dBw at the Earth's
surface. The very low power level of  the GPS signal requires passive antennas of
a few dB gains in the immediate proximity of the receiver or, an active antenna
of minimum 15 dB gain if the antenna is placed on the roof of a car, building,
or boat. To achieve circular polarization from our antenna configuration, we
introduced a 90° delay line between the two branches of the
folded dipole in both sides, top and bottom, as shown in Figure 4.Figure 4
GPS omnidirectional antenna.The top feed is
through a microstrip while the bottom feed is inductive. A shift ofΔz
between the top and bottom layer was added, as shown in Figure 4, to compensate
for the phase shift between the two sides.We built two-prototype
antennas, first on RT-6010LM substrate: 2.45 mm thick, 15 mm wide, 27 mm high, permittivityεr=10.7,
and dissipation factor tgδ=0.0028, and the second on common PCB
substrate: 1.5 mm thick, 18.5 mm wide, 33 mm high, and εr=4.6. For the antenna shown  in Figure 5
the values of (di)  and  (wi), measured in mm, (as shown in
Figure 1) are presented in Table 1.Table 1
di and wi values, measured
in mm, for GPS antenna.
i0123456789101112di2217.610.62.02.01.53.12.01.51.520.820.81.0wi157.11.40.55.80.50.51.52.5————Pictures of GPS
antenna, top and bottom sides.
(a)(b)We tested the antenna performance first in the
lab. After
the lab performance test the overall performance was evaluated on real GPS receiver.
Figure6 illustrates both the simulation and test results for the PCB substrate
GPS antenna. From Figure 6 it can be seen that the return loss S11 is
−25.77 dB at 1.5753 GHz. The measured bandwidth at 1.5753 GHz is around 4 MHz
while at S11=−10 dB the bandwidth is 67 MHz.Figure 6
Simulation and experiment for the GPS antenna return loss.The field test
for the antenna was performed on the Sandbridge Technology SDR platform as
described in Figure7.   The total sensitivity
of the GPS receiver with the passive antenna was estimated at −132 dBm [23]
while adding a 27 dB signal booster right after the antenna, the total
sensitivity came down to −159 dBm.Figure 7
GPS test platform.


### 4.2. PIN Diode Multiband Antenna

The composite
folded dipole antenna can be electronically tuned in frequency by adding a PIN
diode to the matching circuit on the active layer as shown in Figure8. For the
prototype antenna we used the same RT-6010LM substrate and physical dimensions
17×22×2.45 mm. The antenna configuration
is presented in Figure 8.Figure 8
PIN diode antenna
diagram.D is the PIN diode, L=0.2 mH, and C=40 pF.The input RF line
(1) is DC decoupled (2) from the antenna top layer (8). The two branches of the
dipole are phase shifted  by a delay line (4). The PIN diode (3) does the
impedance matching, frequency band selection, and it also changes the dipole
polarization. The diode is tuned through the DC line (5) and the inductance
(6). The bottom layer (7) of the  antenna is totally separated from the top by
the dielectric material of thicknessh as shown in Figure 9. A picture of the
antenna with the PIN-DC connector is presented in Figure 10.Figure 9
Antenna configuration
with PIN diode.Figure 10
Picture of the PIN
diode antenna, top and bottom.For the antenna
shown  in Figure10 the values of (di)  and (wi), measured in mm, (as shown in Figure 1) are presented in
Table 2.Table 2
di  and wi values,
measured in mm, for the antenna with PIN diode.
i0123456789101112di1712.89.51.92.92.05.53.01.51.41.01.251.0wi14.87.11.40.55.80.50.51.52.5————For this design
configuration the return loss measurementS11 is the main feedback
parameter. Figure 11 depicts the return loss of the antenna as a function of
frequency for different PIN diode polarizations, U1<U2<U3, where U1=0 V, U2=0.65 V, and U3=2.00 V. It can be seen that the return loss for different
frequency bands varies with the PIN diode polarization. This property will be
exploited to tune the antenna for various frequency bands required by specific
communication standards.Figure 11
Return loss
measuremenU1=0.0 V, U2=0.65 V, U3=2.00 V are the PIN diode polarization
voltages.The next diagrams
illustrate the measurements performed to characterize the electric field
pattern as a function of the PIN diode polarization in the frequency bands of
interest. The directive gain of the antenna was measured in theXOY plan,
normal to the antenna layers, as shown in Figure 12.Figure 12
Directive gain for
WiMAX at 3.47 GHz.In the WiMAX
frequency band, Figure12 shows a gain improvement of 6 dB corresponding to the
switching of the PIN polarization from U1 to U3. The main radiative direction
remains unchanged at 135° (45° to the OY normal).In
the WLAN frequency band, the PIN diode polarization, switched fromU1 to U2,
caused an increase in the normal direction gain (Φ=90°) from −0.9 dBi to 3.7 dBi
as shown in Figure 13.Figure 13
Directive gain for WLAN
at 2.45 GHz.In the GPS
frequency band the changes with the PIN polarization voltage are more dramatic.
As the polarization voltage changes fromU1 to U2 the directivity pattern will
change from directional at 35° to omnidirectional. The gain rests unchanged
at 35°, but increases by an average of 5 dB in the other directions
(Figure 14). If the voltage is changed from U1 to U3 the directional pattern
stays unchanged but the radiation direction will change with 90°  from
35° to 125°  with a gain improvement of 1.5 dB.Figure 14
Directive gain for GPS
at 1.57 GHz.In the GSM band, a
gain improvement of 6 dB at270° is observed when the polarization
changes from U1  to U2 (practically a reverse in the radiation direction). Also
a change from directional (at U1)  to omnidirectional pattern (at  U3) can be observed
(see Figure 15).Figure 15
Directive gain for GSM
at 980 MHz.


### 4.3. MIMO Systems

Multiple folded dipole composite
antenna can be placed on the same PCB to give way to a MIMO system. Three such
antennas, two on the receive side and one on the transmit side were used on the
Sandbridge Technology form factor SDR platform capable of executing both WiMAX
and WLAN [17].The block diagram of the SDR platform
is illustrated in Figure16 while the picture of the card with the antennas is
shown in Figure 17. The TOP layer includes the microstrip feeding point and the
impedance matching circuit of each antenna in the system to 50 Ohms line impedance.
The bottom layer acts as frequency selective reflector, having similar print
pattern as the top layer. In order to reduce the mutual coupling between antennas,
a free space cut of 2.5 mm was added to the dielectric substrate near the sides
of the middle antenna.Figure 16
SDR platform using
three antennas.Figure 17
Picture of the SDR
platform with three antennas.The
antennas in the MIMO system were positioned such a way to maximize the spatial
diversity. In order to increase the isolation between  antennas, the middle antenna
in the system was positioned to have the polarization perpendicular to the lateral
antennas polarization.The antennas
system was built on 12 layers onIsolaFR406BC PCB with
1.2 mm total thickness, global permittivity εr=3.8 and dissipation factor less than tgδ=0.0140.
The PCB gap between antennas is 2.5 mm wide.For the MIMO
antennas system described in Figure17, the values of (di)  and  (wi),
measured in mm, (as shown in Figure 1) are presented in Table 3. ZSB is open circuit for Rx1 antenna and short circuit (ZSB=0 Ohm) for
Tx and Rx2 antennas.Table 3
di and wi values,
measured in mm, for the three antennas system.
i0123456789101112di1711.010.51.22.82.03.82.41.52.41.250.71.0wi144.51.51.05.01.50.51.50.5————A 2 × 2 MIMO system, capable of both FDD and TDD
communication modes, can be achieved by adding the forth antenna, as shown in
Figure18. The four-antenna system is an improved version of the previous three-antenna
design, where  Rx1 antenna has been
symmetrically duplicated. In this case the complexity of the design will
increase due to mutual coupling between the antennas. To reduce the mutual
coupling a metal shield is added as shown in Figure 18.Figure 18
Four antennas in2×2
MIMO configuration.Two types of
measurement were performed for the MIMO antennas  system
characterizations: return lossS11 and isolation Sij between antennas. The return
loss measurements are illustrated
in Figure 19 for the MIMO system of three antennas.Figure 19
Return lossS11
measurements for MIMO system.In Figure19 it
can be seen that the return loss S11 for the M and R (Figure 17)  antennas
is less than −5 dB in the 2.3–2.6 GHz band. The best antenna in the
system is the Tx   (L in Figure 17) antenna with S11>−10 dB in the 2.3–2.67 GHz
band. In the WLAN band the standing wave ratio is less than −10 dB for all antennas.We estimated the
mutual coupling by using one of the antennas as a transmit antenna and the
other two as receive antennas. The isolation measurements between antennas are
presented in Figure20. It can be seen an isolation better than −15 dB between
all antennas.Figure 20
Isolation between antennas
for the three-antenna configuration.Theoretical analyse of the2×2-antenna configuration
regarding the isolation between antennas show that the worst isolation is
between antennas 1 and 4 and, antennas 1 and 3 as shown in Figure 21.Figure 21
Isolation between antennas
for  the four-antenna configuration.


## 4.1. GPS Antenna

The GPS
satellites transmit right-hand circularly polarized (RHCP)L-band signal known
as L1 at 1575.42 MHz. The minimum signal power level is −160  dBw at the Earth's
surface. The very low power level of  the GPS signal requires passive antennas of
a few dB gains in the immediate proximity of the receiver or, an active antenna
of minimum 15 dB gain if the antenna is placed on the roof of a car, building,
or boat. To achieve circular polarization from our antenna configuration, we
introduced a 90° delay line between the two branches of the
folded dipole in both sides, top and bottom, as shown in Figure 4.Figure 4
GPS omnidirectional antenna.The top feed is
through a microstrip while the bottom feed is inductive. A shift ofΔz
between the top and bottom layer was added, as shown in Figure 4, to compensate
for the phase shift between the two sides.We built two-prototype
antennas, first on RT-6010LM substrate: 2.45 mm thick, 15 mm wide, 27 mm high, permittivityεr=10.7,
and dissipation factor tgδ=0.0028, and the second on common PCB
substrate: 1.5 mm thick, 18.5 mm wide, 33 mm high, and εr=4.6. For the antenna shown  in Figure 5
the values of (di)  and  (wi), measured in mm, (as shown in
Figure 1) are presented in Table 1.Table 1
di and wi values, measured
in mm, for GPS antenna.
i0123456789101112di2217.610.62.02.01.53.12.01.51.520.820.81.0wi157.11.40.55.80.50.51.52.5————Pictures of GPS
antenna, top and bottom sides.
(a)(b)We tested the antenna performance first in the
lab. After
the lab performance test the overall performance was evaluated on real GPS receiver.
Figure6 illustrates both the simulation and test results for the PCB substrate
GPS antenna. From Figure 6 it can be seen that the return loss S11 is
−25.77 dB at 1.5753 GHz. The measured bandwidth at 1.5753 GHz is around 4 MHz
while at S11=−10 dB the bandwidth is 67 MHz.Figure 6
Simulation and experiment for the GPS antenna return loss.The field test
for the antenna was performed on the Sandbridge Technology SDR platform as
described in Figure7.   The total sensitivity
of the GPS receiver with the passive antenna was estimated at −132 dBm [23]
while adding a 27 dB signal booster right after the antenna, the total
sensitivity came down to −159 dBm.Figure 7
GPS test platform.


## 4.2. PIN Diode Multiband Antenna

The composite
folded dipole antenna can be electronically tuned in frequency by adding a PIN
diode to the matching circuit on the active layer as shown in Figure8. For the
prototype antenna we used the same RT-6010LM substrate and physical dimensions
17×22×2.45 mm. The antenna configuration
is presented in Figure 8.Figure 8
PIN diode antenna
diagram.D is the PIN diode, L=0.2 mH, and C=40 pF.The input RF line
(1) is DC decoupled (2) from the antenna top layer (8). The two branches of the
dipole are phase shifted  by a delay line (4). The PIN diode (3) does the
impedance matching, frequency band selection, and it also changes the dipole
polarization. The diode is tuned through the DC line (5) and the inductance
(6). The bottom layer (7) of the  antenna is totally separated from the top by
the dielectric material of thicknessh as shown in Figure 9. A picture of the
antenna with the PIN-DC connector is presented in Figure 10.Figure 9
Antenna configuration
with PIN diode.Figure 10
Picture of the PIN
diode antenna, top and bottom.For the antenna
shown  in Figure10 the values of (di)  and (wi), measured in mm, (as shown in Figure 1) are presented in
Table 2.Table 2
di  and wi values,
measured in mm, for the antenna with PIN diode.
i0123456789101112di1712.89.51.92.92.05.53.01.51.41.01.251.0wi14.87.11.40.55.80.50.51.52.5————For this design
configuration the return loss measurementS11 is the main feedback
parameter. Figure 11 depicts the return loss of the antenna as a function of
frequency for different PIN diode polarizations, U1<U2<U3, where U1=0 V, U2=0.65 V, and U3=2.00 V. It can be seen that the return loss for different
frequency bands varies with the PIN diode polarization. This property will be
exploited to tune the antenna for various frequency bands required by specific
communication standards.Figure 11
Return loss
measuremenU1=0.0 V, U2=0.65 V, U3=2.00 V are the PIN diode polarization
voltages.The next diagrams
illustrate the measurements performed to characterize the electric field
pattern as a function of the PIN diode polarization in the frequency bands of
interest. The directive gain of the antenna was measured in theXOY plan,
normal to the antenna layers, as shown in Figure 12.Figure 12
Directive gain for
WiMAX at 3.47 GHz.In the WiMAX
frequency band, Figure12 shows a gain improvement of 6 dB corresponding to the
switching of the PIN polarization from U1 to U3. The main radiative direction
remains unchanged at 135° (45° to the OY normal).In
the WLAN frequency band, the PIN diode polarization, switched fromU1 to U2,
caused an increase in the normal direction gain (Φ=90°) from −0.9 dBi to 3.7 dBi
as shown in Figure 13.Figure 13
Directive gain for WLAN
at 2.45 GHz.In the GPS
frequency band the changes with the PIN polarization voltage are more dramatic.
As the polarization voltage changes fromU1 to U2 the directivity pattern will
change from directional at 35° to omnidirectional. The gain rests unchanged
at 35°, but increases by an average of 5 dB in the other directions
(Figure 14). If the voltage is changed from U1 to U3 the directional pattern
stays unchanged but the radiation direction will change with 90°  from
35° to 125°  with a gain improvement of 1.5 dB.Figure 14
Directive gain for GPS
at 1.57 GHz.In the GSM band, a
gain improvement of 6 dB at270° is observed when the polarization
changes from U1  to U2 (practically a reverse in the radiation direction). Also
a change from directional (at U1)  to omnidirectional pattern (at  U3) can be observed
(see Figure 15).Figure 15
Directive gain for GSM
at 980 MHz.


## 4.3. MIMO Systems

Multiple folded dipole composite
antenna can be placed on the same PCB to give way to a MIMO system. Three such
antennas, two on the receive side and one on the transmit side were used on the
Sandbridge Technology form factor SDR platform capable of executing both WiMAX
and WLAN [17].The block diagram of the SDR platform
is illustrated in Figure16 while the picture of the card with the antennas is
shown in Figure 17. The TOP layer includes the microstrip feeding point and the
impedance matching circuit of each antenna in the system to 50 Ohms line impedance.
The bottom layer acts as frequency selective reflector, having similar print
pattern as the top layer. In order to reduce the mutual coupling between antennas,
a free space cut of 2.5 mm was added to the dielectric substrate near the sides
of the middle antenna.Figure 16
SDR platform using
three antennas.Figure 17
Picture of the SDR
platform with three antennas.The
antennas in the MIMO system were positioned such a way to maximize the spatial
diversity. In order to increase the isolation between  antennas, the middle antenna
in the system was positioned to have the polarization perpendicular to the lateral
antennas polarization.The antennas
system was built on 12 layers onIsolaFR406BC PCB with
1.2 mm total thickness, global permittivity εr=3.8 and dissipation factor less than tgδ=0.0140.
The PCB gap between antennas is 2.5 mm wide.For the MIMO
antennas system described in Figure17, the values of (di)  and  (wi),
measured in mm, (as shown in Figure 1) are presented in Table 3. ZSB is open circuit for Rx1 antenna and short circuit (ZSB=0 Ohm) for
Tx and Rx2 antennas.Table 3
di and wi values,
measured in mm, for the three antennas system.
i0123456789101112di1711.010.51.22.82.03.82.41.52.41.250.71.0wi144.51.51.05.01.50.51.50.5————A 2 × 2 MIMO system, capable of both FDD and TDD
communication modes, can be achieved by adding the forth antenna, as shown in
Figure18. The four-antenna system is an improved version of the previous three-antenna
design, where  Rx1 antenna has been
symmetrically duplicated. In this case the complexity of the design will
increase due to mutual coupling between the antennas. To reduce the mutual
coupling a metal shield is added as shown in Figure 18.Figure 18
Four antennas in2×2
MIMO configuration.Two types of
measurement were performed for the MIMO antennas  system
characterizations: return lossS11 and isolation Sij between antennas. The return
loss measurements are illustrated
in Figure 19 for the MIMO system of three antennas.Figure 19
Return lossS11
measurements for MIMO system.In Figure19 it
can be seen that the return loss S11 for the M and R (Figure 17)  antennas
is less than −5 dB in the 2.3–2.6 GHz band. The best antenna in the
system is the Tx   (L in Figure 17) antenna with S11>−10 dB in the 2.3–2.67 GHz
band. In the WLAN band the standing wave ratio is less than −10 dB for all antennas.We estimated the
mutual coupling by using one of the antennas as a transmit antenna and the
other two as receive antennas. The isolation measurements between antennas are
presented in Figure20. It can be seen an isolation better than −15 dB between
all antennas.Figure 20
Isolation between antennas
for the three-antenna configuration.Theoretical analyse of the2×2-antenna configuration
regarding the isolation between antennas show that the worst isolation is
between antennas 1 and 4 and, antennas 1 and 3 as shown in Figure 21.Figure 21
Isolation between antennas
for  the four-antenna configuration.


## 5. Conclusions

We
have shown that starting from a basic antenna design there are many possibilities
for practical configurations targeting mobile applications. A single composite
antenna can be configured to be a directional or omnidirectional GPS antenna.
With a few minor changes in the shape of the print and adding an adaptive/tuning
circuit with a PIN diode can significantly improve the antenna gain,
polarization, and selectivity proprieties. A shift and enlargement of the
frequencies bands has also been observed. From directivity diagrams follows
that the antenna directive gain depends on the PIN diode polarization. It can
also be observed an improvement in the antenna gain and a change in the antenna
radiation pattern distribution. We proved that by changing the PIN diode DC
voltage it is possible to select a specific directivity and also to select a
number of simultaneous frequency bands or to enlarge one or more frequency
bands.We
also showed that starting from the single antenna configuration a system of
MIMO antennas can be also obtained with very good performance.



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*Source: 460143-2009-04-23.xml*