Design and Configuration Techniques of a Low-Profile Reconfigurable
Antenna for a Cognitive Radio System
Joseph A. Zammit, Adrian Muscat
Department of Communications
and Computer Engineering
University of Malta
Email: jzam0022@um.edu.mt,adrian.muscat@um.edu.mt
Abstract
Cognitive mobile radios in the longer term will
require broadband small antennas spanning the UHF
and lower microwave mobile radio bands, typically
400MHz to 3Ghz. For these bands to be covered the
antenna requires technology that allows hard switching
and soft tuning mechanisms both in the radiating
structure as well as in the feeding structure to achieve
the desired bandwidth. These antenna controller is
expected to learn in an efficient way how to tune the
antenna that is under its control with the influence of
the physical and operative radio environment.
One way of implementing a tuning algorithm in
a fully reconfigurable antenna is to make use of a
heuristic algorithm. However this technique will result
in a large search space. In this paper a fully reconfigurable antenna consisting of a pixel patch antenna is
presented and several sets of simulations are presented
each showing how the antenna can be tuned to a
desired frequency.In this first order antenna model
several techniques are suggested to prune the resulting
search space. The methods are derived from three
distinct sets of simulations that take into consideration
the effects of sub-patch hard switching, varactor diode
loading and shorting post loading. The results show
that such an antenna is feasible.
Index Terms
antenna, cognitive radio, re-configurable, antenna
switch
1. Introduction
As more wireless services are becoming more common, the available radio spectrum is decreasing even
as higher frequency bands are being opened for use.
Another issue is that different countries have their
own spectrum allocations and spectrum management
techniques which restrict the user’s mobility. Cognitive
Radio technology tries to overcome these issues by
adding a level of computational intelligence to the
terminal to provide a seamless service and dynamic
service. 1 shows the hardware architecture of a modern
Software Defined Radio (SDR).
Figure 1. Hardware Architecture of Software Defined Radio [1]
Many advances have been made in the field of
microelectronics to optimize the RF front end of the
SDR. However modern commercial radio systems are
limited to a few bands such as tri-band mobile phones
or the SpeakEasy [1] system. A cognitive system needs
to have access to the full radio spectrum to be able to
operate in an optimal manner. Currently the limiting
factor in the access to the radio spectrum has been
the antenna. Antennas have been traditionally designed
to operate on a single frequency or to be switched
between a few bands. Reconfigurable antennas aim to
fill this gap by being able to switch between various
frequencies. Various attempts are described in [2], [3],
[4] with a degree of success. However a new class
of reconfigurable antennas is needed which can tune
from the UHF to the upper microwave band (0.4GHz to
3GHz). Moreover antenna control algorithms for rapid
switching and tuning need to be developed. However
before this can be achieved, the various antenna parameters need to be adequately represented in order
for the best solution to be found. This paper describes
attempts to design highly reconfigurable antennas by
the means of ideal switches (hard tuning) and varactor
diodes (soft tuning). A fully reconfigurable pixel patch
patch antenna is presented and techniques are proposed
to reduce the search space generated by such a reconfigurable antenna.
but retained a similar pattern. Due to its small size the
antenna had a low overall gain.
2. Reconfigurable antenna techniques
There are two main methods where the resonant
frequency of the antenna may be changed. The first
is using switches, usually pin diodes [3] or MEMS
switches [5]. This is known as hard switching and
produces an antenna which can switch through various
bands while retaining good gain and efficiency.
Soft tuning techniques utilize reversed biased varactor diodes in order to continuously change the resonant
frequency [?]. Such a method is easy to control but
produces antennas with a very low gain and efficiency.
Many authors have worked on reconfigurable antennas. The slot antenna built by Behdad [2] is a varactor
tuned slot antenna which can resonate from 1.5GHz
to 3GHz. Nikolaou [3]using an annular slot antenna
could switch between 5.8GHz and 6.4GHz. Also the
polar pattern could be reconfigured as well. Holland [4]
built a tunable coplanar antenna that had a 500MHz
bandwidth. Patch antennas could also be frequency
reconfigured such as in the case of Chung et al [6],
the patch antenna had a U shaped notch which could
be shorted using a pin diode. The antenna could switch
between 2.4GHz and 2.62GHz. The antenna was built
but the pin diodes were substituted by metal tapes ,
hence their effect on the antenna was not included
in the experiment. However these antennas though
they had a good performance only had a very limited
tuning range. Cetiner at al [7] describe a pixel-patch
reconfigurable antenna which can change its frequency
and polarization.
The author has investigated both soft tuned and hard
tuned antennas and a highly reconfigurable antenna
consisting of both soft and hard tuned elements [8],
[9], [10]. All the antennas produced good results which
are under further investigation.
In [8] a small antenna measuring 22x20mm was
developed. The antenna could be tuned from 0.55GHz
to 1.5GHz. Such low resonant frequencies were obtained by the use of shorting posts in line with varactor
diodes as shown in Fig. 2. The radiation pattern of the
antenna changed with decreasing resonant frequency
Figure 2. Varactor Tuned shorted post antenna
A different approach was taken in [9] where the antenna was reconfigured by switching parasitic coupling
patches. A total of 13 switch combinations covered a
bandwidth from 1.36GHz to 1.92GHz. However such
a system left some frequency bands which were not
covered. The advantage of using switched coupling
patches was that the antenna exhibited a higher efficiency than the soft tuned antenna. Figure 3 shows the
frequency response of the antenna structure.
Figure 3. Frequency response of the parasitic
switched patch antenna
Both techniques using soft tuning and hard switching were demonstrated in [10]. The proposed structure
consisted of switchable shorting posts and varactor
diodes. The structure of the antenna is shown in Fig
4. The antenna resonated from 0.55GHz to 0.7GHz
and from 1.3GHz to 2GHz. However there was a large
variation in the gain and efficiency over the whole
band.
Figure 5. Fully Reconfigurable antenna
Number of vertical switches =
Pw (Pl − 1)
(2)
Total number of switches in matrix
= (Pw − 1)Pl + Pw (Pl − 1)
= Pw Pl − Pl + Pw Pl − Pw
Figure 4. Reconfigurable antenna using hard and
soft tuning techniques
(3)
If Pl = Pw than equation 3 becomes
3. Antenna Design
In order to have a fully reconfigurable antenna,
the antenna needs to be able to change its shape to
be resonate at the required frequency. In order to
achieve this a series of square patches were fully
interconnected together using ideal switches. Figure 5
shows the whole concept. The patches were selected
to be 4mm x 4mm each and an array consisting of 6 x
4 sub patches was constructed where each sub-patch
was fully connected to its neighbours. The following
is the derivation of the number of switches needed for
full interconnection.
Switches along the width = Pw
Switches along the length = Pl
Number of horizontal switches =
(Pw − 1)Pl
= 2Pw Pl − (Pl + Pw )
(1)
2Pw2 − 2Pw
(4)
It can be noted that equation 4 has a complexity of
O(n2 ). Thus a large number of switches will be needed
in order to increase the reconfigurability of the antenna.
In this case the antenna has a total of 38 switches.
4. Experiments and results
Two types of configurations were run on the antenna.
In the first case the switches were reconfigured in a
way to emulate several classic antennas, in the second
case a dipole shape was chosen and several parasitic
elements were added to it in order to vary the resonant
frequency. The configured shapes are shown in Fig. 6.
Simulations were performed using CST microwave
studio [11]. The antenna was simulated on a Rogers
4003 substrate having an εR of 3.38mm and a height
Figure 6. Reconfigurable antenna regular shapes
Table 1. Resonant frequency for various shapes
Antenna Shape
Rectangular
H shape
Small Octagon
Octagon
Patch with slots
Dipole
Meander line Patch
Short Dipole
Square
Figure 7. Resonant frequency for regular shapes
of 1.524mm. A small ground plane 80x40mm was
used to simulate the size of a mobile terminal. For
all simulations the antenna was fed from at the centre
of the patch by a probe feed. Figure 7 and 1 show
the results that were obtained when the antenna was
reconfigured into various regular shapes. It can be seen
that a high gain and antenna efficiency were obtained
over a large bandwidth stretching from 2.56GHz to
3.92GHz.
Next the ”dipole” antenna was selected and the
effects of switching on various parasitic patches next
Resonant
Frequency
(GHz)
2.56
1.96
4.17
2.77
4.3
2.59
3.57
3.92
3.82
S11
(dB)
Gain
(dB)
Efficiency
(%)
-3.4
-11.6
-32.5
-4.9
-11.62
-11.47
-27.81
-17.73
-13.87
4.5
3.3
6.04
4.05
4.1
4.1
7.26
6.1
6.8
39
54
33
41
56
57
84
75
81
to the dipole were observed. The position and number
of parasitic patches were varied. Figure 8 shows the
combinations of the dipole which were selected. The
results are shown in table 2 and the return loss in figure
9. It can be noted that by switching on and off parasitic
patches the resonant frequency can be changed.
5. Discussion of results
From the above results it can be conclude that a
reconfigurable antenna made up of an array of subpatches can be made to change its resonant frequency
by either reconfiguring the whole shape of the antenna
or by adding parasitic patches to a standard shape. This
Figure 8. Reconfigurable antenna regular shapes
Table 2. Resonant frequency for parasitic
additions to the dipole
Dipole Combination
Combination
Combination
Combination
Combination
Combination
Combination
Combination
Combination
Combination
Figure 9. Return loss of the added parasitic
patches to the dipole
process will happen dynamically in the terminal while
it is searching for a unused part of the radio spectrum.
However the large number of switch combinations
make it lengthy operation to search in real time for
the optimal solution. As shown in 4 the search space
increases in a non-linear fashion and techniques to
prune the resulting search space need to be used.
The simplest method would be to exhaustively
search all switch combinations and produce a look up
table which can be referenced to select the required
frequency. However to make an exhaustive search
would take a long time to complete due to the large
number of simulations required. Also such an antenna
1
2
3
4
5
6
7
8
9
Resonant
Frequency
(GHz)
5.87
2.27
2.09
5.65
2.44
2.2
2.3
2.25
2.26
S11
(dB)
Gain
(dB)
Efficiency
(%)
-11.5
-4.9
-12.9
-9.1
-8.6
-12.0
-1.5
-11.3
-10.4
5
3.6
3.8
6.2
3
3.7
1.8
2.42
2.4
78
40
56
78
45
57
14
43
42
will not be able to adapt to the radio environment
dynamically.
Other techniques such as the use of genetic algorithms [?] and simulated annealing [?] may be used to
find a particular resonant frequency. A weighted fitness
function consisting of the the following parameters
maybe used. For a real time system parameters 3,4,5
made be more difficult to measure.
1) Resonant frequency
2) Return loss
3) -10dB bandwidth
4) Gain
5) Efficiency
The switches could be represented by bits in the
genome so for example the antenna presented in this
paper could be represented as a genome with a length
of 38 bits.
The optimal way to find the best combinations in
the search space is to use evolutionary algorithms and
case based rules to prune the search space. A method
to reduce the search space is to ensure that the when a
switch is put in an ’on’ position a path exists between
the switch and the antenna probe. Another method is
for the search algorithm to estimate the length and
width of the patch to produce the desired resonant
frequency and then add/subtract parasitic patches to
produce the desired result. Also results that have
produced an optimized antenna could be stored to be
used in future cases.
The reconfigurable patch suggested in this paper is
made up is square sub-patches and is a regular shape.
For specialized applications irregular sub-patches may
be used to produced the desired result. It can be also
suggested that and offline evolutionary algorithm could
be used to develop the antenna structure, which is turn
can be used by other evolutionary algorithms during
operation to configure the antenna.
6. Conclusion
This paper has discussed the current literature in
reconfigurable antennas. It has shown a fully reconfigurable pixel-patch antenna made up of an array of
square sub-patches and how they can be re-configured
to resonate at different frequencies. It has also demonstrated how parasitic patches can be added so that
the resonant frequency changes slightly. The results
show that this can be achieved though the matching is
very poor. This could be solved by adding a tunable
impedance matching network as suggested by Mingo
et al. [12].
The resultant reconfigurable antenna consists of a
large number of switches which means a large search
space in order to find the optimal solution. Several
techniques are suggested on how to prune and search
this space.
Acknowledgements
The authors wish to thank the University of Malta
for providing funds for this research project.
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