PIERS Proceedings, Kuala Lumpur, MALAYSIA, March 27–30, 2012
816
Reconfigurable E-shaped Antenna with Bandwidth Control for
Wideband Applications
Hattan F. Abutarboush1 and R. Nilavalan2
1
Electrical Engineering, Physical Science and Engineering
King Abdullah University of Science and Technology (KAUST), Saudi Arabia
2
School of Engineering and Design, Electrical and Electronics Engineering
Brunel University, London, UK
Abstract— A reconfigurable antenna based on E-shaped structure with bandwidth controlled
is proposed in this paper. The bandwidth of the antenna can be increased and controlled from
3.4% to 23% by varying the capacitance of the varactor diodes. The bandwidth can cover the
frequency range from 1.85 GHz–2.33 GHz. The antenna consists of four E-Shaped connected to
50 ohm GCPW line, varactor diodes and 10 pF ship capacitances. The total size of the antenna is
80 × 80 mm2 . The performances of the proposed antenna, in terms of return losses and radiation
patterns, with different DC bias voltages across the varactor diode, have been studied using
computer simulation and measurement. Simulated and measured results are in good agreements.
1. INTRODUCTION
There have been interests in the field of reconfigurable antennas where the multiband capabilities
can be further enhanced to incorporate multiple wireless standards [1]. Future cognitive communication systems will also require antennas capable of operating over wide bandwidth [2]. Modern
wireless communication systems relying on multiband reconfigurable antennas are becoming more
popular for their ability to serve multiple standards using a single compact antenna, allowing a
reduction in the dimensions of the wireless device and more space to integrate with other electronic
components [3–6].
Frequency reconfigurable antennas can be classified into two categories, namely continuous tuning and coarse tuning. Continuous tuning can be achieved using varactors and the antennas are
allowed for smooth transitions within or between operating bands [7]. Coarse tuning can be achieved
using PIN diode switches and the operating frequency is switched among different services, depending on the switching states [8].
In this paper, the proposed antenna has E-shaped radiators, a GCPW and four varactor diodes
connecting the E-Shapes. Through the use of DC bias voltage across the varactor diodes, the
bandwidth of the antenna can be controlled to be wide or narrow.
2. DESIGN CONCEPT
The structure of the proposed reconfigurable E-shape antenna is shown in Fig. 1(a), where four
radiators are optimized to allow the bandwidth of the antenna to be controlled over a wide range.
The varactors operate like variable capacitors. The positions of the varactor on the structure are
optimized using the HFSS simulator software to achieve the widest controlled range. The antenna
was fabricated using an FR-4 substrate with thickness of 1.57 mm and a relative permittivity of 4.4,
as shown in Fig. 1(b). To prove our design concept, a practical varactor diode, BB184 from NXP
with a capacitance value ranging from 2 pF to 14 pF controlled by a DC bias voltage varying from
1 V to 14 V was employed. The dimension of the varactor diode is approximately 1 × 1 mm2 . In
the simulations, the varactor diodes were modelled using Resistance, Inductance and Capacitance
(RLC) boundary sheet when the capacitance of the varactor diodes varied from 2 pF to 14 pF. The
characteristic of Fig. 2 are used for the varactor diode in the simulation tests.
3. SIMULATED AND MEASURED RESULTS
Results in Fig. 3 show that, when all the varactors are fixed at 2 pF, the bandwidth of the antenna
is 3.4% (for reflection coefficient S11 < −10 dB). The bandwidth of the antenna can be further
increased to 5%, 8% and 23% when increasing the varactor diodes to 4, 5 and 10 pF respectively.
This is summarized in Table 1. 10 pF chip capacitance is used to prevent the DC signal from flowing
to the main feed line but allow the RF current to pass through.
Progress In Electromagnetics Research Symposium Proceedings, KL, MALAYSIA, March 27–30, 2012 817
(a)
(b)
Figure 1: (a) Layout of proposed antenna and (b) fabricated prototype.
Figure 2: Capacitance versus DC bias voltage
for varactor BB184 (obtained from BB184 data
sheet).
Figure 3: Effect of varactor capacitance on the bandwidth.
Table 1: The achieved bandwidth VS value of varactor diodes.
Varactor diodes
2 pF
4 pF
5 pF
10 pF
Bandwidth (−10 dB)
3.4%
5%
8%
23%
To validate the simulation results, the proposed antenna has also been fabricated as shown in
Fig. 1(b) and the S11 in have been measured using Agilent N5230A vector network analyzer. Results
of the measured S11 are shown in Fig. 3. The simulated and measured results were in acceptable
agreements. There were small discrepancies between the simulated and measured results and this
could be attributed to the fabrication accuracy of the prototype and the losses of the varactor diodes.
Further results of simulations have reviewed that varying the capacitance values of the varactor
diodes alters the surface current distributions of the antenna, hence changing the bandwidth. It can
be seen that, meandered line technique has been used in part of the E-shape in the four radiators
to allow the antenna to operate at lower frequencies without using larger radiators.
PIERS Proceedings, Kuala Lumpur, MALAYSIA, March 27–30, 2012
818
(a)
(b)
Figure 4: Simulated radiation patterns with different biasing values (a) E-plane and (b) H-plane.
It should be noted that the bandwidth-tuning concept proposed here can be extended to cover
more bands or to serve the lower frequencies by using meandered line in all the E-shape. Since our
target is for the mobile and Wireless applications, we have limited the meandered line structure to
just in a single arm of the E-shape.
Figure 4 shows the simulated co- and cross-polarization patterns for E- and H-planes of the
antenna in different biasing state at 2.3 GHz with results normalized to the maximum values. It can
be seen that the shape of the radiation patterns does not change when applying different biasing
voltage. The cross polarization is lower than −10 dB at the broadside.
4. CONCLUSIONS
A reconfigurable wideband antenna, employing four E-shapes and meandered structure with a
compact volume of 80 × 80 × 1.57 mm3 , has been presented and studied using simulation and
measurement. Four varactor diodes are used to control the operating bandwidth of the antenna over
a wide range. The bandwidth of the antenna can be increased to a maximum of 23%. The simulated
results have shown that radiation patterns at 2.3 GHz are stable in different modes. The main
advantages of the proposed antenna include low profile, lightweight and easy to fabricate simple
structure targeting future smaller wireless communication devices and cognitive radio applications.
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