Frequency Reconfigurable Antenna Using Active Capacitors I. ROUISSI1,2, I. BEN TRAD2 J.M. FLOC‟H2, H. RMILI3, H. TRABELSI1 1
URCRFS, FST, University of Tunis El Manar, 1002 Tunis, Tunisia (
[email protected]) IETR, INSA Rennes, 20 Avenue Buttes de Coësmes, 35043 Rennes, France (
[email protected]) 3 King Abdulaziz University, Faculty of Engineering, and Computer Engineering Departement, P.O. Box 80204, Jeddah 21589, Saudi Arabia (
[email protected],
[email protected]) 2
Abstract—The development in wireless communication requires the use of tunable antenna adapted to the RF environment and providing optimal connectivity. In this paper, the design of frequency reconfigurable antenna is discussed. The reference square patch structure was optimized to operate at 2.45GHz. The frequency agility was achieved by integrating a varactor diode, a tuning range of 0.5-2.03GHz is realized. Then to improve the antenna performances, the varactor diode was replaced by a digital capacitor. A tuning range of 1.65- 2.23GHz was achieved with better radiation properties (impedance matching, gain...) due to the capacitor’s ability to dynamically command the antenna using a binary code. Prototypes were realized for both cases. Simulated and measured results are presented and discussed.
exhaustive study using lumped capacitor and varactor diode was made [11, 12]. It was found that the most suitable position to set the varactor diode was in the corner of the antenna between the patch and the ground plane as shown in Fig.1. In the aim to enhance the antenna agility behaviour, the varactor diode was replaced by a digital capacitor as presented in Fig.2. Antenna performances are significantly improved by minimizing losses and avoiding the use of bias circuit. Actually, depending on the enabled binary code (6 bits- 16 states), one capacitance value is selected from the 16 states (from 0.854 to 3.454pF at shunt configuration).
Keywords—Frequency reconfigurable antenna; varactor diode; digital capacitor.
I. INTRODUCTION The expansion of wireless communication applications require a new generation of antennas able to cater the need of such applications [1]. The new era of antenna design must generate antennas that are able to tune their operating characteristics (frequency, polarization, radiation pattern) according to the ever-changing communication requirements [2-3-4]. Reconfigurable antennas receive an increasing interest in telecommunication field thanks to their flexibility and their electrical performances. The reconfigurable antenna requires switching techniques to change the antenna properties and to achieve the tuning capability such as RF-MEMS [5-6], PIN-diode [7-8] or varactor diode [9-10]... In this perspective, a frequency reconfigurable square patch antenna which can achieve large tuning frequency ranges is proposed. The frequency agility is obtained by loading firstly a varactor diode in the corner of the antenna, then a digital capacitor was employed in order to eliminate the diode biasing circuit, improve antenna performances, provide optimal connectivity and have a simplified structure with a contribution of intelligence. II. ANTENNA DESIGN The proposed antenna is a 39.2×39.2mm² square patch printed in a Cuclad substrate with thickness h=1.6mm and relative permittivity ɛr=2.2. The size of the ground plane is 80×80mm². In order to produce the frequency agility, an
Fig.1: The antenna with varactor diode
Binary code Wire connection at the corner of the patch
Digital capacitor
Command circuit
Fig.2: The prototyped antenna using digital capacitor
III. RESULTS The square patch antenna was designed and performed using the electromagnetic simulator HFSS 15. A wide tuning frequency range was achieved by loading the varactor diode BB833 from Infineon in the corner of the antenna. Then, to further improve the obtained results, a digital capacitor developed by Ethertronics was used instead of the varactor diode.
A. The varactor diode Simulated and measured return losses of proposed structure are presented in Fig.3, a good agreement can be observed. Indeed, a second resonance frequency is generated in addition to the first resonant frequency F=2.45GHz of the reference antenna when we insert the varactor diode at the corner. By varying the bias voltage from 0 to 20V (equivalent to the variation of the capacitance C from 20 to 0.9pF), a tuning simulated frequency range from 0.78 to 2.08GHz was reached. The measured resonance frequency shifts to lower frequencies and varies between 0.5 and 2.03 GHz (75.3%).
antenna has a quasi-omnidirectional radiation pattern and stable radiation properties for different resonance frequencies. The assessment of the measured radiation pattern depicted in Fig.5-c shows that when fixing the bias voltage at 1V (C=9.6pF), the antenna radiation was disturbed leading to the efficiency decrease thus the gain value decreased drastically from 0.7 to -17dBi. The low gain values may be due to the losses related to the varactor diode and its bias circuit.
(a)
(a)
(b)
(c) Fig.4: 3D simulated radiation patterns at (a) 1.86GHz (16V), (b) 1.42GHz (8V) and (c) 0.8GHz(1V) (b) Fig.3: Return loss (S11) of the antenna: (a) simulated and (b) measured results.
We present in Fig.4 and Fig.5 respectively the simulated and measured radiation patterns for different capacitance values (corresponding to bias voltage: 16V, 8V and 1V). The
better input impedance matching. A tuning frequency range from 1.65 to 2.23 GHz is easily achieved when the capacitance value C decreases from 3.454 to 0.845 pF. Obtained simulated frequencies are summarized in Table I.
(a)
(b)
Fig.6: Simulated return loss (S11) of the proposed antenna
TABLE I.
(c) Fig.5: 3D measured radiation patterns at (a) 1.9GHz (16V), (b) 1.52GHz (8V) and (c) 0.77GHz (1V)
B. Digital capacitor The use of an digital capacitor helps us to diminish losses and to outwit the difficulties associated to the bias circuit design. In fact, to shift the resonance frequency, the inserted capacitance value is controlled through a binary code of 6 bits transmitted from a connected command circuit. The used digital capacitor (developed by Ethertronics) can alter its capacitance value C from 0.845 to 3.454pF (16 configurations). Simulated return loss of the proposed structure is depicted in Fig.6. Actually, the main resonance frequency of the reference antenna F=2.45GHz still maintained while generating a second resonance frequency by exciting the mode TM01. This frequency can be adjusted by changing the controlled capacitance value from 0.845 to 3.454pF. As it can be seen, the antenna is able to keep the same behavior as the previous case (using the varactor diode) with a
Simulated resonance frequencies.
Bin
C (pF)
F1 (GHz)
F2 (GHz)
10 0000 10 0010 10 0100 10 0110 10 1000 10 1010 10 1100 10 1110 11 0000 11 0010 11 0100 11 0110 11 1000 11 1010 11 1100 11 1110
0.845 1.009 1.213 1.345 1.391 1.547 1.759 1.899 2.210 2.373 2.602 2.747 2.806 2.973 3.207 3.454
2.45 -
2.23 2.19 2.13 2.08 2.07 2.04 1.98 1.95 1.87 1.84 1.78 1.77 1.75 1.72 1.68 1.65
Fig.7 presents the simulated 3D radiation patterns of the reconfigurable antenna for few capacitance values C=0.845, C=2.21and C=3.454pF. A good gain is achieved, the simulated obtained values are 7dBi (2.23GHz- 0.845pF), 6dBi (1.87GHz- 2.21pF) and 5dBi (1.65GHz- 3.454pF).When the capacitance value increases the gain decreases and therefore, we have degradation of the efficiency. It can be concluded that the proposed antenna still able to maintain the same radiation properties as with the varactor diode (a quasi-omnidirectional radiation pattern) With a better impedance matching, gain and efficiency for most antenna‟s operation states. Measured results will be presented in the final paper.
code. It avoids employing bias circuits and allows enhancing significantly the antenna performances (by reducing losses, increasing gain and efficiency...) to cover more frequency bands such as GSM, WIMAX, LTE… REFERENCES [1]
(a)
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[6] (b)
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(c) Fig.7: 3D simulated radiation patterns at (a) 2.23 (0.845pF), (b) 1.87 (2.21pF) and (c) 1.65GHz (3.454pF).
[11]
[12]
IV. CONCLUSION [13]
A frequency reconfigurable antenna designed for multistandards wireless communication systems is presented. The frequency agility was achieved using, first, a varactor diode then a digital capacitor developed by Ethertronics. A digital capacitor is easily integrated and controlled using a binary
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