EuCAP 2012 1569522339
A Single Feed Reconfigurable Polarization Printed Monopole Antenna M.H. Amini#1, H.R. Hassani #2, S. Mohammad ali nezhad#3 Electrical & Electronic Engineering Department, Shahed University Persian Gulf Highway, Tehran, Iran 1
[email protected], 2
[email protected],
[email protected] Abstract—A dual polarized reconfigurable printed monopole antenna excited by single feed is proposed. This structure consists of a conventional monopole antenna fed by a microstrip transmission line. On the other side of the substrate lies the ground plane in which a narrow L-shaped slot is created. The overall antenna is designed to operate at 5.8 GHz which is the wireless local area network frequency band. Using a pair of PIN diodes, one along the microstrip line and the other along the Lshaped slot, and by switching the PIN diodes, horizontal or vertical polarization can be obtained. The proposed antenna is on a FR4 substrate and has a small size of 20×35 mm2. Keywords- dual polarized; reconfigurable; monopole; slot; PIN diode
I.
INTRODUCTION
Polarization reconfigurable printed antennas have attracted many attentions over the past few years. A phenomenon every communication system may deal with is multipath fading. Through polarization diversity this fading can be improved. Dual polarized antennas can also increase the channel capacity. A quadric-polarization switchable microstrip antenna is reported in [1], where by two PIN diodes, the polarization is switched among LHCP, RHCP, and two orthogonal linear polarizations. In [2] a compact U-slot microstrip patch antenna with reconfigurable polarization is proposed. PIN diodes are properly positioned to change the length of the U-slot arms, which causes polarization diversity. In [3] a slot antenna with switchable polarization is reported whereby circular and linear polarization is achieved. A CPW-fed square slot antenna is also proposed in [4]. By using two PIN diodes the polarization is switchable between LHCP and RHCP. A reconfigurable polarization slot antenna is designed in [5]. By adjusting two PIN diodes vertical polarization and horizontal one is achieved. To the knowledge of the present authors, there is no paper in the open literature offering a single feed monopole antenna with polarization diversity. In this paper a printed antenna consisting of a monopole antenna and a narrow L-shaped slot antenna providing dual polarization is described. The antenna fed by microstrip transmission line and has two PIN diodes working at 5.8 GHz for wireless local area networks (WLANs). By placing two pin diodes in appropriate positions and by switching them to “ON,” or “OFF,” state the sense of polarization can be controlled. The vertical polarization is
Fig. 1. The structure of the proposed antenna.
excited by conventional patch and have -10 dB return loss bandwidth of 1.4 GHz while horizontal one is excited by slot and has -10 dB bandwidth of 1.5GHz. Good omnidirectional pattern is obtained thorough this design at both polarization. The simulation results are carried out by commercially available software package HFSS. II.
ANTENNA DESIGN
The structure of the proposed antenna is shown in Fig 1. The antenna is printed on FR4 substrate with a size of 20×35 mm2, thickness of 1 mm and a relative permittivity of 4.4 with loss tangent of 0.02. The radiating elements consist of a conventional monopole antenna and a narrow L-shaped slot created on a ground plane. Both antennas are fed through a single microstrip transmission line. The monopole antenna produces vertical polarization while the slot antenna provide horizontal polarization. To have one sense of polarization in each time, two PIN diodes is applied one placed along the microstrip transmission line and another placed along the center of the slot. By properly switching the PIN diodes, one
1
Fig. 3. The reflection coefficient of the monopole antenna and the slot antenna
Fig. 2. Electric field vector when the patch is excited.
of the radiating elements is exited and one case of polarization can be achieved. When both diode 1 and diode 2 is on, current flows over the patch via microstrip transmission line. In order to have the desired resonant frequency we should choose the length of W2 such that W2= g/4
EQ(1)
Note that turning diode 2 to “ON,” state, causes the L-shaped slot not to resonate at desired frequency of 5.8 GHz. Fig. 2 shows the electric field vector over the upper surface of the substrate. As shown in this figure, the electric fields have oriented in the x direction in position of the patch. So in this case vertical polarization is achieved. Due to match the input impedance of the port to 50 ohm, the inset fed is applied. In fact the size of S2 is proportional to the amount of reflection coefficient of the monopole antenna. The reflection coefficient of monopole antenna is plotted in Fig .3. It can be found from this figure that the monopole antenna operates over the 5-6.4 GHz and make it suitable for WLANs. The reflection coefficient of this antenna is -22 dB at centre frequency of 5.8 GHz. Another polarization is obtained when both two PIN diodes go to “OFF,” state. With switching the state of diode 1 and diode 2 to “OFF,”, the current path bounded to only Lshaped slot. The L-shaped slot consists of two parts, horizontal part and vertical one. Horizontal part of the L-shaped slot has small length of S5 and vertical part of it has length of W3. Note that an element which provides horizontal polarization is vertical part of the L-shaped slot. To excite this part of the Lshaped slot which has length of W3, current distribution must be change in place of it. This is done by applying Horizontal part of the L-shaped slot. In fact the horizontal part of the
Fig. 4. The current distribution over the ground plane.
L-shaped slot which has length of S5, leads the current distribution to the vertical part of the L-shaped slot and causes the latter part to radiate. On the other hand the vertical slot must be excited by current with high amplitude. So the location of the horizontal slot is also important and this slot must be created at a position that the current has maximum amplitude. Fig .4 shows the current distribution over the antenna ground plane without the presence of slot and when the diode 1 is “OFF,”. As shown in this figure the current has high amplitude at near the feed point. So the best position of the horizontal part of the slot with respect to the feed point which is determined by length of S6, is 2.5mm. In order to have the desired resonant frequency, equation(2) must be satisfied. W3+S5 g/2
2
(2)
TABLE I. STATES OF DIODES FOR GENERATING POLARIZATION DIVERSITY State of diode 1
State of diode 2
Vertical polarization
ON
ON
Horizontal polarization
OFF
OFF
Fig. 5 indicates the electric field distribution over bottom surface of FR4 substrate. It can be seen from this figure that the electric fields have oriented in the y direction which indicates that horizontal polarization is excited. Table.1 shows the states of PIN diodes with corresponding polarization. Impedance matching will occur by changing the size of the parameter S5. It is noticed that horizontal length of the Lshaped slot produces opposite polarization to the vertical part of it. In fact increasing in the length of S5 causes vertical polarization and then the length of S5 should not be long, so to have a better cross polarization in this case the length of S5 must be optimized.. In table 2 the values of parameters L1, W1, L2, W2, S1, S2, L3 and W3 are shown. The reflection coefficient of the slot antenna is shown in Fig. 3. It is found from this figure that the slot antenna has reflection coefficient of -24 dB in the centre frequency. The slot antenna operates over 5.3-6.8 GHz which covers the WLAN band. Fig. 6 shows the radiation pattern of the monopole antenna and that of the slot antenna at 5.8 GHz. It can be found from this figure that the diversity of polarization is carefully achieved. As shown in this figure the E-plane pattern of the
(a)
(b) Fig. 6. The E-plane (blue) and H-plane (red) radiation pattern of the (a) monopole antenna, and (b) slot antenna.
monopole antenna is bidirectional while the E-plane pattern of slot antenna is omnidirectional. Also the H-plane pattern of the monopole antenna is omnidirectional while the H-plane of slot antenna is bidirectional. It is noticed that good omnidirectional pattern is obtained at both polarizations.
TABLE II. PARAMETERS VALUE OF THE ANTENNA L1 20 S1 3 S3 2
W1 35 S2 1.5 S4 2
L2 9 L3 4 S5 3.5
W2 11.5 W3 13.5 S6 2.5
III.
CONCLUSIONS
In this paper a single feed reconfigurable polarization printed monopole antenna is presented. The proposed antenna consists of a monopole patch and a narrow L-shaped slot created on a ground plane. The antenna is designed at 5.8 GHz for wireless LANs. By switching two PIN diodes to “ON,” or “OFF,” state, dual orthogonal linear polarizations is achieved . The monopole antenna produces the vertical polarization while the slot antenna provides horizontal one. The monopole antenna and slot antenna operates over 5-6.4 GHz and 5.3-6.8 GHz respectively. Good omnidirectional pattern is achieved thorough this design. REFERENCES [1]
[2]
[3]
Fig. 5. Electric field vector when the slot is excited.
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R.-H. Chen and J.-S. Row, “Single-fed microstrip patch antenna with switchable polarization,” IEEE Trans. Antennas Propag., vol. 56, no. 4, pp. 922–926, Apr. 2008. Pei-Yuan Qin, Andrew R. Weily, Y. Jay Guo, and Chang-Hong Liang, “Polarization Reconfigurable U-Slot Patch Antenna,” IEEE Trans. Antennas Propag., vol. 57, no. 10, Oct 2009. M. K. Fries, M. Grani, and R.Vahldieck, “A reconfigurable slot antenna with switchable polarization,” IEEE Microw. Wireless Compon. Lett., vol. 13, no. 11, pp. 490–492, Nov. 2003.
[4]
[5]
Y. B. Chen, Y. C. Jiao, and F. S. Zhang, “Polarization reconfigurable CPW-fed square slot antenna using pin diodes,” Microw. Opt. Technol. Lett., vol. 49, pp. 1233–1236, Jun. 2007. Yue Li, Zhijun Zhang , Wenhua Chen, and Zhenghe Feng, “Polarization Reconfigurable Slot Antenna With a Novel Compact CPW-to-Slotline Transition for WLAN Application” IEEE Trans. Antennas Propag., vol. 9, 2010.
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