A compact radiation pattern diversity antenna for WLAN applications
Nicolas Boisbouvier*, Françoise Le Bolzer and Ali Louzir THOMSON multimedia, Corporate Research 1, avenue de Belle Fontaine, BP 19, 35511 Cesson-Sévigné Cedex - France e-mail :
[email protected]
Abstract This paper proposes a novel planar antenna structure with a radiation pattern diversity. Such an antenna is designed for wireless indoor network applications and is particularly suitable for improving system performances in a multi-path propagation environment. The new antenna structure is based on two tangential annular slot antennas etched on one side of a dielectric substrate and fed by electromagnetic coupling to microstrip feeding lines. This structure also incorporates an integrated switching function which allows control of the radiation pattern. Actually three radiation pattern configurations can be checked thanks to two PIN diodes.
I. Introduction An important issue in wireless radio communication and especially in indoor environments is multi-path fading. In order to minimize possible outage and thus assure better quality and reliability of the communication, several techniques have been proposed. Antenna diversity techniques are one of them [1]. Several antenna diversity techniques exist such as spatial diversity (use of two or more same antennas sufficiently spaced [2]), polarization diversity (use of two antenna output providing orthogonal polarization [3]) and radiation pattern diversity [4]. The last technique consists of an antenna output with two or more radiation patterns, resulting for example from different switch settings. In this paper a novel compact and low-cost printed antenna with a radiation pattern diversity of order three is presented. Details of the antenna design and experimental results are presented at 5.8 GHz.
II. Antenna description The novel structure is based on the well-known microstrip fed Annular Slot Antenna (ASA, [4],[5]). This antenna radiates two lobes on both sides of the substrate. Key points of the design of such an antenna are, on the one hand, the radius R of the annular slot which should be about λs/2π and, on the other hand, the feeding microstrip line which extends beyond the slot about lm=λm/4 (λs and λm refer respectively to wavelength in slot line and under the microstrip line). The microstrip slot-line transition is designed following [6],[7].
Figure1 shows the topology of the novel reconfigurable antenna : The idea is to etch two coupled Annular Slot Antennas (s1,s2 on figure2) on the same side of a dielectric substrate. The feeding of each ASA is controlled by using the microstrip-slot line transition described above and a PIN diode located at the end of the microstrip line. Those diodes are shunted to ground through metallic holes (radius, r) as shown figure1. Two microstrip line sections (m3,m4 on figure2) are used for impedance matching of the structure to 50Ω. Due to its magnetic nature, the coupling of the microstrip line and the slot is maximum when the diode is in his OFF-state and minimum when in the ON-state. Thus three configurations are possible as shown on figure3. Those three configurations are controlled by a simple DC voltage V brought by the SMA connector. Diodes are placed head to tail in order to be on opposite state when for an example V>V1 is applied on the ground plane (V1 corresponds to the forward voltage of the PIN diode). Metallic inserts, added in a short circuit plane of each annular slot, allow electric continuity of the ground plane.
III. Measured and simulated results An antenna prototype has been designed at 5.8 GHz on a 0.81mm height RO4003® substrate (εr=3.38) using the electromagnetic software Ie3D (Zeland). PIN Diodes used here are HSMP489B from HP. Dimensions of the antenna are as follows : R=6.5, Ws=0.4, lm=5.75, Wm=0.3, Lm’=3.7, Wm’=0.3, Lm”=8.6, Wm”=0.25, r=0.4, d=l=1.5, h=3, LGND=40, lGND=30, where all units are in mm. Table 1 sums up performances of the antenna obtained for the three states. The measured bandwidth is wider than 870 MHz (15%) for the three states which is in good agreement with prediction. The three different radiation patterns corresponding to the three switching antenna states are also confirmed by the 3-D simulation shown in table 1. For the first configuration the angle of the main beam is θ max= -18.5° and 15.5° for the second configuration. The radiation pattern of the third configuration is roughly omni-directional.
IV. Conclusion A novel planar radiation pattern diversity antenna of order three is proposed. This planar antenna is compact (30mmx40mm), low-cost (only two PIN diodes used for the control of three radiation pattern) and well suited for wireless LAN systems. Evaluation of the diversity performance is in progress through a propagation measurement campaign.
Fig. 1 : The fabricated antenna prototype
V. References [1] P. Irazoqui-Pastor, J.T. Bernhard, “Examining the performance benefits of antenna diversity systems in portable wireless environments”, IEEE Antenna Applications Symposium, Allerton Park, Sep. 15-17, 1999 [2] C.B. Dietrich, K. Dietze, J.R. Nealy & W.L. Stutzman, “Spatial, polarization, and pattern diversity for wireless handheld terminals”, IEEE Trans. on Antennas and Propagation, vol. 49, N°9, pp. 1271-1281, Sep. 2001 [3] R.G. Vaughan, “Polarization diversity in mobile communications”, IEEE Transactions on Vehicular Technology, vol. 39, N°3, pp. 177-186, Aug. 1990 [4] J.S. Kot, N. Nikolic, R.A. Sainiti & T.S. Bird, “Aspects of antenna designs for indoor wireless millimetre-wave systems”, Journal of electrical and electronics engineering, Australia-(JEEEA), Vol. 15, N° 2, June 1995. [5] P. Mastin, B. Rawat, M. Williamson, “Design of a microstrip annular slot antenna for mobile communications”, URSI Radio Science Meeting and Nuclear EMP Meeting (Chicago), vol.1, July 1992. [6] J. B. Knorr, “Slot Line Transitions”, IEEE transactions on Microwave Theory and Techniques, may 1974. [7] R. Janaswamy, D. H. Schaubert, “Characteristic impedance of a wide slot line on low permittivity substrates”, IEEE Transactions on Microwave Theory and Techniques, vol 34, n°8, aout 1986.
dielectric substrate
LGND
metallic insert
diode 2
Ws
metallic hole
m1
m2 d Lm
Ground plane / annular slot
Lm h Wm'
Lm' R
l
m3
lGND
Wm
diode 1 Y
s1
Lm''
Wm''
m4
s2
microstrip line
50Ω Ω line Z
X
Fig. 2 : The novel radiation pattern diversity antenna topology
ON
OFF
ON
Configuration 1 (V > V1)
OFF
OFF
Configuration 2 (V < V1)
OFF
Configuration 3 (V = 0)
Fig. 3 : schematic view of the switching principle
Config. diode1 diode2
Radiation Pattern (3-D top view)
Input Return Loss (S11) 0
-5
-10
1
ON
OFF
S11 (dB)
-15
-20
-25
-30
simulated measured
-35
-40 4
4,5
5
5,5
6
6,5
7
Frequency (GHz)
tilt : 18.5°
BW(-10dB) : 870 MHz 0
-5
-10
2
OFF
ON
S11 (dB)
-15
-20
-25
-30
simulated
-35
measured -40
4
4,5
5
5,5
6
6,5
7
Frequency (GHz)
BW(-10dB) : 885 MHz
tilt : 15.5°
0
-5
-10
3
OFF
OFF
S11 (dB)
-15
-20
-25
-30
simulated
-35
measured -40
4
4,5
5
5,5
6
6,5
7
Frequency (GHz)
BW(-10dB) : 1.32 GHz Table 1 : main characteristics of the three different configurations
