A Reconfigurable Spiral Antenna with Wide Beam Coverage

Liang Gong*, King Yuk Chan*#, Rodica Ramer* *School of Electrical Engineering and

Telecommunications UNSW

Sydney 2052, NSW Australia liang.gong@unsw.edu.au

#ICT Centre, CSIRO, Cnr Vimiera & Pembroke Roads,

Marsfield, NSW, 2122, Australia Eric.chan@csiro.org

Abstract—A novel reconfigurable spiral antenna capable of beaming toward five different directions is proposed in this paper. RF-MEMS switches are incorporated into a single-arm rectangular spiral antenna to steer the beam. Return loss and radiation patterns are generated for each case by Ansys HFSS. A good matching is achieved around 3.3 GHz. The gain varies from 4.8 dBi to 5.9 dBi for individual cases at this frequency.

I. INTRODUCTION Smart antennas are essential components for

communication systems that require receive/transmit wireless signals from/to specific directions, but suppression in other directions. Spiral antennas provide cost effective solutions for such reconfigurable antenna. References [1-4] pioneer the scan-beam rectangular spiral with switches attached. Nevertheless, most of the directions they beam to cluster together to a limited space. The novel design proposed in this article examines the possibility of wide beam coverage in azimuth plane using radio frequency microelectromechanical system (RF-MEMS) switches.

II. THE DIMENSIONS OF THE SPIRAL ANTENNA The schematic and top view of the proposed antenna is

shown in Figure 1 (a) and (b). The structure is very similar to those proposed in [1-4]. The spiral arm is 1.4 mm in width. The first segment has a unit length of a=4.8 mm. Then the arm extends outward with sequence of a, 2a, 2a, 3a, 3a … 7a, 7a, 8a with the last segment 3 mm long. Rogers 4350 is chosen as the substrate which has a dielectric constant of 3.66. The thickness is set to 12 mm; approximately the quarter-wavelength at the operational frequency. In this design a metal pin contacted with the first segment of the spiral is connected to the inner conductor of the coaxial cable; the outer conductor is attached to the ground.

Better steering capability than the already published is achieved by placing 3 switches (SW1, SW2, and SW3) near the feeding point and 2 bridges (BR1 and BR2) between different turns of the spiral (Figure 1(b)). Switches have the role to alter the current flow along the spiral arm. Seven switches can be arranged in 128 combinations theoretically, but most of them are meaningless because of poor matching, split radiation pattern, or low directivity. Table I lists 5 of the useful cases

with beam directions spaced around 90 degree plus an end-fire one.

(a) Schematic view

(b) Top view

Figure 1. A beam steering spiral antenna.

TABLE I. SWITCHES’ OPERATION. SW1 SW2 SW3 SW4 SW5 SW6 SW7

Case 1 OFF ON OFF OFF ON OFF OFF Case 2 OFF ON OFF ON OFF OFF OFF Case 3 ON OFF ON ON OFF OFF ON Case 4 ON OFF ON OFF ON ON ON Case 5 ON OFF ON OFF OFF ON OFF

206978-1-4673-5317-5/13/$31.00 ©2013 IEEE AP-S 2013

III. SIMULATION RESULTS Simulation work is performed using Ansys HFSS. Switches

are assumed to be RF-MEMS switches which have the following Lumped RLC boundary conditions: a capacitance of 0.04pF for ON-state or 3.76pF for OFF-state. This setting is accordance with the MEMS switches in [1]. An on/off capacitance ratio of 94 leads to unparalleled performance outweighing PIN diodes or varactors.

A. Return loss Fig. 2 presents curves of S11 parameters in dB for all of the

five cases. It can be seen that all of them have return loss better than 10 dB from 3.2 GHz to 3.4 GHz.

Figure 2. Return loss for each case.

B. Gain and Radiation patterns. Fig. 3 illustrates the equivalent structures based on

switches’ states and the simulated radiation patterns for both right hand circular polarization (RHCP) and left hand circular polarization (LHCP). Table II summarizes the maximum RHCP gain, maximum beam direction, half power beam width (HPBW), and axial ratio (AR) at maximum beam directions. It is worth noticing that gain for cross polarization, i.e. LHCP, is much lower than that for RHCP in the same direction. Low axial ratio is attained by case 2 and 3 which indicate good circular polarization purity.

TABLE II. SUMMARY OF MAXIMUM BEAM DIRECTION, GAIN AND HPBW.

Max beam direction Gain (dBi)

HPBW ( -plane)

HPBW ( -plane)

AR (dB) Theta Phi

Case 1 38˚ 7˚ 4.9 103˚ 128˚ 7.6 Case 2 41˚ 87˚ 5.9 89˚ 116˚ 2.1 Case 3 39˚ 189˚ 5.4 95˚ 185˚ 2.3 Case 4 38˚ 274˚ 5.8 93˚ 130˚ 4.0 Case 5 13˚ - 4.8 92˚ - 8.8

IV. CONCLUSION A modified single-arm reconfigurable spiral antenna is

designed and analyzed. Simulation results show that good matching is achieved around 3.3 GHz for all cases, and its five beam directions are separate with each other at the operation frequency. This design can be a choice for smart communication systems especially those are less sensitive to polarization characteristic.

(a) Case 1 -plane at ˚ -plane at ˚

(b) Case 2 -plane at ˚ -plane at ˚

(c) Case 3 -plane at ˚ -plane at ˚

(d) Case 4 -plane at ˚ -plane at ˚

(e) Case 5 -plane at ˚ Figure 3. Equivalent structures and radiation patterns.

REFERENCES [1] J. Chang won, L. Ming-jer, G. P. Li, and F. De Flaviis, "Reconfigurable

scan-beam single-arm spiral antenna integrated with RF-MEMS switches," Antennas and Propagation, IEEE Transactions on, vol. 54, pp. 455-463, 2006.

[2] P. Deo, A. Mehta, D. Mirshekar-Syahkal, and H. Nakano, "An HIS-Based Spiral Antenna for Pattern Reconfigurable Applications," Antennas and Wireless Propagation Letters, IEEE, vol. 8, pp. 196-199, 2009.

[3] A. Mehta, D. Mirshekar-Syahkal, and H. Nakano, "Beam adaptive single arm rectangular spiral antenna with switches," Microwaves, Antennas and Propagation, IEE Proceedings, vol. 153, pp. 13-18, 2006.

[4] H. Nakano, J. Eto, Y. Okabe, and J. Yamauchi, "Tilted- and axial-beam formation by a single-arm rectangular spiral antenna with compact dielectric substrate and conducting plane," Antennas and Propagation, IEEE Transactions on, vol. 50, pp. 17-24, 2002.

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