Octagonal ESPAR with Cone Reflector Shizuo Mizushina† and Takashi Watanabe‡

† Professor Emeritus, Shizuoka University, 800-8 Tomitsuka, Naka-ku, Hamamatsu, Shizuoka, 453-8002 Japan

‡ Information Science, Faculty of Informatics, Shizuoka University, 3-5-1 Johoku, Naka-ku, Hamamatsu, 432-8011, Japan

To recall basic features of a monopole antenna with flat disc

reflector, a farfield radiation beam pattern produced by such an antenna is shown in 3D and 2D in Figs. 1 and 2, respectively.

Abstract — An octagonal ESPAR built on cone reflector is proposed. Results of simulation on the proposed structure showed that it could produce a farfield radiation pattern having four beams in the horizontal plane at 0°-90°-180°-270° or at 45°-135°-225°-315° directions, where each beam had angular width (3dB) at 34.8° (horizontal) and 55.9° (vertical) with elevation angle at 0° for an optimized design. The horizontal beam directions were steered by voltage settings of varactor-diodes mounted on the parasitic elements. The structure could also produce a single beam radiation pattern in a direction at 0°, 45°, 90°, 135°, 180°, 225°, 270° or 315° with angular beam widths at about 77° (horizontal) and at 73° (vertical) and elevation angle at 0°. The proposed structure can be applied to the wireless LAN to allow flexibility in network design.

x

Index Terms — Octagonal ESPAR, cone reflector, farfield radiation pattern, 3D simulation results.

I. INTRODUCTION

ESPAR (Electronically Steerable Parasitic Array Radiator) antenna has been proposed and investigated by Ohira et al. [1, 2]. Applications of ESPAR to Ad Hoc and other wireless networks have been investigated by Watanabe et al. [3, 4]. Basic structure of the ESPAR reported so far employs a flat circular metallic disc for reflector along with a λ/4 antenna rod at its center and six parasitic antenna elements on a concentric circle. The parasitic elements are loaded with varactor-diodes for beam steering. With the flat reflector, the farfield radiation pattern naturally has an elevation angle at around 45°. It is desirable to make the elevation angle nearly 0° when the ESPAR is applied to wireless networks on ground surface. Use of cone reflector instead of flat one is proposed to control the elevation angle. In addition, the six parasitic elements arranged on a hexagonal single ring are replaced with an octagonal double ring configuration to obtain narrower beam angle width (at 3dB), say, less than 40°, in the horizontal plane.

A structure of octagonal ESPAR with cone reflector is described in this paper along with results of a 3D EM wave simulation study of the structure using CST Microwave Studio, V.5.2, which is based on the finite integration method. Results of the simulation supported the above approach.

II. MONOPOLE ANTENNA WITH FLAT REFLECTOR

(a) (b)

z

y

Theta z

y

Phi x

Fig.1. (a) A monopole antenna with flat disc reflector. (b) 3D representation of farfield radiation beam pattern produced by the antenna shown in (a).

z

x

45°

y

x

(a) (b) Fig.2. Farfield radiation patterns in 2D polar plots (linear scale) (a) in horizontal plane and (b) in vertical plane.

Fig.2(b) indicates that the farfield radiation beam has an elevation angle at about 45°.

III. STRUCTURE OF PROPOSED ESPAR

Structure of an octagonal ESPAR with cone reflector is illustrated in Figs.3 and 4. The frequency of operation is set at 2.45GHz (λ=12.24cm). The cone reflector has: Top Radius=5.331mm, Bottom Radius=91.775mm, Height =57.00mm. The skirt has: Outer Diameter=91.775mm (=λ/2)

©2011 IEEE 2011 Korea-Japan Microwave Conference

FR2-4-1

and Height=50.00mm. The antenna element A placed on the axis is the active element. The rod of active antenna measures R=1.224mm × L=27.00mm, and is connected to a 50Ω coaxial line through a metal joint measuring R=1.224mm × L=2.00mm.

The 50Ω coaxial lines measure: Outer conductor OD=5.331mm, ID=4.331mm, Inner Conductor R=1.224mm, and εr=2.3. Lengths of the coax lines are: L=18.00mm for the active element and L=17.00mm for the parasitic elements. The parasitic antenna elements P11, P12, …, P18 are placed on the concentric ring with radius of 30.592mm (=λ/4), and P12, P22, …, P82 are placed on the concentric ring with radius of 61.184mm (=λ/2). (P11, P12) pair is placed in the x-z plane. Similarly, (P21, P22), (P31, P32), …, (P81, P82) pairs are placed in the corresponding vertical planes in the radial directions 2, 3, …, 8. The antenna rods of P11, P12, …, P18 measure

R=1.224mm × L=25.00mm. The antenna rods of P12, P22, …, P82 measure R=1.224mm × L=19.50mm. These dimensions were determined through an optimization procedure. Criteria for the optimization procedure were (1) to make the beam elevation angle nearly 0° and, simultaneously, (2) to make the horizontal beam angle as small as possible. Finally, the bottom ends of the 50Ω coaxial lines connected to the parasitic elements are terminated with capacitances Cij, which are in a range from 1.135pF to 7.6025pF. A pair of varactor diodes, 1SV278 (TOSHIBA), connected in series give C=1.135pF and C=7.6025pF at bias voltages of 25V and 2V, respectively.

A: Active element. Pij: Parasitic elements.

IV. RADIATION BEAM PATTERNS

A. Four-beam Farfield Radiation Pattern

A four-beam farfield radiation pattern in a 3D representation is shown in Fig.5.

Fig.5. A 3D representation of 4-beam farfield radiation pattern on the right. Capacitances Cij on parasitic elements Pij are given in the inset.

Cij AssignmentsCij = 7.6025pF

i2 4 6 8

C21,C22C41,C42C61,C62C81,C82

2D polar plots (linear scale) for the 4-beam pattern in Fig.5 are shown in Fig.6. The 4-beam radiation pattern at 0°-90°-180°-270° can be rotated by 45° to radiate at 45°-135°-225°-315° as shown in

Parasitic element radial directions: i = 1, 2, …, 8

Cij = 1.135pF

Fig.3. External view of the ESPAR with cone reflector.

i1 3 5 7

C11.C12C31,C32C51,C52C71,C72

z

y

Theta

Phi x

Fig.4. A cross-section of the ESPAR with cone reflector.

Fig.6. 2D polar plots (linear scale) (a) in the horizontal plane at z = 0 and (b) in the vertical plane at y = 0 for the 3D pattern shown in Fig.5.

Angular width (3dB) = 34.8° (a)

y

x

Angular width (3dB) = 55.9°

Elevation angle = 0°. (b)

z

x

Fig.7. The Cij assignments to Pij for this case are given in the inset of the figure. Please note that Cij =1.135pF/7.6025pF assignments to Pij in the inset of Fig.7 are the reversal of that of Fig.5. This is done by switching bias voltages of varactor-diodes.

2D polar plots (linear scale) for the 3D four-beam pattern in Fig.7 are shown in Fig.8. Angular beam widths in horizontal and vertical planes are 34.8° and 55.9° in Fig.6, while they are 34.6° and 56.1° in Fig.8. The small differences (34.8-34.6) and (55.9-56.1) reflect errors in numerical computations which use rectangular cells set up in the x-y-z coordinates in which the beam rotates. The horizontal angular width can be regarded as quite narrow. The beam elevation angle is 0° from Figs.6 and 8. The objectives of our proposal are met, according to the simulation results.

B. One-beam Farfield Radiation Pattern

The octagonal ESPAR can also be made to produce one-beam farfield radiation pattern by applying a proper set of bias voltages to varactor-diodes. One-beam farfield patterns

radiating in radial directions at 0° and at 45° are shown in Figs.9-10 and in Figs.11-12, respectively.

Fig.9. A 3D representation of 1-beam farfield pattern radiating in the direction at Phi = 0°. Capacitances Cij on parasitic elements Pij are given in the inset.

z Theta

y

Phi x

Cij AssignmentsCij = 7.6025pF i 1 C11,C12

Cij = 3.800pF i 8

2 C81,C82C21,C22

Cij = 1.135pF

i

3 4 5 6 7

C31,C32C41,C42C51,C52C61,C62C71,C72

Fig.7. A 3D representation of the four-beam farfield radiation pattern rotated by 45°. Capacitances Cij on parasitic elements Pij are given in the inset.

Theta

z

y

Phi x

Cij Assignments Cij = 7.6025pF i

1 3 5 7

C11,C12 C31,C32 C51,C52 C71,C72

Cij = 1.135pF i

2 4 6 8

C21,C22 C41,C42 C61,C62 C81,C82

Note that the radial direction i = 1 is set at Phi = 0. Pij with i = 1 are loaded with Cij = 7.6025pF, i = 8 and 2 with Cij = 3.800pF, and others with Cij = 1.135pF.

Fig.8 2D polar plots (linear scale) for the 4-beam radiation pattern shown in Fig.7: (a) in the horizontal plane at z = 0 and (b) in the vertical plane at y = 0.

Angular width (3dB) = 56.1°Elevation angle = 0°. (b)

z

r

Phi=135° Phi=315°

Angular width (3dB) = 34.6°. (a)

Phi=135°

y

x

Phi=315

Fig.10. 2D polradiation pattern

ar plots (linear scale) for the farfield shown in Fig.9: (a) in the horizontal plane

at z = 0 and (b) in the vertical plane at y = 0.

Angular width (3dB = 76.6°. (a)

y

Angular width (3dB) = 73.3°

Elevation angle = 0°. (b)

x

z

)

x

Fig.11. One-beam farfield pattern radiating in the radial direction at Phi = 45°. Capacitances Cij on parasitic elements Pij are given in the inset.

Ci z Theta

y

Phi

x

j AssignmentsCij = 7.6025pF

i 2 C21,C22

Cij = 3.800pF i 1

3C11,C12C31,C32

Cij = 1.135pF

i

45678

C41,C42C51,C52C61,C62C71,C72C81,C82

VI. INPUT IMPEDANCES To set the beam at Phi = 45°, Pij with i = 2 (45°) are loaded with Cij = 7.6025pF, i = 1 and 3 with Cij = 3.800pF, and others with Cij = 1.135pF. Input impedances of the cone ESPAR operating in the 4-

beam and 1-beam radiation modes are presented in Fig.15 and Fig.16, respectively.

Fig.12. 2D polar plots (linear scale) for the radiation pattern given in Fig.11: (a) in the horizontal plane and (b) in the vertical plane

Angular width (3dB) = 77.0°. (a)

Phi=45°

x

y

Angular width (3dB) = 73.6°

Elevation angle = 0°. (b)

r

z

Phi=45°

Likewise, the beam direction can be set at Phi = 90°, 135°, 180°, 225°, 270°, or 315° by assigning proper set of Cij to Pij.

V. ELECTRIC FIELD IN NEAR-FIELD RANGE

Peak electric field (absolute) patterns in near-field range corresponding to Fig.5 and Fig.9 are shown in Fig.13 and Fig.14, respectively.

VII. Conclusion Octagonal ESPAR with cone reflector is proposed. Results of 3D EM simulation showed that it could produce 4-beam farfield radiation pattern at 0°-90°-180°-270° or at 45°-135°-225°-315° with horizontal angular width (3dB) < 40° and elevation angle at about 0°. It could also produce 1-beam farfield radiation pattern at 0°, 45°, 90°, 135°, 180°, 225°, 270°, or 315° with horizontal angular width (3dB) ≈ 77° and elevation angle at 0°. The structure can be applied to wireless LAN networks to allow flexibility in their design.

REFERENCES

[1] T. Ohira and K. Gyoda, “Electronically steerable parasitic array radiator ntenna for low-cost analog beam forming,” IEEE Int. Conf. Phased Array Sys. Tech., pp. 101-104, May 2000.

[2] T. Ohira and K. Iigusa,, “Electronically Steerable Parasitic Array Radiator Antenna,” IEICE Trans. C, vol. J87-C, no.1, pp. 12-31, January 2004.

[3] M. Takata, M. Bandai, T. Watanabe, “A MAC Protocol with Directional Antenna for Deafness Aviodance in Ad Hoc Networks,” IEEE Global Comm. Conf., CD-ROM, 2007.

[4] T. Furukawa, M. Bandai, H. Yano, S. Obana, T. Watanabe,”Multi-Lobe Directional Transmission for Network Coding in Multi-Rate Ad Hoc Networks,” APSITT, CD-ROM, 2010.

(a) (b) Fig.14. Peak E-field (absolute) patterns in (a) horizontal and (b) vertical planes for the 10-beam radiation shown in Fig.9.

z y

x

z y

x

(a) S11 Smith chart. (b) S11 vs. frequency. Fig.15. Input impedance of the cone ESPAR in 4-beam mode.

1

∞0

-1 -0.4

0.4

Frequency (GHz)

S11

1

(a) S11 Smith chart. (b) S11 vs. frequency. Fig.16. Input impedance of the cone ESPAR in 1-beam mode.

0 ∞

-1

0.4

-0.4 Frequency (GHz)

S11

(a) (b) Fig.13. Peak E-field (absolute) patterns in (a) horizontal and (b) vertical planes for the 4-beam radiation shown in Fig.5.

y

x

z y

x

z