Advances in Conformal Antennas Based on High Impedance and EBG Metamaterial Surfaces

J (Yiannis) C. Vardaxoglou(1) and George K. Palikaras (2,3)

(1) School of Electronic, Electrical and Systems Engineering, Loughborough University, LE11 3TU, Loughborough, United Kingdom.

j.c.vardaxoglou@lboro.ac.uk (2) School of Electronic Engineering and Computer Science, Queen Mary University of London,

E2 0SZ, London, United Kingdom. (3) Lamda Guard Limited, QMB Innovation Centre, 42 New Road, E1 2AX, London, United Kingdom.

george.palikaras@lamdaguard.com

Abstract— The ability of novel and emergent high impedance conformal metamaterial surfaces to reduce the diameter of cylindrical microwave antennas while improving antenna operating performance characteristics will be experimentally explored for various structures. A full-wave analysis has been applied to an infinitely long cylindrical cavity that provides physical insight in the HIS/EBG coexisting operation. The proposed structures include resonant-cavity type antennas, of different profile thickness: λ/2, λ/4 and λ/6 operating around 2.3GHz.

I. INTRODUCTION Resonant cavity (Fabry-Perot) type antennas have been proposed in the past as a way of producing high-gain antennas from low gain sources [1, 2]. They are typically composed of a low directivity source (e.g. a patch antenna) placed between a PEC (i.e. ground plane) and a Partially Reflecting Surface (PRS) (as shown in Fig. 1), producing a highly-directive antenna. The PRS is made of periodic conducting (or aperture) elements and supports a leaky-wave resonant mode between itself and the ground that can be efficiently studied in terms of multiple reflections and leaky rays as shown in Fig. 1. This simple plane wave analysis provides useful design guidelines for the realisation of high-gain antennas as well as further low-profile designs employing high impedance surfaces. Recently, novel metamaterial based antenna systems employing High Impedance Surfaces (HIS) and Electromagnetic Bandgap (EBG) have proven to be suitable solutions in reducing the antenna size as well as enhancing antenna performance such as gain, directivity and multi-band operating performance. High impedance surfaces fully reflect incident waves with a near zero degrees reflection phase and can be utilised as ground planes for antenna applications. A new class of low-profile highly-directive planar antennas has been developed based on EBG superstrates and HIS ground planes [3]. Furthermore, using EBG as a superstrate to form an EBG resonator antenna has been successfully applied to a number of high gain antenna designs [4-8]. Cylindrical antennas with directive E-plane patterns formed by conformal EBG supertstrates have also been presented at microwave [9-10] and millimetre-wave, [11-12].

In this paper, we present experimental results on novel microwave conformal/cylindrical antennas formed by HIS and EBG surfaces, resulting in reduced diameter/profile thickness structures while maintaining good antenna performance characteristics, such as narrow E-plane patterns, high directivity and wide azimuth beamwidth.

Fig. 1. A Fabry-Perot antenna

II. HIS AND EBG STRUCTURES AS RESONANT CAVITIES The use of High Impedance Surfaces (HIS) and

Electromagnetic Bandgap (EBG) structures in the design of conformal directive antennas is studied. In order to achieve a reflection coefficient of zero phase at about 2.4 GHz (Fig. 2) the following dimensions were computed for the HIS substrate: LD=31mm, WD=13mm, with periodicity DX=14mm and DY=32m. The EBG array consists of conducting patch elements as in [9], sized LD=61mm, WD=26mm, with periodicity DX=28mm and DY=64mm. The patches are cylindrically arranged surrounding a metallic ground cylinder at a specific resonant distance, in order to increase the

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directivity of a simple radiating source positioned between the periodic array and the ground. It has been demonstrated that this specific distance is approximately half wavelength, for a simple metallic ground. For an accurate full-wave analysis, the infinite conformal structure has been modeled using 3-D Transmission Line Model (TLM) based software (CST Microstripes) and imposing periodic boundary conditions along the z-¬axis. The conformal EBG and HIS periodic arrays are printed on very thin (32µ) dielectric layers for support. The arrays are shaped around cylindrical polystyrene foam (εr=1.05) for support which has not been taken into account in the simulations since the permittivity value is very close to that of air. The resonant modes supported by the cavity are identified and are relatively close to the plane wave approximation prediction. Conformal HIS are designed and utilized here in order to reduce the diameter of the cylindrical antenna to about λ/4 and λ/6.

Magnitude

Phase

Fig. 2a Complex Reflection Coefficients of HIS

Fig.3 The unit cells of the proposed cylindrical (a) EBG,

Magnitude

Phase

Fig. 2b Complex Reflection Coefficients Three structures have been simulated, all of which had a

resonance frequency of about 2.4GHz: Fig. 3(a) below depicts the unit cell of the proposed conformal antenna with an EBG superstrate placed at a distance of λ/2 (66mm) from the ground (120mm diameter), where λ is the wavelength at 2.4GHz. Fig. 3(b) illustrates the unit cell of the hybrid conformal antenna with an EBG superstrate placed at a distance of λ/4 (32mm) from the HIS substrate that surrounds the same sized ground as in Fig. 3(a) and at a distance of 6mm, Fig. 3(c) shows the unit cell of the proposed conformal antenna with an EBG superstrate placed at a distance of λ/6 (20mm) from the HIS substrate that surrounds the same ground and at a distance of 8mm.

(b) EBG+HIS (λ/4) and (c) EBG+HIS (λ/6) antennas

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III. HYBRID HIS AND EBG ANTENNA PERFORMANCE The three conformal structures with ground

diameter=120mm have been manufactured into finite cylindrical structures (Fig. 4) of approximate legth 2.5 λ. The feeding of the antennas is done by a balanced dipole antenna of 61mm length placed 20mm from the ground and in the middle of the cavity. The measured results are presented in Fig. 5 and 6. These figures present the measured radiation patterns of the antenna with and without the use of the cylindrical EBG array as well as with and without the use of the high impedance surfaces. In the case without the EBG array the maximum directivity is 4.1 dB at 2.28GHz. In the presence of the EBG a maximum directivity of 12.7dBi occurs at about 2.33 GHz, three times higher than that of the dipole alone.

The measured return loss for the proposed structures is

presented in Fig. 5. A wide bandwidth has been achieved for the reference dipole antenna by using a type III balun configuration. A good impedance match was achieved at 2.33GHz for all considered antenna structures. Figure 6 above shows the measured radiation patterns for the proposed conformal structures at 2.33GHz. The E- and H- Plane patterns for the conformal EBG superstrate (λ/2 design) antenna Fig.4 (b), the conformal hybrid HIS-EBG antennas (λ/4 design) shown in Fig.4(d) and (λ/6 design), are plotted and compared against a reference dipole antenna Fig. 4(a) placed over an identical cylindrical ground plane. It is evident that for all proposed designs the azimuth beamwidth (Fig. 6(a)) is broader due to the conformal/cylindrical geometry of the proposed antennas and the larger radiating aperture size compared to the reference dipole antenna. The E-plane radiation patterns (Fig. 6(b))for all designs become highly-directive when compared to the reference dipole antenna and as expected the beamwidth increases as the antenna diameter reduces from λ/2 EBG to λ/4 and λ/6 HIS-EBG designs. The Side Lobe Level (SLL) is at the relatively low value of around -15dB.

Fig. 4: Manufactured conformal metamaterial-inspired

antenna systems: (a) Photo of the conformal reference dipole antenna with no Metamaterial, (b) Photo of the conformal EBG superstrate Metamaterial Antenna (λ/2) , (c) Photo of the conformal HIS Metamaterial surfaces, (d) Photo of the proposed new conformal EBG+HIS hybrid Metamaterial antenna with profile reduction λ/4 and λ/6 (λ/4 design shown above).

Fig. 5 Measured return loss: Comparison for different

structures: Dipole (without arrays), Conformal EBG antenna (λ/2), Conformal hybrid HIS+EBG antennas (inc. λ/4 and λ/6 designs).

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Fig. 6 Measured radiation patterns: (a) H- and (b) E- Plane

Co-Polar radiation pattern comparison for different structures: Dipole (without arrays), Conformal EBG antenna (λ/2), Conformal hybrid HIS+EBG antennas (inc. λ/4 and λ/6 designs).

IV. CONCLUSION In this paper we presented applications of fabricated

microwave conformal EBG and HIS metamaterial antenna structures. Numerical full-wave simulations were performed on infinite size periodic EBG and HIS arrays of conducting elements that yielded physical insight and provided initial design guidelines. The conformal hybrid EBG-HIS antennas were fabricated and measured, and showed a good agreement with the predicted results. Directive elevation plane microwave antennas utilizing cylindrical metalodielectric EBG arrays as superstrates, placed λ/2 from a cylindrical metallic ground, has been experimentally demonstrated. The new EBG superstrate has been combined with a HIS substrate to form new, hybrid, reduced profile cylindrical cavity antenna designs. The performance of the EBG-HIS antennas has been demonstrated for λ/4 and λ/6 reduced profile lengths maintaining highly directive elevation plane patterns, good

matching at the desired frequency and broad horizontal plane radiation patterns.

ACKNOWLEDGMENT This work was supported by a development studentship of

Loughborough University.

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[10] G. K. Palikaras, A. P. Feresidis and J. C. Vardaxoglou, “Cylindrical EBG Surfaces for omni-directional wireless LAN antennas”, IEEE APS Int. Symp. Dig., vol. 4B, pp. 339-42, July 2005.

[11] Y. Lee, X. Lu, Y. Hao, S. Yang, R. Ubic, J. R. G. Evans, and C. G. Parini, “Directive millimetre-wave antenna based on free formed woodpile EBG structure” Electron. Lett., vol. 43 no. 4, 195-6, 2007.

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