1348 PIERS Proceedings, Marrakesh, MOROCCO, March 20–23, 2011

Design of a Multilayer Composite-Antenna-Structure by Spiral Type

D. Kim1, J. Kim1, J. Kim3, W.-S. Park2, and W. Hwang1

1Department of Mechanical Engineering, POSTECH, Republic of Korea2Department of Electronic and Electrical Engineering, POSTECH, Republic of Korea

3Antenna R&D Group, Mobile Comm. Samsung Electronics Co., Ltd., Republic of Korea

Abstract— We study a composite-antenna-structure (CAS) having high electrical and mechan-ical performances that we have designed and fabricated. The CAS, consisting of a glass/epoxyface sheet and a honeycomb core, acts as a basic mechanical structure, in which a spiral antennatype is embedded. To increase the intensity, a carbon fiber plate is used as a bottom sheet. Thisstructure of the 0.5 ∼ 2GHz band has a gain of 5 ∼ 9 dBi with circular polarization characteristicsand reflection loss below −10 dB within the desired frequency band.

1. INTRODUCTION

In the last 15 years there has been much research into the embedding of antennas in load-bearingstructural surfaces of aircraft, so as to improve both structural efficiency and antenna perfor-mance [1–3]. Structural, material and antenna designers have collaborated to develop a novelhigh-payoff technology known as a Conformal Load-bearing Antenna Structure (CLAS) [3]. Thistechnology shows great promise for enhancing the performance and capability of aircraft, by reduc-ing weight, improving the structural efficiency of airframes that contain antennas, and improvingthe electromagnetic performance of antennas. To develop the load-bearing antenna structure, weproposed the use of antenna-integrated composite structures of sandwich construction, specificallythe surface-antenna-structure (SAS) [4–6] and the composite-smart-structure (CSS) [7–9]. In thosestudies, we designed and fabricated a microstrip antenna structure which implemented satellitecommunication in the X band (8.2 ∼ 12.4 GHz) and the Ku band (12.4 ∼ 18 GHz). At such highfrequencies the microstrip antenna has only a small bandwidth. In the present paper we report anew CAS based on a spiral antenna type giving good performance in a low frequency band withhigher bandwidth [10, 11]. A sandwich composite consisting of a glass/epoxy face sheet and hon-eycomb core is used as a basic mechanical structure, in which a spiral antenna type is embedded.To increase the intensity, a carbon fiber plate is used as the bottom sheet.

2. STRUCTURE AND MATERIALS

The basic design concept of the CAS panel is an organic composite multi-layer sandwich panel intowhich spiral antenna elements are inserted. This concept originates mechanically from a compositesandwich structure, and electrically from a spiral antenna, as shown in Figure 1. The sandwichstructure consists of two thin load-bearing facesheets, bonded to either side of a moderately thickand lightweight core that prevents the face sheets from buckling. The sandwich structure gainsits bending rigidity mainly by separating the facesheets, and has very high structural efficiency(ratio of strength or stiffness to weight). The SAS panel consists of several basic layers. Eachlayer must meet its own combination of structural and electrical design requirements, as well as themanufacturing and assembly requirements. The basic panel layers are: an outer facesheet, antennaelement, honeycomb core, and supporter elements. These are shown in Figure 2 in an explodedview, which also specifies the materials chosen in each layer. The layers are bonded by adhesiveto form the final assembly. The outer facesheet must carry a significant portion of the in-planeloads, since it contributes to the overall panel buckling resistance, and it also provides low velocityimpact and environmental resistance. This outer facesheet must also permit the transmitting andreceiving of RF signals. The facesheet material must be low loss and only weakly dielectric inorder to minimize signal attenuation and reflection loss. The honeycomb cores transmit shearloads between layers induced by bending loads in the panel, and support the facesheet againstcompression wrinkling. They also provide impact resistance and increase the overall panel bucklingresistance. The thickness of the honeycomb cores contributes significantly to the overall rigidity, andis involved in the balance between panel thinness and structural rigidity. The supporter also carriesa significant portion of the in-plane loads together with the outer facesheet, as well as supporting thewhole structure. It can be selected without need to consider electrical performances, and thereforehas the best mechanical properties of any layer in the CAS construction. Spiral antennas [10, 11]

Progress In Electromagnetics Research Symposium Proceedings, Marrakesh,Morocco, Mar. 20–23, 2011 1349

Figure 1: Design concept of the composite antennastructure.

Figure 2: Structure and materials of the compositeantenna structure.

can be used in high-performance aircraft, spacecraft, and in satellite and missile applications,where constraints include size, weight, cost, performance, ease of installation, and aerodynamicprofile. These antennas are low-profile, conformable to planar and nonplanar surfaces, simple andinexpensive to manufacture using modern printed-circuit technology, and compatible with MMICdesigns. Our CAS design is based on a spiral antenna type with a bottom layer of carbon fiberplate. The antenna does not work well if the the spiral antenna current is interrupted. To overcomethese problems, we used an electromagnetic wave absorber on a hole in honeycomb core. The spiralantenna is placed on the absorber, which absorbs electromagnetic waves. Unwanted radiationcannot pass through the absorber, reducing the effect of the reflector.

3. DESIGN AND EXPERIMENTAL PROCEDURE

The antenna is to be designed for low frequency and broadband communication. The antennarequirements are: frequency range 0.5 to 2 GHz (bandwidth 1.5 GHz), and gain at least 10 dBiwith circular polarization. In designing the antenna elements, a computer-aided design tool (CSTMicrowave Studio) is used to select a large number of strongly interacting parameters by means ofintegrated full-wave electromagnetic simulation. The resulting antenna elements and their dimen-sions are shown in Figure 3. The facesheet is used to a FR-4 glass/epoxy radiating patch comprisinga spiral antenna, 1 mm thick. This spiral antenna, set in a circle of diameter 30 cm, is 2 mm thickand has 10.5-turns. FR-4 glass epoxy is a popular and versatile high pressure thermoset plasticlaminate grade with good strength to weight ratio. FR-4 undergoes negligible water absorptionand is commonly used as an electrical insulator possessing considerable mechanical strength. Themain objective is to obtain a good impedance match as seen by the feedline, in the range of fre-quencies from 0.5 to 2 GHz. The feedline connected to the input port has characteristic impedance50Ω, chosen for impedance matching at the port. Coaxial cable is used to feed the antenna inthe center. Manufacture of the CAS is a sequential process. The facesheet, including antennaelements, are first prepared by a photolithographic process. The honeycomb cores and each layermust be aligned prior to permanent bonding, in order to give precise electromagnetic coupling. Foralignment, four guide holes are made near the edge of all layers. These are confirmed to have noeffect on antenna performance. The CAS is assembled by aligning these holes using a plastic nutand bolt. Each layer is bonded to the top and bottom of its neighbors in the designed sequence,using epoxy film adhesive. The assembly, covered by a vacuum bag, is then cured in an autoclaveaccording to the recommended curing cycle for this adhesive (125C for 90 minutes at a pressureof 3 kg/cm2). Figure 4 shows the appearance of each layer and the top view of the final assemblyafter fabrication. The size of the CAS is 300×300×27mm. Antenna performance of the fabricatedCAS is determined by electrical measurements. The return loss characteristic, which measures themismatch or the ratio of the reflected power to the incident power at the input port, is measuredusing a Network Analyzer 8510 under laboratory conditions. The radiation patterns are measuredin an anechoic chamber at four frequencies, 0.5, 1, 1.5 and 2GHz, in order to show the bandwidthpatterns. Gains and axial ratios are calculated by comparing the magnitude of the electric fieldagainst a standard-gain horn antenna.

4. RESULTS AND DISCUSSION

We have studied an antenna embedded in a structural surface, which provides good structuraland good electrical efficiencies at the same time. The design is a composite sandwich structure

1350 PIERS Proceedings, Marrakesh, MOROCCO, March 20–23, 2011

Figure 3: Design of the spiral antenna element. Figure 4: Structure and fabrication of the compositeantenna structure.

(a) Reflection coefficient (b) Gain (c) Axial ratio

Figure 5: Electrical performance of composite antenna structure.

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Figure 6: Radiation patterns in bandwidth.

in which a spiral antenna element has been inserted. This design provides antenna performancesthat meet our requirements. Figure 5 shows the electrical performance of the composite antennastructure. Figure 5(a) shows the return loss characteristic; a bandwidth of approximately 1.5GHz isseen, corresponding to the frequency range of interest (0.5–2 GHz). Figure 5(b) shows the antennagain and the gain reduction occurs at low frequencies and is caused energy loss due to absorption.Figure 5(c) shows the axial ratio, and measuring less than 3 dB within the band and get the value ofcircular polarization is well formed. Figure 6 shows the radiation patterns at 0.5, 1, 1.5 and 2GHz.The radiation pattern indicates that emissions from the front of the spiral antenna is LHCP (LeftHand Circularly Polarized), RHCP (Right Hand Circularly Polarized) include coming out the back.The back radiation is low because of the carbon fiber plate, and front radiation is large with a LHCPcharacteristic.

5. CONCLUSIONS

In this paper, we have designed and fabricated a spiral antenna of honeycomb sandwich construc-tion. The final demonstration article is a 300 × 300× 27mm flat antenna panel with an antenna

Progress In Electromagnetics Research Symposium Proceedings, Marrakesh,Morocco, Mar. 20–23, 2011 1351

element. Electrical measurements of the fabricated structure show that is satisfies the design re-quirements, with a bandwidth above 1.5 GHz and a high gain with circular polarization. Thedesign concept can be extended to give a useful guide for manufacturers of structural body panelsas well as antenna designers, and promises to be an innovative future communication technology.

ACKNOWLEDGMENT

This research was financially supported by the Ministry of Education, Science Technology (MEST)and Korea Institute for Advancement of Technology(KIAT) through the Human Resource TrainingProject for Regional Innovation.

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