Mechanics of Composite Materials, Vol. 40, No. 3, 2004
DESIGN AND FABRICATION OF COMPOSITE SMART STRUCTURES WITH HIGH ELECTRIC AND MECHANICAL PERFORMANCES FOR FUTURE MOBILE COMMUNICATION
C. S. You and W. Hwang
Keywords: smart structures, sandwich structures, microstrip antenna, composite laminate, Nomex honeycomb In this paper, we have developed a load-bearing outer skin for antennas, which is termed a composite smart structure (CSS). The CSS is a multilayer composite sandwich structure in which antenna layers are inserted. A direct-feed stacked patch antenna is considered. A design procedure including the structure design, material selection, and design of antenna elements in order to obtain high electric and mechanical performances is presented. An optimized honeycomb thickness is selected for efficient radiation and impedance characteristics. High gain conditions can be obtained by placing the outer facesheet in the resonance position, which is at about a half wavelength distance from the ground plane. The measured electrical performances show that the CSS has a great bandwidth (over 10%) and a higher gain than an antenna without a facesheet and has excellent mechanical performances, owing to the composite laminates and honeycomb cores. The CSS concept can be extended to give a useful guide for manufacturers of structural body panels and for antenna designers.
1. Introduction The designers of structures, materials, and antennas have recently joined their forces to develop a new high payoff technology called conformal load-bearing antenna structure (CLAS) [1-4]. The embedding of radio-frequency (RF) antennas in a load-bearing skin is a new approach to the integration of antennas into structural body panels. It emerged from the need to improve the structural efficiency and antenna performances. It demands an integrated product development from disparate technologies, including structures, electronics, materials, and manufacturing, in order to generate a realistic design. The classic division between structures and antennas is bridged in the CLAS, and the technical challenge is to satisfy the structural and electric requirements, which often conflict. Fig. 1 shows the progressive evolution of antenna and avionics packaging technology [1]. In 1999 [2], the design, fabrication, and structural validation of a load-bearing multifunction composite antenna panel subjected to realistic aircraft flight loading conditions was successfully demonstrated. The loading conditions were representative of those of the mid-fuselage F-18-class fighter composite panel installation. The wideband electric performance of the composite antenna panel was validated. The implementation of the CLAS technology extends beyond the military field of application of multiple antennas. The load-bearing ability in commercial applications has to be balanced carefully with safety, Department of Mechanical Engineering, Pohang University of Science and Technology, Pohang, Korea. Russian translation published in Mekhanika Kompozitnykh Materialov, Vol. 40, No. 3, pp. 369-382, May-June, 2004. Original article submitted January 30, 2004.
0191-5665/04/4003-0237 © 2004 Plenum Publishing Corporation
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Fig. 1. Evolution of antenna and avionics packaging technology. 1 — level 0 (old technology); 2 — level 1 (conformal/integrated antenna); 3 — level 2 (emerging technology), smart-skin structures; 4 — level 3 (load-bearing electronics); 5 — level 4 (completely integrated smart vehicle); 6 — increasing integration.
cost, and complexity, but the manufacturing and material processes developed for the CLAS are common to almost all applications. Fig. 2 shows a conceptual model of the Delphi multiple antenna reception system [5]. The use of a composite roof structure provides a possibility to satisfy the communication and entertainment reception needs with a lightweight and durable self-contained structure. This approach eliminates the multiple mast antennas currently used for CB, AM/FM, television, cellular, and GPS reception. Through the innovative integration of antenna elements, amplifiers, and the ground plane, the reception quality and manufacturability of vehicles is improved significantly. The most important problem connected with these structures is the selection of materials of high electric loss without lowering the antenna efficiency, because materials fully satisfying both mechanical and electrical requirements have not appeared up to the present. Load-bearing antenna structures designed neglecting the electric and mechanical performances will not be of a greater benefit than the ordinary mast antennas in vehicles, due to their inefficiency. The aim of the present study is to design an electrically and structurally effective antenna structure, termed a composite smart structure (CSS), for the future mobile communication and the next generation of structural surface technology [6, 7]. Structurally effective materials with a high electric loss must not lower the antenna efficiency in order to obtain both high electric and mechanical performances. The design procedure is focused on a high gain and a great bandwidth in the electric part and a high strength, stiffness, and environmental resistance in the mechanical part. A direct-feed stacked patch antenna is considered, and a composite sandwich structure including composite laminates and Nomex honeycombs is used for high mechanical performances. The design procedure and the measured electric performances are presented.
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Fig. 2. Delphi multiple antenna reception system: 1 — cellular antenna; 2 — secure short range; 3 — Ku-band; 4 — top surface; 5 — country-specific services; 6 — RFA antenna; 7 — AM/FM/TV antennas; 8 — S-band terrestrial and satellite, 9 — bottom surface; 10 — signals to the head unit (via an optical bus); 11 — receiver electronics; 12 — GPS.
2. Composite Smart Structures A fundamental design concept for the CSS is an organic composite sandwich panel in which microstrip antenna elements are inserted, as shown in Fig. 3. Microstrip antennas [8] can be used in high-performance aircraft, spacecraft, satellite, and missile applications, where the size, weight, cost, performance, ease of installation, and aerodynamic profile are constraints. These antennas are low-profile, conformable to planar and nonplanar surfaces, simple, and inexpensive (can be manufactured by using the modern printed-circuit technology), and compatible with MMIC designs. One of disadvantages of the original microstrip antenna configuration is its small bandwidth. However, this problem is solved by means of aperture-coupled feeding [9] or stacked radiation patches [10], or both the methods simultaneously [11]. The conventional sandwich construction [12, 13] consists of two relatively dense and stiff facesheets bonded to a low-density core. The facesheets carry the bending-induced axial loads, and the core sustains the shear stresses and the compressive stresses normal to the plate. It also helps to resist the buckling of the facesheets under axial compressive loading. The core usually has low in-plane and flexural stiffnesses, compared with those of facesheets, but it can have a significant transverse stiffness and an adequate shear stiffness. The presence of a core removes the facesheets away from the neutral axis, thus increasing the bending resistance provided by the facesheets.
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Fig. 3. Basic concept of composite smart structures. 1 — structure; 2 — antenna; 3 — sandwich structure: 4 — facesheet, 5 — adhesive, 6 — honeycomb; 7 — microstrip antenna: 8 — patch, 9 — dielectric, 10 — ground plane.
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Fig. 4. Configuration of a composite smart structure: 1 — facesheet (1 mm); 2 — honeycomb (h mm); 3 — upper patch; 4 — dielectric (0.25 mm); 5 — honeycomb (2.54 mm); 6 — lower patch; 7 — dielectric (0.76 mm); 8 — feedline; 9 — ground.
3. Structure and Materials As shown in Fig. 4, the basic panel layers of a direct-feed stacked patch antenna are two facesheets, two honeycomb cores, and antenna elements made of dielectrics. The material properties are shown in Table I.
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TABLE I. Properties of Constituent Materials Materials Facesheet Glass/Epoxy [0/90]2S (UGN200, SK chemicals) Honeycomb Nomex Honeycomb HRH-10-1/8-6 (Hexcel Composite) Dielectric Glass/PTFE (Duroid 5880, Rogers Co.)
Dielectric constant tan d
Ultimate strength
Elastic modulus
4 0.03
574 MPa
25.4 GPa
1.1 nearly zero
7,76 ÌÏà
414 MPa (compressive properties in the thickness direction)
2.2 0.0009
28 MPa
1 GPa
The facesheets mechanically carry a significant portion of in-plane loads, contribute to the overall panel buckling resistance, and provide a low-velocity impact and environmental resistance. The outer facesheet, which is placed above the radiating patch, provides signal losses by its high electric loss (tan d) when a signal passes through it. However, it can make the antenna to radiate more efficiently, without electrical losses, when it is placed at the resonance position. Glass/epoxy laminates are used as facesheet materials. The honeycomb cores mechanically transmit the shear load induced from bending loads in the panel, prevent the outer facesheet from compression wrinkling, provide an impact resistance, and increase the overall panel buckling resistance. They also create an air gap without signal losses and adjust the position of the outer facesheet for maximum antenna performances. The thicknesses of honeycomb cores contribute to the antenna efficiency as well as the overall rigidity. The thickness h of the honeycomb core between the upper radiating patch and the outer facesheet is selected for the most efficient antenna performances. Two radiating patches are used for a great bandwidth, which is achieved by coupling the dual resonance system. The upper and lower patches are separated by an air gap, which is provided by a Nomex honeycomb 2.54 mm thick. The duroid 5880 used for layers of antenna elements has good electric properties (a small dielectric constant and low electrical loss) but it does not contribute to structural performances. Thus, the CSS is a composite sandwich structure in which the antenna part is inserted between the honeycomb core and the lower facesheet. In order not to lower the antenna efficiency by the outer facesheet, which is made of a structural material with a high electric loss, the thickness of the honeycomb core, which determines the position of the outer facesheet, must be adjusted in the design procedure. 4. Design Procedure The antenna performances were aimed at the Ku-band satellite communication in the frequency range from 11.7 to 12.75 GHz with linear polarization. At first, the antenna elements made of dielectrics without facesheets were designed for a central frequency of 12.2 GHz. A computer-aided design tool (IE3D V. 9.35) was used to select a large number of strongly interacting parameters by an integrated full-wave electromagnetic simulation. The designed antenna elements with their dimensions are shown in Fig. 5. The upper radiating patch is smaller than the lower one and the impedance at central edge of the lower patch is 76 W, which is transformed to a 50-W feedline when fabricated. Fig. 6 and 7 show the reflection coefficients c and radiation patterns, respectively, obtained by computer simulation for honeycomb cores of thickness h under the outer facesheet and the performances of an antenna without facesheets (h is infinite). Beginning with h = 8 mm, the radiation in the broadside direction becomes stronger, and the gain reaches a maximum at h = 10 mm, at which the outer facesheet is approximately at a half wavelength distance from the ground plane. At h = 10 mm, the
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Fig. 5. Designed antenna elements: 1 — upper patch; 2 — lower patch; 3 — feedline.
Fig. 6. Reflection coefficients c vs. frequency f for of honeycomb cores of various thickness h: (m) — infinite; (l) — 4, (F) — 8, (n) — 10, (v) — 14, and (t) — 18 mm.
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Fig. 7. Gain G as a function of radiation direction at 12.2 GHz for honeycomb cores of various thickness h. a — E-plane; b — H-plane. Designations as in Fig. 6.
reflection coefficient is also satisfactory in the desired frequency range with a bandwidth of 1.5 GHz at VSWR 2. When h (h > 10 mm) increases further, dips in the main beam appear in H-plane patterns, which result in two maximum radiation directions symmetric with respect to the broadside direction. The greater the thickness h, the wider and deeper the dip and the more tilted
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Fig. 8. Gain variations with frequency changes: ( without a facesheet.
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) — with à facesheet (h = 10 mm); (m) —
b
c
Fig. 9. A fabricated CSS 48 ´ 48 ´ 16 mm (a) with an upper patch (b) and a lower patch with a feedline (c).
the beams. Fig. 8 shows the gain G as a function of frequency f. It is seen that the gain varies less than by 1 dB on the frequency range from 11.7 to 12.75 GHz. 5. Measured Electric Performances The optimized honeycomb thickness under the outer facesheet is 10 mm, when the maximum gain is obtained from the resonance condition and the bandwidth needed is also achieved if stacked radiating patches are used. After fabricating the designed structure, the reflection coefficient was measured with a Network Analyzer 8510 in laboratory conditions, the radiation patterns were found in an anechoic chamber, and the gains were calculated from the field values of a reference horn antenna measured under the same conditions. Fig. 9 shows a fabricated CSS. The measured electric performances of antennas with and without an outer facesheet are shown in Figs. 10 and 11. Their impedance characteristics are almost same, and they all satisfy design requirements: the VSWR is less than 2 in the frequency range from 11.7 to 12.75 GHz. The corresponding radiation patterns are presented in Fig. 11. The E-plane pattern is unsymmetric due to the feedline effect.
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c, dB
0 -10 -20 -30
f, GHz -40
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Fig. 10. Measured reflection coefficient c vs. frequency f. Designations as in Fig. 8.
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Fig. 11. Measured radiation for antennas patterns in the E (a) and H (b) planes at 12.2GHz. Designations as in Fig. 8.
The radiation pattern of the antenna with an outer facesheet is narrower and the gain is higher by 3.5 dB (11.2 dB) than that of the antenna without a facesheet (7.63 dB). 6. Discussion A design of CSS with structurally effective materials, without a compromise with electrical properties, is possible by gain enhancement methods where a superstrate having a great dielectric constant is placed at the resonance position (at a half-wavelength distance) [14, 15] or multiple substrates are located above the antenna [16, 17]. However, the bandwidth varies inversely to the gain. Using gain enhancement methods, the antenna efficiency can be improved by placing the outer
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facesheet at the resonance position. In this case, the impedance characteristic over a 10% bandwidth under VSWR 2 is also satisfied by using stacked radiating patches. The radiation improvement by the outer facesheet can be investigated with the use of a transmission line analysis [16, 17] or an aperture field analysis [18]. From a the standpoint of optics, the rays emanating from an antenna and incident on an interface of different media are refracted according to Snell’s law. Using the transmission line analysis, it is found that, by a proper selection of layer parameters, most of the rays going into the free space can be inclined to a prescribed direction. The maximum gain, as in a transmission line, corresponds to the maximum voltage transfer from the incident voltage wave in the air to the line voltage at the radiating patch. In the aperture field analysis, the outer facesheet, at the resonant distance, rearranges the field distribution on an aperture in an optimal way. The field amplitude distributions on the top surface with and without a facesheet are almost the same, whereas the phase distribution for the structure with a facesheet is much smoother than that without a facesheet. An electric field with a smoother phase distribution creates a higher gain when the area of the aperture and the amplitude distribution are kept unchanged, i. e., the increasing gain comes from the rearranged aperture field on an area larger than the original antenna itself. 7. Conclusion Composite smart structures having high electrical and mechanical performances have been designed. A CSS is a composite sandwich structure in which a microstrip antenna is placed between a honeycomb core and the lower facesheet. For a great bandwidth, two radiating patches of different sizes is used. The honeycomb thickness under the outer facesheet is selected from the resonance condition, at which the maximum radiation is obtained. The outer facesheet is placed at about a half wavelength distance above the ground plane. The outer facesheet rearranges the radiation field distribution. It can be considered as a structure creating multiple reflections, which can be used to concentrate the radiated power in the broadside direction. The measured electric performances show that the CSS has a great bandwidth (over 10%) and higher gain than an antenna without a facesheet, and excellent mechanical characteristics owing to the composite laminates and honeycomb cores. The CSS concept makes it possible to design antennas from structurally effective materials for the communicative body panel in vehicles. It can be extended to give a useful guide for manufacturers of structural body panels, as well as antenna designers, promising an innovative future communication technology.
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