Characterization of Carbon Fiber Composite and Standard Structural Beam Shapes for UAS VHF Antenna Applications

Abstract

Radar remote sensing from both manned and unmanned airborne vehicles is undoubtedly an effective tool for collecting data in inhospitable regions or regions that are otherwise difficult to reach. Large very high frequency (VHF) antenna systems are challenging to integrate into small- to medium-scale unmanned aerial systems (UASs) due to limited space availability and payload limitations. Though lightweight Carbon Fiber Reinforced Plastic (CFRP) composite materials are appealing for UAS structures, they significantly affect the antenna performance due to the Electromagnetic Interference (EMI) caused by the CFRP UAS structures. Thus, a trade-off between aircraft and electrical performance generally occurs when integrating antenna system into UASs. This motivated the research on finding solutions to mitigate or possibly eliminate the EMI caused by the CFRP UAS structures. An innovative approach of utilizing shapes typical of CFRP primary structures of UAS as the radiating structural antennas is proposed to overcome the EMI problem. If electrically effective, and if properly integrated structurally such multifunctional structures could then address size and weight limitations of UAS. This study addresses several fundamental aspects of CFRP antennas for typical structural shapes and materials, such as characterizing the conductivity of the material, determining antenna performance sensitivity to a variety of laminate variables, and investigating whether certain common structural shapes offer advantages over others. The potential of UAS CFRP structural antennas is also demonstrated through several prototype examples, with an emphasis on electrical performance over detailed structural characterization. In this research study, the electrical conductivity of carbon fiber composite material is first characterized. Carbon fiber composite material is indispensable for aerospace applications due to its excellent specific strength and stiffness. However, the electrical conductivity and other electrical properties are not well established, especially in the very high frequency range. Knowing the conductivity can help estimate electrical performance of carbon fiber antennas through simulations, as well as determine the electromagnetic interference of carbon fiber composite structures in the vicinity of antenna elements. Thus, the DC (direct current) and AC (alternating current) electrical conductivity of carbon fiber composite material is first established in the VHF range. The DC conductivity values are calculated by using physical parameters and the measured resistance values of carbon fiber specimens in different product forms. The DC conductivity is determined to be 7,380 S/m and 5,135 S/m for uncured and cured carbon fiber composite cloth material (AS4/DPL862), respectively. Based on the skin effect theory, the AC conductivity in the VHF range is expected to be comparable to DC conductivity of carbon fiber composite material. To verify this hypothesis, the AC conductivity values are determined through a new novel experimental approach. Multiple dipole antennas resonating at 200 MHz are fabricated from carbon fiber fabric by varying the number of layers and the length of the antenna feeding metal strips. Since the antenna response is sensitive to various physical parameters, such as feeding technique, the carbon fiber composite conductivity found in this study is referred as an “effective” conductivity. An S-parameter matching methodology is developed to determine the effective carbon fiber conductivity by matching experimental and simulated results of multiple carbon fiber antenna designs. The effective conductivity range for the single- and double-layer carbon fiber antennas (made of AS4/DPL862) at 200 MHz is determined to be 4,800-20,000 S/m for +/-0.5 dB S11 variation. The effective conductivity of carbon fiber composite improves with increasing contact between the feeding metal strip and carbon fiber plies. The S11 matching methodology is validated experimentally using the Wheeler Cap method by comparing carbon fiber conductivity at a VHF frequency. The S11 matching methodology is also verified for an ultra-high frequency, and the effective conductivity of carbon fiber composite (AS4/DPL862) at a frequency of ~700 MHz is found to be 880 S/m. The effective carbon fiber conductivities at both very high frequency and ultra-high frequency are within the conductivity values found in the past literature. The carbon fiber composite antennas provide gains within 2-10% of a geometrically identical copper antenna. After characterizing the carbon fiber composite conductivity, for the first time a comprehensive assessment is conducted to characterize the sensitivity of carbon fiber antenna performance to laminate design variables such as feeding technique, product form, fiber orientation, ply stacking sequence, and thickness. Initially, the sensitivity to the fabrication process is assessed, and the effects of manufacturing tolerance are determined for key antenna parameters. The antenna feeding location is varied between the top and mid-plies of the carbon fiber antenna laminate. Significant improvement of 1.33 dB and 0.15 dB in the S11 and gain at resonance, respectively, is observed when antennas operating at or near 200MHz are fed at the laminate mid-plies. Application of silver-epoxy paste at the feed location offers small performance improvement for a single-layer carbon fiber antenna; however, no significant improvement is seen for the multi-layer antenna. Carbon fiber antennas made of cloth product form consistently perform better than their equivalent tape antennas, and the cloth antennas are found to be less affected by the fiber orientation and ply stacking sequence as compared to the tape antennas. However, variability in carbon fiber antenna performance due to the product form, fiber orientation and ply stacking sequence, is significantly reduced by adding one or more 0° plies to a non-zero plies composite laminate. Carbon fiber cloth and tape laminates with all 0° plies offer the best electrical performance as expected, with improvements of up to 7 dB and 1.7 dB in the S11 and gain at resonance, respectively, as compared to the antennas with all 45° plies. Carbon fiber tape antennas with all 45° or 90° plies symmetric laminate do not resonate, while the tape antennas with all 45° plies asymmetric laminate demonstrate a downward shift of about 10 MHz (~5%) in resonant frequency due to an increase in the effective electrical length. The S11 and gain performance of both the cloth and tape antennas improve gradually with addition of CFRP plies which also increases the antenna thickness. Relatively thicker carbon fiber antennas with 6-12 plies offer S11 and gain performance similar to the copper antenna. Most of the carbon fiber antennas with at least one 0o ply provide similar bandwidths as a copper antenna regardless of any variations in the physical parameters. Considering the improved fiber-to-metal contact during the laminate sensitivity study, the effective CFRP conductivity is reassessed for improved S11 performance. The conductivities of single- and double-layer designs are found on the higher end of their original conductivity range spectrum. The conductivities of both the antennas are found within the originally recommended effective conductivity range of 4,800-20,000 S/m. The reassessed CFRP conductivities are also within the conductivity range seen in the past literature. It is also noted that adding 2-10 plies to the double-layer design further improves an effective conductivity by 2,000-10,000 S/m. However, this study does not differentiate whether the overall electrical improvement is due to increased number of plies or due to increased antenna thickness. Antennas with common structural cross-sections are investigated to verify if an antenna shape or size offers any advantages in terms of electrical performance. Extensive simulated and experimental analyses are conducted for antennas which could be structural operating at very high frequencies, specifically for a center frequency of 200MHz. In the simulated analysis, antennas with five different shapes, sixteen different sizes, and two different feed locations are simulated. The examined antenna shapes include circular, box, C-channel, I-channel, and airfoil shapes. Antenna sizes include 1 in., 3 in., 5 in., and 8 in. tall antennas, with width-to-height ratio of 0.25, 0.5, 0.75, and 1. The box, C-, and I-channel antennas are also fed at the cap and web of the sections. A relatively small variation of 10-30% in bandwidth is observed for the 1 in. and 3 in. tall antenna designs, whereas a large variation of 25-125% in bandwidth is observed for 5 in. tall designs as cap-fed open cross-section designs offer dual resonance. From the dual resonance study, a potential antenna principle/theory is proposed which shows that when the antenna width/perimeter is approximately equal to a quarter-wavelength corresponding to second resonating frequency, the antenna exhibits dual resonance. A wide bandwidth of up to 115% is achieved by maintaining L/P ratio of 2.6 for both 2D planar and 3D C-channel antennas, and this specific antenna size ratio results into a dual resonance with two closely spaced resonances. The circular tube-shaped antennas offer better overall electrical performance at 200MHz as compared to most antenna shapes and sizes. There are a few exceptions, such as a 5 in. C- and I-channel cap-fed antennas, which offer bandwidths up to 62% higher than the circular design. When considering non-circular cross-sections, a W/H ratio of 1 is recommended to achieve larger bandwidth than the circular antenna. Circular tubes are recommended for small- to medium-scale vehicles, due to their comparable electrical performance as non-circular shaped antennas and they are also generally easy to integrate into the vehicles. The gains of 5 in. and shorter antennas are not affected by the variation in physical antenna parameters, as all the antennas consistently provide gains of 2-2.6 dBi. For structural antennas operating around ~200MHz, open section cap-fed designs are more effective for heights of about ~5 in. and are shown to be less effective for taller sections. The majority of the 8 in. tall antennas and the airfoil shaped antennas exhibit poor electrical performance, as there are certain antenna size limitations which affect these antennas’ abilities to radiate. For the structural beam shaped antennas operating at a VHF frequency of ~200MHz, a length-to-perimeter (L/P) ratio of 1.5-2.5 for C- and I-sections; 1.3-1.6 for box and circular sections; and 1.2 for airfoil shaped antennas are recommended to enhance the antenna impedance bandwidth performance. Finally, measurements are conducted for circular and C-channel carbon fiber antennas to verify the simulated performance results. A circular-tube antenna is also installed on a small-scale unmanned vehicle, and the antenna measurements are compared to observe if any electronics in the vicinity affect antenna performance. In addition, as a part of ongoing research, a near-high frequency electrically small antenna was developed for an unmanned helicopter. After extensive simulated and experimental analyses, a prototype was built which demonstrates the potential of an innovative electro-mechanical approach of utilizing aircraft carbon fiber structures as antennas rather than simply embedding antennas into structures. The antenna performance is further improved by adding matching networks to the antenna system. The experimental and simulated results show good agreement, in terms of equal number of resonances, similar resonant frequencies, and bandwidths for all the fabricated antennas. Research addressed herein characterizes the potential of developing multi-functional/synergistic CFRP structural antennas by utilizing primary UAS spar structures, and it is expected to enhance overall electrical performance. Carbon fiber antennas of common structural shapes demonstrate electrical performance similar to a typical metallic copper antenna. The promise of such antennas, if well integrated and shown to be structurally efficient, is that it opens the door to exciting opportunities for achieving desired electrical performance while reducing structural and payload weights for unmanned aircraft systems. Carbon fiber structural antennas could offer lightweight and low-drag designs for airborne remote sensing applications. Potential applications of carbon fiber antennas could extend into other fields that would benefit from lightweight and highly stiff antenna structures, such as space antenna structures, structural health monitoring of windmill blades, structural sensors for self-driving automobiles, as well as wearable sensors for health monitoring.