The Army Digitized Force requires a robust communications infrastructure for its superior IT/C4ISR (Information Technologies/Command, Control, Communications, Computers, Intelligence, Sur veil lance, and Reconnaissance). Ballistic radomes protecting communications antennas will increase the survivability and maintain the lethality of combat platforms. Legacy antennas on combat platforms are vulnerable to small arms fire and munitions fragments. Antennas on platforms with active threat protection systems have the added threat from the munitions fragments generated by the system that can shoot down and pre-detonate incoming warheads. Current radomes do not protect the antenna from these threats, as they are usually thin-walled composite structures to minimize RF transmission loss.
A combined ballistic and electromagnetic design and testing methodology has been formulated to develop radomes capable of providing ballistic protection to the antenna system with low RF loss. Technologies developed are applicable to frequencies from 225 MHz up to 45.5 GHz, with specific receive and transmit bands for dish, phased array, and other types of low-profile antennas. Results demonstrated the potential for efficient radome designs (less than 1 dB transmission loss), while meeting ballistic requirements using multi-layer, thick-section sandwich construction.
General requirements for radomes capable of ballistic protection include minimal RF transmission loss, ability to withstand standard environmental conditions, vehicle mobility loads, and costs. The radome must not only be able to stop the projectile, but also be stiff enough to minimize deflection. This requirement reduces the ability to use material deflection as an energy absorbing mechanism.
Initial efforts focused on the selection of fibers and resins that have the potential to meet both energy absorption and dielectric requirements. Composite materials were fabricated from selected fiber/resin combinations and characterized for both energy absorption capability and high-frequency dielectric properties. Candidate materials were selected based on combined energy absorption and dielectric performance. In parallel, electromagnetic simulations were performed for radome design to evaluate the candidate materials.
Dielectric Property Characterization
In conventional non-ballistic radomes, typical resins used are cyanate esters (especially at high frequencies), primarily for their low dielectric loss characteristics. However, cyanate esters are generally poor ballistic resins due to their low elongation and strength, and are also very expensive. Standard resin systems used in ballistic applications are generally thermosets, which also tend to have relatively high dielectric loss factors. Thermoplastics are inherently tougher polymers compared to thermosets and are also good candidates for ballistic applications.
Fiber/resin combinations were fabricated for measurement of high-frequency dielectric properties, in the range 15- 20 GHz, using a cavity-based method. A fiber with a thermoplastic polymer provides a good balance between energy absorption and dielectric properties.
Using the selected candidate materials, designs for ballistic energy-absorbing radomes were developed. The conventional design process involved minimizing material thicknesses to reduce transmission loss, and this can result in either monolithic or sandwich designs. Radome design was performed by evaluating monolithic and sandwich constructions of the selected materials. A simple, multilayered radome simulation code was developed for this purpose, which can predict the transmission loss for a specified multi-layer sandwich construction.
The low-frequency radome covers a 2:1 bandwidth at L-band, and is designed to protect a low-profile communications antenna. This radome is about 12" tall, and 12" in diameter at the base. It is designed to be mounted to an armored vehicle (see photo). RF performance was optimized by genetic algorithm to minimize degradation of the pattern or impedance.
Multifunctional radomes that can provide both electromagnetic performance and energy absorption are desirable for the next-generation antenna systems. Radomes are uniquely different from standard armor due to the need to provide complete protection with minimal dynamic deflection of the radome (to prevent damage to antenna).
A methodology involving materials selection, characterization of dielectric properties, and energy absorption capability per unit areal density was developed to screen candidate composite materials that can meet the desired radome requirements. Electro magnetic simulations based on the down-selected materials have developed radome designs that can meet both electromagnetic and ballistic performance requirements.
This article was written by S. Yarlagadda and B. Gama of the Center for Composite Materials at the University of Delaware, Newark, DE; D. Linden and J. Lilly of JEM Engineering, LLC, Laurel, MD; and T. Fung, L. Coryell, and S. Goodall of the US Army Communications Electronics Research Development and Engineering Center, Fort Monmouth, NJ. For more information, visit http://info.hotims.com/22928-544.