Predicting the relative permittivity of water-contaminated glass/epoxy laminates at X-band

The effect of low levels of absorbed water on the relative permittivity, or dielectric constant, of glass-fiber-reinforced epoxy laminates is examined by means of a split post dielectric resonant technique at 10 GHz. The two laminates studied utilize the same matrix material, and are commonly used as the radar-protecting structure, or radome, for military and civilian aircraft and ground-based radar systems. The relative permittivity of the composite material dictates the radome wall thickness and geometry which is crucial to unimpeded propagation of the electromagnetic wave without appreciable signal loss. The cumulative effect of absorbed moisture due to humid air, precipitation, and condensation is a significant increase in relative permittivity. The suitability of existing dielectric mixing models is investigated for prediction of the effective relative permittivity of the multiple component system in which absorbed water is treated as one of the constituents. An effective medium model formed by a power law approximation is proposed for predicting the effective relative permittivity of the water-contaminated composites. The increase in relative permittivity at 10 GHz as a result of absorbed moisture is recorded up to 1.8 percent water content by weight, at which point a 13% increase in dielectric constant is observed relative to the dry state. The model is used to predict the effective dielectric constant of both moisture-contaminated laminates using a common exponential term, recovered from a least-square fit to the experimental data. Prediction is based solely on moisture content and dielectric constant of the laminates in the dry state, which is measured at 3.42 and 4.62 depending on the type of fiber reinforcement. The ability to predict the relative permittivity of in-service composite laminates used as radar-protecting structures is crucial to the design and accuracy of these systems. The proposed model is capable of predicting the relative permittivity of two glass-fiber reinforced epoxy radome materials based on the level of moisture contamination due to practical, long-term usage.