The effect of moisture contamination on the relative permittivity of polymeric composite radar-protecting structures at X-band

The impact of absorbed moisture on the X-band relative permittivity of three composite materials used in aircraft radar-protecting structures (radomes) is investigated, quantified, and analyzed with respect to existing effective medium theories. Two glass-fiber-reinforced epoxy laminates and a quartz-fiber reinforced bismaleimide laminate are fabricated, contaminated with water, and analyzed with a split-post dielectric resonator in order to determine moisture-induced relative permittivity changes at 10 GHz. In the most significant case, a 3.7% increase in water content by weight resulted in a 43% increase in relative permittivity for BMI/quartz laminates. Such an increase may be a root cause of radome-induced radar performance loss for long-service-life aircraft. The relationship between absorbed water volume and relative permittivity is not well described by the existing effective medium theories considered. Support is given to the existence of conditions which lead to restricted molecular dipole rotation of water within a polymer network and a resulting “effective” relative permittivity of water based on polymer chemistry and morphology. A simple power-law expression predicated upon this assumption successfully describes the relative permittivity increase as a result of absorbed water prior to equilibrium moisture condition.