The materials most widely used for radome fabrication are composites consisting of a woven substrate fabric protected with applied coatings. In these composites, it is the substrate which provides the majority of tensile strength and tear resistance. Most applications today are required to have a 20-year maintenance-free life. The long-term durability of any composite is imparted by the coating and/or film used on the outer surface. Their function is to protect the substrate from the effects of the environment, allowing them to retain the greater part of their original strength.
Coatings and films typically used in RF applications are polytetrafluoroethylene (PTFE - Teflon®), polysiloxane compounds (silicones), polyvinyl chloride (PVC, vinyl), polyvinyl fluoride (Tedlar), chloroprene (neoprene), and chlorosulfonated polyethylene (Hypalon). Of these, only two show little degradations in functionality during a 20-year period; PTFE and silicone. Silicone accumulates dirt and requires frequent cleaning, but does not lose its hydrophobic (water shedding) characteristic. Other coatings and films require maintenance on a regular basis. Tedlar, which has only fair hydrophobicity, loses much of that after brief weathering, leading to the formation of a water film as opposed to a droplet condition. Hypalon, which does not have good inherent hydrophobicity, requires regular painting with a protective coating. Vinyl is quickly deteriorated by sunlight which results in cracking, discoloring, and fading.
Only PTFE (Teflon®) retains its hydrophobic performance indefinitely with virtually no cleaning and never requires coating or painting (see Figure).
Teflon® composites are naturally flexible and their physical properties remain stable over a wide range of temperatures. The material is inert and translucent, unaffected by most chemicals and contaminates. Dust and dirt do not stick to the surface, but typically wash clean in rain.
The key to high performance is Saint-Gobain’s SHEERGARD® family of microwave transmissive PTFE (Teflon®) composites. They combine low dielectric constant and loss tangent with long-term structural integrity, weather resistance, nonflammability, and permanent hydrophobicity. Saint-Gobain’s SHEERGARD® composites incorporate a PTFE (Teflon®) outer coating. The PTFE surface is an integral part of the fabric, and is not a painted coating or a resin gel coat. This surface provides excellent hydrophobic characteristics to assure it will meet transmission performance requirements in all weather conditions. Additionally, the PTFE surface requires no recoating or refurbishment throughout its life.
These high performance PTFE coated composites have been in use worldwide since the early 1970’s. With hundreds of systems installed, from the tropics to the artic, these materials have a proven performance history.