Radome Hydrophobic Surfaces
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When a radome is subjected to rain, a water film forms over the radome surface. For RF signals passing through the film, power absorption and noise temperature increase in direct proportion to rain rate. To reduce these rain effects, a property known as surface hydrophobicity is enhanced. Good surface hydrophobicity effectively creates a dryer radome --- reducing power loss, additive rain noise temperature and G/T degradation. Further, hydrophobicity significantly reduces ice adhesion.
Various water film thickness buildup and flow mechanisms over radome surfaces were first proposed by D. Gibble at the Bell Telephone Laboratories to account for the increase in radome attenuation at the Andover, Main site. Gibble's work was later disputed by C.C. Mei of MIT for leading to transmission loss results which are to excessive. Mei's analysis accounted for the turbulent water flow over the radome. Even Mei's transmission loss theory proved prohibitively high to account for modern hydrophobic surfaces. In contrast, R.H.J. Cary, on the other hand, proposed a water film thickness mechanism that nearly achieves constant film thickness independent of rain rate. Cary's formulation hints at hydrophobic surfaces that appear dryer by shedding water and puddling. Measurements undertaken by Dr. E. Joy of Georgia Tech. further support Cary's theory. He measured the statistical significance of rain puddle diameters, height and frequency per unit area as a function of rain rate and hydrophobicity. For broad rain rate ranges, Dr. Joy’s results suggest that hydrophobic surfaces continue to behave effectively dryer with increasing rain rate----again approaching a equivalent constant water film thickness. His work clearly identifies the degradation of hydrophobic surfaces as the root cause of increasing rain absorption loss. In AFC's opinion both Cary's and Mei's results bear truth. A radome with a perfect hydrophobic surface, having minimal added rain attenuation calculated by Cary, deteriorates rapidly to Mei's results when the hydrophobic surface degenerates by dirt, abrasion, ultraviolet light and/or corrosive chemicals in our atmosphere.
| Additive Rain Transmission Loss | Additive Rain Noise Temperature |
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Surface chemistry is a unique and complex study. AFC does not profess to be an authority on this subject. However, since our introduction into the radome business, extensive research has been pursued towards a hydrophobic surface. After working with several companies who work with low critical surface energy materials, including Dow Corning and DuPont, we have reached the following conclusions: The only commonly used material capable of achieving 20 ergs/cm2 free surface energy is pure Polytetraflouraethylene (TEFLON ®). However, fluorinated products are very hazardous for use in an industrial environment (ultimately increasing the price of the final product). Products in the Teflon family are typically modified to enhance some specific property, such as bonding ability. Modified Teflon is likely to exhibit a surface energy of 21-23 ergs/cm2, or more. Fumed silicon dioxide coatings exhibit a low surface energy. AFC has seen one such product with a claimed contact angle of 140 degrees. They do, in fact, produce an extremely hydrophobic surface. To date, however the above family of coatings, Vellox for example, is fragile. They are easily rubbed off, and susceptible to some airborne, industrial pollutants. Due to ease of removal, surface products would most likely have to be applied after radome assembly, and reapplied every year.
Members of the silicon family typically achieve free surface energy in the low 20's, 22-23 ergs/cm2. Many silicones remain rubbery, giving a surface which retain some contaminants and subsequently naturally washed away by rain.
The industry "standard" radome coatings, gelcoat, paint and TEDLAR ®, exhibit surface energies in the high 20's when new. They rapidly degrade to a totally wettable surface. Within 2 years after exposure, water removal is accomplished by sheeting. This is the least desirable method. Such materials, without modification, are obviously unacceptable for long term hydrophobicity.
AFC has worked extensively with the manufacturer of a proprietary modified silicon coating. It is most commonly described as a silicon polymer, in that it cross-links with the substrate to achieve a chemical bond. It works well with gelcoat and Tedlar, leaving a hard, water repellent clear surface. Radome surface color is unchanged. The product is abrasion resistant, and inert to all common solvents. The product has a measured water droplet contact angle of 111 to 113 degrees, and a free surface energy of 22-23 ergs/cm2. Weathering data available suggests that the product will last until a sloughing of the substrate occurs. Should this happen, it is easily reapplied in the field.
Considering performance and cost parameters, the proprietary modified silicone coating on gelcoat or Tedlar is the superior choice. It is a tough surface and water removal is accomplished by water droplets and narrow rivulets. The coating should last for years until natural etching occurs from bombardment by our dirty atmosphere. Should radome color be important, the transparency of the modified silicon coating does not change the color of the Tedlar or gelcoat.
AFC manufactures, markets and sells worldwide satellite dish antennas, conical horn antennas, radomes, antenna feeds, microwave and waveguide components, ultra low loss waveguide transmission line TallGuide ®, and shelters. Our customers serve the broadcast, communications, radar, weather and cable industry, defense, government, and government agencies worldwide.
A complete Internet WWW AFC document index may be found in Antennas for Communications (AFC) Home Page Document Summary List. Additional radome information is contained in AFC's Dilectric Radome Data Sheet and AFC's Radome Capability Brochure.
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