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The Importance of Hydrophobic Coating

The effect of rain on radome surfaces with regards to antenna system performance has been well documented for many years. Water has a high dielectric constant and a very high loss tangent for microwave and millimeter wave frequencies. As a result, even thin films of water on the surface of antennas, radomes or feed waveguides can cause large attenuation of transmitted or received signals.

Although a radome provides a rain free environment and eliminates losses due to water deposited on antenna surfaces, the radome itself can allow water to collect and increase systems losses if the radome surface itself is not hydrophobic.

Hydrophobic literally means, “to repel or fail to mix with water” and in this context, it means that the water on the radome surface is repelled from the surface in the from of beads rather than in sheets. Without a hydrophobic surface, water forms a film on the surface, which will absorb much of the radiated signal, leading to high losses. If the water is repelled from the surface in beads, the radiated energy will be slightly diffracted because the water droplets have dimensions much less than a wavelength at these frequencies.

To understand the importance of hydrophobic coating, consider the graph below which compares a radome coated with ESSCOLAM 10™, a proprietary hydrophobic coating developed by ESSCO, designed especially for radomes, compared to a dry radome and a non-hydrophobic coated radome. As can be seen from the graph, as the frequency of operation increases, the effect of the rain becomes more significant on a non-hydrophobic radome as compared to the significant improvement in performance gained by the use of ESSCOLAM 10 hydrophobic coating.

For example, at Ku-Band, the transmission loss for a dry radome is about .7 dB while the transmission loss in 10 mm rain per hour increases to about 5 dB for a non-hydrophobic surface versus an increase to only about 1.5 dB for a radome treated with ESSCOLAM 10™.



MEASURING HYDROPHOBICITY

The standard measure of hydrophobicity for a surface is the contact angle between that surface and a water droplet sitting on it as determined by a contact goniometer. The angle is measured at the base of the water droplet as illustrated below.



A surface is generally considered to be hydrophobic if the contact angle is greater than 90°. By this measure, pure Teflon®, with a contact angle of 107° is hydrophobic. ESSCOLAM 10, the hydrophobic coating developed by ESSCO exclusively for its radomes, has superior hydrophobicity with a measured contact angle of 120° and is considered to be super hydrophobic. This superior hydrophobicity gives ESSCO radomes a quantitative edge in performance in the rain. The simplest and most direct measure of this difference is the comparison of radome transmission loss as seen in the figure above. Radome transmission loss versus frequency is plotted for a dry radome and for the same radome, but with different surface treatments, in rain falling at a rate of 10 mm / hour. Transmission loss for the non-hydrophobic surface (60° contact angle) is significantly worse than for the ESSCOLAM 10 (120° contact angle) surface. The performance for surfaces of intermediate hydrophobicity will fall between these bounds.

The ESSCOLAM 10 hydrophobic coating system has been employed in actual field sites since 1985. It has a rated life of 20 years, unlike most other hydrophobic coatings that last only months before expensive recoating is necessary.



This photo shows water beads on an ESSCOLAM 10™ surface.

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