Comprendre les dangers de l'humidité

So, what if water does get into a coaxial radio frequency (RF) network? Unfortunately, its presence is not always obvious and its impact can be elusive and difficult to manage. Here are some tips to help you trouble-shoot a persistent moisture problem:

Know your risks

Water doesn't often enter your cable or connector in the ways you may expect. A sudden rain shower, or even flooding, doesn't always result in moisture problems. Sure, a gash in the jacket or loose port seals are a welcome mat for moisture, but it usually takes time, as well as the right chemicals, temperature and pressure conditions for water to seep through the very microscopic openings, which are the only entry points to a typical connector and cable. Knowing when and how moisture can penetrate your connectors and cables will allow you to troubleshoot issues more quickly.

Corrosion occurs most quickly in environments with an abundance of moisture, as well as electrolytes or ionic compounds. Salt air in coastal areas certainly is an obvious culprit, but localized pollution, such as exhaust gasses or industrial fumes, may also provide the conditions for accelerated damage.

Cabling in manholes, particularly manholes that share ventilation with sewers or storm drains, are at heightened risk of exposure to all sorts of corrosive chemical compounds, as well as seal- and jacket-melting solvents.

Satellite TV installations are vulnerable to moisture-related corrosion because they not only carry RF signals on the coaxial cable, but they also power the amplifier on the dish through the cable. A high-resistance or intermittent contact anywhere in that circuit results in loss of power to the amplifier. This can often trigger a complete reboot.

The presence of direct current (DC) voltage on the line also speeds up the corrosion process if a steady supply of moisture is available. A copper-clad steel center conductor can completely dissolve in less than a day when submerged in water while voltage is applied.

Corrosion on either conductor can result in harmonic interference of the cable signal with itself, known as common Path Distortion or Passive Intermodulation. This sort of interference can be difficult to identify without a spectrum analyzer, and will often be chalked up to "noise." But, it is unmistakable on the analyzer screen due to its regular and periodic distribution on the frequency axis.

Store your cables and connectors properly

Most coaxial cable has a foam dielectric bonded to the center conductor and first layer of foil, so if water does get in, its only path for migration is to wick along the metal braid. For this reason, best practices should be followed, even when cable is just being stored. If cable ends are not capped and sealed, a leaky warehouse roof or even high humidity can spoil cable before installation.

In addition, if disconnected cable is left exposed, oxidation damage to the braid can go back several inches - or even feet -under the jacket from the free end, and may result in a poor connection. The heavy oxidation on the braid will prevent proper electrical contact with the post of the new connector.

Some types of cable are treated to inhibit corrosion of the braid, or block moisture migration through the braid. If you are going to put a fresh connector on this kind of cable, and it hasn't been stored sealed, special care should be taken so that no moisture remains trapped in the braid. Just because you can't see any corrosion, doesn't mean the water isn't there.

The physical properties of water vary more than your network components

Ill-effects of moisture on coaxial RF systems go beyond the more obvious and direct damage from corrosion. Water vapor trapped within an interface changes the dielectric constant of the air in that space, and can significantly alter the return loss of that interface, resulting in high reflective losses.

What's more, that loss will vary with daily or seasonal temperature changes as the vapor thins out or condenses (or even freezes). The cyclic response of the water to temperature also produces physical force in the form of internal pressure or suction within the cable, which can exacerbate an existing leak by creating a pressure gradient.

For example, hot, sunbaked cable that has been momentarily cooled by a passing rain shower literally sucks in moisture droplets hanging around the interface (if not properly sealed), because the volume of moist air within the cable contracts significantly as it cools. An RG-6 jumper of only two feet can exhibit temporary gradients of 1-2 pounds per square inch (psi) when the temperature swings from day to night.

Understanding these principles will help you to identify and mitigate the dangers of moisture damage to your cables and connectors.