![]() You can picture the spaces for “stuff” to get into. In effect, the layers of thin Teflon® tape wound around the center conductor resemble a roll of corrugated cardboard. It has to do with the porosity of the dielectric. Their lower loss, smaller diameter, and lighter weight are important elements in selecting cables for airborne applications, but there is a price to pay. Over time, it will equalize again, but there is no way of knowing what “over time” might be.Īt the risk of downplaying the advantages of the newer, softer, more porous dielectric materials, it appears these, too, may be implicated in this. All this happens differentially conditions inside the connector will not change as quickly as those around it. During descent, though the warming of the skin tends to return the moisture to the air, some of it can be drawn into the connector as the pressure returns to higher levels. Such temperature changes can precipitate moisture in the air, and a drop in cabin pressure can create a partial vacuum in and around the connector. Points of entry for moisture shown for a conventional unsealed TNC connector Pressure may change from about 30 inches of mercury at sea level to about 7 inches (a 75% drop) at 35,000 feet cabin pressurization, however, can reduce the effect to about a 15% drop.įigure 1. (See Figure 1.) In commercial air transport jet aircraft (which account for most TCAS II installations), air temperatures at the skin can cycle from as much as, say, +120☏ on a Phoenix ramp to -60☏ at 35,000 feet. Moisture can become trapped inside the antenna connector as a result of repeated exposure to significant swings in temperature and atmospheric pressure in the presence of significant relative humidity. As a result, a sensitive system may become unreliable or, worse, fail.īefore making a blanket indictment of the connectors, cables, cable assembly techniques and/or installation practices, let’s examine the factors that can create this problem. Where there is a change in impedance, there is an increase in signal reflection (VSWR). However, even if corrosion were not to develop, moisture present in the connector or dielectric can change the characteristic impedance of the cable. The shields and conductor can suffer corrosion, especially if cleaning is less than meticulous when the connector is applied. But be assured the cable assembly will be affected if moisture enters. Moisture trapped inside a connector can produce effects ranging from unnoticed to serious, depending on the demands of the system. The cause of these problems has been traced to moisture either in the coaxial cable at the antenna connector or inside the connector itself. ![]() There have been reports of TCAS II installations showing signs of RF cable deterioration after only a few years of operation.
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