Correct Handling Low PIM Components
Whether connectors, cables or components, low PIM components are precision building blocks of RF networks. Low PIM products require manufacturing processes that meet highest standards, 100 % quality sampling, carefully handling and shipping with sufficient protection. These components have to be treated carefully also in the field to avoid degrading or damage. Since components like duplexers or loads are often hermetically sealed, their internal elements are relative protected, but their connectors are exposed. This is also the case for cables. This chapter is about treating connectors of cables and component ports.
Mechanical damage can be inflicted by a variety of events. Dropping a component is the most obvious mishap, but it can occur already in the factory and during shipment if loose components are allowed to bump into each other. A less obvious cause is improper connector tightening. RF connectors are designed in a way that they can be screwed on almost completely by hand. Wrenches are to be used only for the last half turn. It clearly indicates non-parallel mating of connectors if it is too tough to screw them on by hand. This can be caused by too short or too long cables, which apply sideways pulling or pushing forces to connectors.
If RF connectors do not turn easily during mating, their threads and connecting parts are forced against their counterpart surfaces, causing extreme friction and even deforming. High forces can chip of parts of the plating. These – conducting – chips are alien bodies that interfere with the current flow in the RF path, causing passive intermodulation distortions. Cables’ geometry is paramount for proper functioning. It can be damaged by external force ( denting) or too tight bending radii.
A good practice is to install release cable loops to avoid both, forces at the connectors and too tight cable bents. Such loops cost a bit more material, but the investment goes a long way as the installation tends to be more reliable over time. After connectors are pre-tightened by hand, they have to be mated with a torque wrench to apply exactly the right tightening force. Connections with too little torque result in insufficient contact force, too much torque causes contact areas to deform. Both are consecutively resulting in passive intermodulation distortion.
Alien bodies like dust, dirt, metal chips can very easily find their way into connectors. Base station sites or in-building installations are never dust free, and dust and dirt kernels are difficult to avoid. Keeping protection caps on connectors helps. It is suggested to wipe connectors always with alcohol wipes and dry them with moist free canned air before mating. While connector dust might not be visible to the bare eye, dark areas of used wipes will clearly show that it has been there. Connectors of test equipment, measurement cables and low PIM terminations have to be cleaned frequently with alcohol wipes and dried with moist free air.
Humidity and moisture are creeping enemies of low PIM networks. Over time they cause oxidation. While initial measurements may look good, connectors with accidentally enclosed humidity and moisture will degrade. An often overseen but common source of humidity is human breath. It is tempting to blow into a connector to remove a little dust fluff. Never do it. Exhaled air has a relative humidity of 100 %!
No Touching of RF conductors
Sweat cools the body and skin lubricates itself with oily matter. What is helpful to maintain our health is adverse to proper function of RF connectors. Even minuscule amounts can alter PIM performance of connector contact areas. Low PIM RF conductors are very susceptible to such external influences. There is a good reason why manufacturers of low PIM components require their workers to wear gloves.
Electrical damage is easily overseen but often cause of serious PIM problems. It can happen by applying power levels to a device that exceed its actual power rating. Without question, that has to be avoided. Another occurrence that happens sometimes unintentionally is mating and disconnecting connectors under RF power. If this happens, spark discharges are unavoidable. They cause craters in the material, altering current flow significantly, which again is a source of PIM.