Passive intermodulation (PIM) is a form of interference created when downlink signals at a cell site mix at “non-linear” objects in the RF path. Any loose metal-to-metal contact or dissimilar metal contact in front of or in the reactive near field behind an antenna can generate PIM.

Passive Intermodulation (PIM) sources located beyond an antenna, referred to as “external” PIM sources, can be a challenge to locate. PIM analysers typically include a Distance-to-PIM (DTP) function that is very useful in aiding PIM hunts. Unfortunately, these measurements only identify how far the PIM source is from the antenna with no indication of direction. When the distance indicated by a PIM analyser is less than 15 FT (4.5m), experience has shown that the PIM source can be located directly behind or beside the antenna. As a result, guesswork is required to determine where to begin the PIM hunt.

The Absorber Kit System is an additional tool that can be used during a PIM hunt to help determine PIM source direction when using a PIM analyzer or when using a CPRI analyzer to measure progress. If the PIM magnitude or uplink noise level reduces when the Front Absorber Kit (901124) is installed on the front of an antenna, the PIM source is likely in front of the antenna. If the PIM magnitude or uplink noise level does not reduce (or increases) when the Front Absorber Kit is installed on the front of the antenna, the PIM source is likely behind or beside the site antenna.

The Rear Absorber Kits and Side PIM Blanket complete the Absorber Kit System and can be used with the Front Absorber Kit to refine PIM source direction.

In addition to PIM Hunting, the Absorber Kit System can be used while site testing to help isolate the antenna and its feed system from the environment. If the PIM magnitude or uplink noise level decreases when the complete Absorber Kit System is installed on the antenna, it is a good indication that the largest noise source is beyond the antenna. If the PIM magnitude or uplink noise level does not change or increases when the complete Absorber Kit System is installed, it is a good indication that the largest noise source is inside the antenna or its feed system.

PIM blankets and absorption kits were developed to provide mobile operators a method to temporarily suppress PIM on rooftop cell sites. PIM blankets work by reflecting RF energy away from the covered area.

If the energy from the antenna is not able to reach a PIM source under the blanket, no PIM is produced by that source. With the PIM sources suppressed at a site, operators can determine the level of improvement that could be achieved with a permanent repair to the covered area.