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Passive Intermodulation (PIM) Solutions

Passive intermodulation (PIM) is a well-known problem in cellular systems. Downlink signals at the cell site mix at passive, non-linear junctions in the RF path, creating new signals. If these new signals (intermodulation products) fall in an operator’s uplink band, they can elevate the noise floor and degrade system performance.

PIM Shield™ is a family of products developed by ConcealFab that reduce external PIM at cell sites. These products eliminate PIM by doing one or more of the following:

COMMON SOURCES OF EXTERNAL PIM
Using ConcealFab’s test and measurement solutions many sources of external passive intermodulation (PIM) have been identified. These PIM sources fall both in the main beam in front of the base station antennas as well as in the reactive near field region below, above and behind antennas.

Metal objects in front of antennas:
Cable trays, air handling units, sky lights and vent pipes are just a few of the metal objects commonly found on rooftops in front of base station antennas. Many of these items are made from sheet metal with loosely touching metal-to-metal surfaces.

Roofing materials / construction:
Roofs are engineered to prevent water ingress and provide thermal insulation for buildings. They are not designed with PIM in mind. Some of the common problems found include overlapping steel deck members, plates and screws used to secure insulation and over-lapping sheet metal used for flashing. Many of these issues are below the roof membrane and not visible just looking at the roof.

Galvanic corrosion:
Stainless steel and galvanized steel are at opposite ends of the galvanic series and small pockets of corrosion will form where these materials touch. These corrosion pockets do not present a structural concern but they do generate significant PIM when close to base station antennas.

Metal Snap-in hangers:
Metal snap-in style cable hangers are very convenient given that they require no tools for installation. Unfortunately, the loosely touching metal-to-metal interfaces between hangers are prime sources of PIM, as are the galvanic corrosion pockets created when hangers are attached to galvanized steel support brackets.

Hose clamps:
Stainless steel hose clamps are deployed by the thousands in wireless infrastructure to secure cable support brackets. Unfortunately, the galvanic mismatch between the stainless-steel banding and galvanized steel pipe generates PIM. Hose clamps also have a “tail” that can lightly touch other steel members and cause PIM.

Metal strut fasteners:
Metal strut is used at cell sites to support a wide variety objects including RF cables, radios and fiber distribution boxes. While the strut itself is generally low PIM, the interfaces between the strut and just about every other metal object is not low PIM. Nut retaining springs, snap-in stainless-steel fasteners, bolted connects between struts and connections between the strut and round members at sites can all be sources of PIM.

Angle adapters:
Galvanized steel angle is commonly used as a structural member on both rooftop sites and tower sites. Angle adapters are deployed along the length of these angles to secure RF, power, RET and fiber cables. PIM is generated by the galvanic mismatch between stainless-steel angle adapters and galvanized steel angle. PIM also occurs where the stainless-steel set screw digs into the galvanized steel surface.

PIM SHIELD PRODUCTS FROM CONCEALFAB
PIM Shield™ is a family of products developed by ConcealFab that reduce external PIM at cell sites. These products eliminate PIM by doing one or more of the following:

Eliminate Galvanic Mismatches at Junctions
Galvanic mismatches can be eliminated by making sure the same metal & finish is used on both sides of a junction.

Insure High Contact Pressure at Junctions
It is perfectly okay for metal to touch metal as long as the junction between parts is designed to maintain high contact pressure. Large contacting surfaces should be avoided.

Insulate Junctions
If you can’t guarantee high contact pressure between metal parts or can’t prevent a galvanic mismatch, insulate the parts to prevent electrical contact.

Block RF from Reaching the Junction
If you can’t fix the metal-to-metal junction using one of the above methods, install an RF barrier material to prevent RF energy from reaching the non-linear junction.