Identifying Sources of External PIM

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.

Factory PIM test equipment has been commercially available for over 20 years. RF equipment manufacturers use this equipment to verify that the components they produce are low PIM. Third order intermodulation product (IM3) levels below –107 dBm (–150 dBc) when tested using 2x 43 dBm (20 W) test signals are commonly achieved. Field PIM test equipment was introduced approximately a decade later and is used to ensure that the completed cell site is low PIM. The field test not only verifies the condition of the RF components but also verifies that the installation crew applied proper workmanship techniques while assembling the site. IM3 levels below –100 dBm (–143 dBc) are often specified and achieved.

External PIM is typically caused by loose metalto-metal contacts. 

Some of these PIM sources are relatively easy to identify:

• Air handling equipment on the rooftop
• Overlapping layers of metal flashing
• Sheet metal cable trays
• Sheet metal roof vents
• Rusty metal objects

Other external PIM sources are not so obvious:

• Loose metal-to-metal connections hidden from view by roofing materials
• Loose cable hangers behind the antenna