J
Member
Chasing PIM issues at a macro site. External sources (rusty fences!) vs internal. What's your systematic approach to PIM hunting?
Passive intermodulation (PIM) - hunting sources in the field
L
Admin
Good question — rusty fences are a classic. A systematic approach that works for many teams:
1. Confirm it is PIM: PIM shows up as a rise in uplink noise floor that correlates with downlink traffic or transmit power. A worked example: with PCS downlink carriers at 1930 and 1990 MHz, the third-order product 2·1930 − 1990 = 1870 MHz lands inside the PCS uplink (1850–1910 MHz).
2. Test the line system first: a PIM analyser with two 43 dBm (20 W) tones (the IEC 62037 setup) and typical acceptance thresholds around −150 to −153 dBc. Tap connectors lightly while measuring (dynamic PIM test) to find loose or contaminated joints.
3. Separate internal vs external: replace the antenna with a low-PIM load. If PIM disappears, the source is the antenna or beyond. Distance-to-PIM (DTP) tools on modern analysers estimate the distance to the source, which helps to distinguish "in the jumper" from "on the roof".
4. External sources: rusty bolts, fences, metal flashing, loose roof hardware within the main beam. A directional PIM hunter (handheld antenna plus receiver) helps locate them.
Question: is your PIM issue present on all sectors or only one, and does it vary with weather?
— rfforum 编辑部 / Editorial Team
1. Confirm it is PIM: PIM shows up as a rise in uplink noise floor that correlates with downlink traffic or transmit power. A worked example: with PCS downlink carriers at 1930 and 1990 MHz, the third-order product 2·1930 − 1990 = 1870 MHz lands inside the PCS uplink (1850–1910 MHz).
2. Test the line system first: a PIM analyser with two 43 dBm (20 W) tones (the IEC 62037 setup) and typical acceptance thresholds around −150 to −153 dBc. Tap connectors lightly while measuring (dynamic PIM test) to find loose or contaminated joints.
3. Separate internal vs external: replace the antenna with a low-PIM load. If PIM disappears, the source is the antenna or beyond. Distance-to-PIM (DTP) tools on modern analysers estimate the distance to the source, which helps to distinguish "in the jumper" from "on the roof".
4. External sources: rusty bolts, fences, metal flashing, loose roof hardware within the main beam. A directional PIM hunter (handheld antenna plus receiver) helps locate them.
Question: is your PIM issue present on all sectors or only one, and does it vary with weather?
— rfforum 编辑部 / Editorial Team