Published on 10/13/2026
A cellular site passes every VSWR sweep, yet the uplink noise floor is 8 dB higher than predicted. Technicians chase the antenna, the radio and the filters. The fault ends up being one loose, slightly oxidized jumper connector that rectifies the two downlink carriers into a spurious signal that lands in the uplink band.
That effect is passive intermodulation (PIM). It is not an active device behaving badly. It is a passive joint or material that behaves slightly non-linearly when carrying high power, and the product falls into a receive channel.
This guide explains the math, the typical specs, the real causes in cables and connectors, and a checklist to avoid it. For related reading on loss and return loss see our article on insertion loss, return loss and VSWR.
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When two or more carriers pass through a perfectly linear passive device, nothing new is created. When a contact, a rusty bolt or a ferromagnetic plating layer introduces even a tiny non-linearity, the device mixes the carriers. The strongest and most troublesome product is the third order:
PIM is expressed in dBc (relative to the carrier) or dBm, measured with two high power tones. The common test condition is two 43 dBm (20 W) carriers, as in IEC 62037.
| Level | Typical value | Meaning |
|---|---|---|
| Ordinary connector or cable | -100 to -130 dBc | Not suitable for cellular receive paths |
| Low-PIM component | -155 dBc | Common spec for macro sites |
| Premium low-PIM | -160 dBc or better | DAS and multi-operator sites |
At 2 x 43 dBm, -155 dBc corresponds to a PIM product of about -112 dBm, which is near the noise floor of a cellular receiver. Each 3 dB of margin matters.
| Cause | Why it creates PIM | Fix |
|---|---|---|
| Loose connector | Micro-arcing at the contact | Torque to spec (see torque specs article) |
| Dirty or oxidized contacts | Thin oxide films act as non-linear junctions | Clean and keep dust caps on |
| Ferromagnetic materials | Nickel and steel have non-linear magnetic response | Use silver, tri-metal or non-magnetic plating |
| Poor solder or crimp | Cold joints and voids | Controlled process and inspection |
| Cracked or damaged cable | Shield or center conductor discontinuity | Replace the cable |
| Corrosion | Water and chemical film at joints | Seal and use compatible metals |
| Rusty hardware near the antenna | Passive rectification in nearby metal | Remove or replace rusty clamps and brackets |
Specifying a low-PIM assembly?
Share the spec and we will confirm what we can build.
Talk to an EngineerPrevention checklist
PIM-critical assemblies, such as DAS jumpers and macro-site feeders, are built with 7/16 DIN, 4.3-10 or N connectors, tri-metal plating and process control. SigmaRF assemblies are VNA-swept for VSWR and insertion loss on every unit. If your project has a certified PIM requirement, tell our engineers early so we can confirm the construction and the test method before quoting.
Cellular macro sitesDASSmall cellsRepeatersPublic safety radioMulti-operator rooftops
What is PIM in RF?
Passive intermodulation is the unwanted mixing of two or more signals in a passive component such as a connector or cable, producing new frequencies that can fall into a receive band.
What causes PIM in coax cable?
Loose or dirty connectors, ferromagnetic plating such as nickel, poor solder or crimp joints, cracked cable, corrosion and nearby rusty metal.
What is a good PIM value?
-155 dBc at 2 x 43 dBm is a common low-PIM specification. Premium components reach -160 dBc or better.
How do you calculate third-order PIM?
Use 2 x f1 minus f2 and 2 x f2 minus f1, where f1 and f2 are the two carrier frequencies.
Does VSWR show PIM problems?
Not directly. VSWR is measured at low power and can look fine while the link produces PIM at working power.
Which connectors are best for low PIM?
7/16 DIN and 4.3-10 for high power, and N type with non-magnetic plating for lower power. Fewer, cleaner, correctly torqued joints matter more than the type.
Can PIM be fixed after installation?
Often yes: find the offending joint with a PIM analyzer, clean or replace the connector, and retest.
Why is nickel plating bad for PIM?
Nickel is ferromagnetic and responds non-linearly to RF current, which creates intermodulation products. Silver and tri-metal platings are preferred.
PIM is a high power, non-linear effect that low-power sweeps miss. Reduce joints, use non-magnetic plating, clean and torque every connector, and test finished links with a PIM analyzer when the specification calls for it.
SigmaRF builds assemblies from our India (Vadodara) volume facility and USA (Cleveland area) precision facility, each VNA-swept for VSWR and insertion loss before shipment. For PIM-critical jobs, talk to us before you order so the construction and test method match your specification.
Have a PIM spec to meet?
Tell us the PIM level, power and connectors.
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