A cell site with multiple 5G antennas on top of a red building

Understanding External PIM: Radiated vs. Conducted

Passive intermodulation (PIM) is a major source of noise and signal interference, particularly for 5G networks. PIM occurs when two or more signals at a cell site mix in a non-linear system or device to produce new, undesired signals that, depending on their strength and proximity to the original signal, can cause interference. This interference can reduce wireless coverage and performance — and ultimately degrade the customer experience.

And while PIM can be found in many locations within and outside of the system, external PIM sources can be particularly difficult to find, much less address. Common sources include loose metal-to-metal contact or junctions of unmatched metals, oxidation, rust and more — all of which can be found anywhere outside the system. In addition, external PIM can be further broken down into two more categories: radiated and conducted PIM, both of which are unique and require their own solutions.

External PIM: Conducted vs. Radiated

While the wireless industry often tends to think of PIM as either external or internal, external PIM is a bit more complex than simply being generated by objects outside of the system. The best way to address external PIM depends on the specific electromagnetic circumstances that create the interference. With this in mind, external PIM can be either conducted or radiated:

Line graphic showing how currents are induced in a non-linear junction of a cell site

Conducted: Conducted PIM is caused by induced currents reaching a non-linear junction when flowing in conductive structural elements near the antenna. That is, currents are induced in these structural elements when they are exposed to electromagnetic energy created by the antenna. When those currents flow through a non-linear junction, such as antenna mounting brackets, rusty bolts or hose clamps, they can create PIM currents. These induced currents can then cause radiation of PIM from the conductive structural elements back to the antenna. The efficiency of conducted PIM radiation depends on the details of the structure, but in many cases this mechanism is the source of PIM. This type of issue cannot be addressed by existing solutions that cover PIM sources with conductive shields.

Line drawing showing an antenna radiating a signal that is then radiated back to the antenna by a non-linear object like a bolt

Radiated: Radiated PIM, on the other hand, is generated by a non-linear device directly irradiated by the antenna. In these cases, no induced currents are involved; the antenna itself emits signals that excite nearby non-linear objects, which then radiate signals back to the antenna and cause PIM. For this mechanism, covering the PIM source with conductive shields is effective.

While these differences may seem inconsequential, they present an overlooked variable in PIM reduction — their inciting mechanism. Understanding the difference could help you improve your network and address PIM-related issues in the future.

Why should conducted PIM be addressed?

Regardless of where it comes from, PIM can have a huge impact on network performance, reducing receive sensitivity, data rates, reliability, capacity and more. And let’s face it, PIM is a problem that won’t go away on its own. In fact, as wireless networks densify, additional spectrum is allocated and site colocations continue to increase, it’s very likely PIM will only get worse — unless promptly and properly addressed.

Enter the new 3M™ PIM Reduction Kits

A picture of 3M PIM Reduction Kit 1300, including all three key components: PIM absorber, tape, and label

These new kits from 3M are the first known and currently the only known solution for conducted PIM. Where conventional PIM mitigation solutions shield the PIM source, the 3M™ PIM Reduction Kits absorb PIM-creating currents (not affected by shielding) before they reach a potential non-linear junction. This means users can address potential conducted PIM that otherwise would have been left unabated.

Discover the 3M™ PIM Reduction Kit 1300, which is comprised of two functional layers and an informative label for quick and easy installation:

  1. 3M™ External PIM Absorber 1000: Composite material consisting of a carrier resin, magnetic fillers and an acrylic pressure-sensitive adhesive (PSA). These absorbers can help reduce PIM-inducing currents throughout the structure, not just at one specific source.
  2. Scotch® Linerless Rubber Splicing Tape 130C: Provides environmental protection with the combination of an aggressive rubber resin adhesive and a self-fusing, ethylene propylene backing.
  3. “Do Not Remove” Label: An extra layer meant to remind technicians not to tamper with the product during routine tasks, helping secure the site for the long term.

The all-in-one 3M PIM reduction kit effectively addresses conducted external PIM while being easy to install — without impeding antenna adjustment or replacement. It all adds up to improved performance and robust support for 5G networks, without any downtime during install.

Due to increased spectrum allocations, it’s no longer possible to ignore or avoid conducted external PIM. But by staying up to date and investing in next-generation PIM-mitigation strategies like the 3M PIM reduction kits, you can sustain your signal quality for now — and into the future.

Did you know?
Modern radio access networks (LTE/5GNR) are more vulnerable to PIM effects than legacy systems due to greater bandwidth, increased number of frequency bands, increased data density and other factors. This makes it much more important to stay on top of every possible PIM source, including conducted PIM.