A test chamber for assessing conducted PIM

Conducted PIM: What it is, why it occurs and what you can do

Conducted PIM can negatively affect your wireless network performance. Learn about its key characteristics versus more conventional forms of PIM and how you can quickly and easily mitigate it with new 3M material solutions.

Passive intermodulation (PIM) is a growing problem for network operators. And while the wireless industry often tends to think of PIM as either external or internal, external PIM is more complex than simply being generated by objects outside of the system. Depending on how it is generated, external PIM can be radiated or conducted.

Radiated PIM involves energy being delivered to the PIM source and back to the antenna over the air. A well-known issue in the wireless industry, conventional conductive shields are an effective solution for this type of external PIM.

Conducted PIM, on the other hand, involves energy being delivered to the PIM source through induced currents in conductive hardware, such as an antenna mast or mounting brackets. The PIM source in this case is typically located in or close to the reactive near field and can even be in direct electrical contact with the antenna.1

When an antenna radiates an EM wave and that wave encounters a metallic support structure (e.g., antenna mast), the EM wave induces an electrical current that flows along the long axis of the mast. Eventually, these currents may reach a PIM source, which can result in PIM being generated and radiated from the structure, negatively affecting signal integrity. This phenomenon is known as conducted PIM.

The science behind induced currents

When an antenna is installed on a typical rooftop or tower and radiates an electromagnetic (EM) wave, the wave will encounter various structures, components and materials that have certain electrical and magnetic properties. These structures may consist of linear and/or non-linear media. When the non-linear media is in the presence of an EM wave, harmonics and passive intermodulation can be generated. This may occur through several mechanisms including galvanic corrosion, surface roughness, the Schottky Effect and others. Thanks to advanced modeling capabilities, you can see a typical example of induced currents traveling along an antenna mast, eventually generating PIM:

A boxplot showing PIM reduction for 3M PIM Absorbers across two different receiver branches.

The figure above illustrates the difference in PIM distribution with or without a 3M absorber applied. With any electrical current flow, there is an associated magnetic field. If a material with high permeability is placed within this magnetic field, the current flow will be reduced since the material will present a high inductive impedance.

When conventional solutions don’t work, 3M can help

In contrast to radiated PIM, conducted PIM cannot be addressed by existing solutions that cover PIM sources with conductive shields. A PIM source may be shielded from the EM wave, but the wave can propagate over the conductive shield and couple into other parts of the structure and induce PIM from other sources. To address this, it would require network operators to shield all structural elements – a monumental task.

3M scientists understood this dilemma and discovered that absorbers were an effective way to help mitigate conducted PIM and, as a result, developed the 3M™ PIM Reduction Kit.

Most of the energy absorbed by this material will be converted to a negligible amount of heat and not reflected. If the magnitude of the current is reduced, then the resulting PIM will also be reduced and the amount of reduction is approximately proportional to the order of the PIM product.

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

3M PIM Reduction Kit 1300

It’s no secret that PIM is – and will continue to be – an issue that needs to be addressed. What’s most important, though, is understanding the nature of the specific PIM you’re dealing with and the most optimal solution. At 3M, we are ready to work with you to understand your specific site configuration and external conducted PIM issues.


1 Antenna Near Field & Far Field Distance Calculator (everythingrf.com)