External Passive Intermodulation (PIM)

Passive intermodulation (PIM) is a problem for network operators that is not going away anytime soon.  In fact, it’s getting worse. To review, PIM is interference created when the transmit signals at a site mix at “non-linear” objects in the RF path and generate new frequencies.  If these new frequencies fall in an operator’s uplink they can raise the noise floor and degrade system performance.

With the deployment of new 600 MHz and 700 MHz “digital dividend” frequency bands, “external” PIM has become a much more serious problem.  At locations where these new frequencies can combine with existing 700 MHz and 800 MHz frequencies, 3^rd order intermodulation products (IM3) can be generated that fall in one or more operator’s uplinks.  These IM3 signals are thousands of times higher in magnitude than the 7^th and 9^th order products operators were experiencing before the new frequencies were added.  Since the new bands are deployed using separate antennas and separate radios, the most common location where the signals can mix is beyond the antenna.

Loosely touching metal surfaces in the main beam in front of site antennas are a common source of external PIM. As cell sites get closer to users to improve data rates, the number of potential PIM sources found in front of antennas increases. Powerlines, billboards and metal flashing on buildings are only a few of the PIM producing objects operators are having to deal with.

In addition to areas in front of the antenna, metal objects above, below and behind antennas have also been found to generate PIM. This has come to be known as the “High-Risk” PIM zone. Even though this region is well outside the antenna’s main beam, enough RF energy is present to excite any non-linear objects present. The most common PIM sources found in this region are the metal brackets and hardware used to support cables running between radios and antennas. Remote Electrical Tilt (RET), fiber, power and RF cables all need to be mechanically supported periodically along their length to prevent movement in the wind. The antenna mounting pipe and other steel support structures near the antenna provide convenient locations to secure these cables.

Traditionally, all sorts of stainless-steel brackets have been used to secure the growing number of cables at cell sites.  And why not?  They are strong, won’t rust and are inexpensive to produce. These brackets are almost always secured using stainless-steel “round member adapters” (a fancy name for hose clamps.) Snap-in style cable hangers then grab the cable and “snap” into the support bracket allowing cables to be secured and re-positioned easily without having to use any tools.

Unfortunately, there are many potential PIM problems with these traditional cable supports.  First, if hose clamps are not tight, PIM will be generated at all loosely touching metal surfaces. Second, even if tight, PIM can be generated at the free end of the hose clamp where the “tail” can lightly touch itself and other metal objects. Third, stainless-steel is at the opposite end of the galvanic series than galvanized steel. Where the two metals touch a “battery” effect is created and small pockets of white, powdery galvanic corrosion forms. This type of corrosion is self-limiting, so it does not create structural problems.  However, th corrosion products are highly non-linear and able to generate extremely high magnitude PIM when exposed to even low levels of RF energy.

ConcealFab has been developing new cable mounting solutions to address this problem since 2016. The methods used to correct these problems are well known by RF equipment manufacturers and have been deployed for decades in antennas, filters and RF connectors.  Keys to achieving low PIM mounting solutions are:

• ELIMINATE GALVANIC MISMATCHES AT JUNCTIONS – Galvanic mismatches can be eliminated by making sure the same metal & finish is used on both sides of a junction
• ENSURE HIGH CONTACT PRESSURE AT JUNCTIONS – It is perfectly okay for metal to touch metal if the junction between parts is designed to maintain high contact pressure. Large contacting surfaces should be avoided.
• INSULATE JUNCTIONS – If you can’t guarantee high contact pressure between metal parts or can’t prevent a galvanic mismatch, insulate the parts to prevent electrical contact.

ConcealFab’s line of low PIM cable support solutions are branded PIM Shield® and can be found in the Interference Mitigation products catalog.  In future posts we will address specific mounting challenges and point out the appropriate PIM Shield® solution for that problem.