July 28, 2017
Mobile carriers are continually looking for new ways to prevent passive intermodulation (PIM) from degrading network performance. As we have discussed in many posts, PIM sources can be both internal and external. One major cause of PIM is loose connections. To address this issue, a new roofing material technology has been introduced for cell sites that, when used in conjunction with Anritsu test and measurement tools, has proven extremely effective at reducing external PIM. At the site described here, PIM levels were reduced by an average of >40 dB per line in the 700 MHz band.
The PIM Shield™ technology was co-developed by ConcealFab Corp and Johns Manville, a global leader in the roofing industry. PIM Shield was designed to be installed on rooftops to greatly improve wireless network performance by reducing the amount of PIM that affects the system. In the case described in this post, Anritsu’s PIM Hunter™ probe technology played an integral role in achieving the improvements at this site.
PIM Hunting Process
PIM Shield™ roofing technology solves a common issue. Though PIM often comes from loose metal-to-metal contacts, these connections can often be hidden below the existing roof surface. Another factor that makes rooftop sites particularly difficult for locating PIM sources is that the antennas are often recessed from the edge of the building or hidden behind concealment panels for aesthetic reasons.
Because of the many PIM hunting difficulties faced with this site, technicians from Vertical Limit Construction used the PIM Hunter probe and traditional interference hunting techniques to locate the sources of external PIM. This process involved using a PIM analyzer, such as the PIM Master™ MW82119B, to generate two high-power CW test signals injected directly into the system. After traveling through the antenna feed system, these signals radiated the PIM sources in the RF path.
PIM sources beyond the antenna acted as point source radiators, sending intermodulation (IM) product signals in all directions from the point of origin. These signals then travelled back in the direction of the antenna, through the antenna feed system and arrived at the PIM analyzer receiver. Then, the analyzer measured and displayed the intermodulation product frequency (IM3).
At this site, the technicians recorded starting PIM levels and estimated the distance between the PIM source and antenna by using Distance-to-PIM (DTP) technology in the PIM analyzer. With this information, Vertical Limit technicians determined an approximate starting distance for the external PIM hunt. The PIM analyzer uses two CW test signals to excite PIM sources in the system, so the IM3 signal generated is also a CW signal.
At this point, the technicians connected the PIM Hunter probe and an appropriate band-pass filter to the RF IN port of a MS2720T Spectrum Master™ spectrum analyzer and tuned the center frequency to match the measured IM3 frequency Limit lines were set at approximately 10 dB lower than the desired IM3 levels. While hunting for the PIM, the technicians listened for audible alarms when the IM3 level increased above the limit line, indicating they were near a PIM source (figure 1). As it turned out, all the PIM sources were located in front of and directly below the sector antennas under the roofing surface.
Covering the PIM Sources
The next step was placing PIM blankets, available from ConcealFab, over the locations where the PIM sources were found. These blankets reduced the sector’s PIM to about -90 dBm on all ports. Figure 2 shows the improvements made to each port.
Though these PIM blankets are useful aides during test and measurement, they do not make for a suitable long-term mitigation solution. Because of this, PIM Shield™ roofing membrane, available through ConcealFab, was permanently installed using MBR bonding and flashing adhesive from Johns Manville. After this installation, PIM was measured again. The next round of measurements is shown in Figure 3. After applying the PIM Shield™ material, a layer of Johns Manville TopGuard 4000 acrylic elastomeric coating was applied over the membrane for increased solar reflectivity and for aesthetic purposes.
The PIM mitigating material installed at this site was DynaLastic™ 180 FR PIM, which is a multiply Styrene-Butadiene-Styrene (SBS) modified bitumen roof membrane. It has ceramic-coated roofing granules bonded to the top surface with a special PIM mitigating layer between the two SBS modified bitumen layers.
Anritsu’s PIM Hunter technology was crucial for this specific case of site improvement. By using Anritsu’s solutions to locate the PIM sources, Vertical Limit was able to accurately locate PIM sources on the sector and suppress them with PIM Shield™ roofing material to improve network performance.
For more information on PIM Shield™ roofing technology, you can download a success story on the project. To learn more about the measurement process used in this application, read this application note.