Passive intermodulation (PIM) is a well-known problem in cellular systems. Downlink signals at the cell site mix at passive, non-linear junctions in the RF path, creating new signals. If these new signals (intermodulation products) fall in an operator’s uplink band, they can elevate the noise floor and degrade system performance.
RF equipment manufacturers have spent decades developing methods to improve the PIM performance of the RF components deployed at cell sites. In addition, the crews installing these components have fine-tuned their skills to the point that PIM levels below -150 dBc (with 20W test tones) is now readily achievable within antenna feed systems. Unfortunately, a low PIM feed system by itself doesn’t insure that a site will perform to expectations. PIM sources beyond the antenna are now often the limiting factor for site PIM performance.
ConcealFab is a leader in the wireless industry in developing solutions to mitigate external passive intermodulation. ConcealFab began by developing its proprietary PIM Kote™ finish to reduce PIM at metal-to-metal contacts in the antenna mounting system and later developed our PIM Shield™ line to suppress radiation of near field PIM sources in small cell installations. ConcealFab is expanding the PIM Shield™ family of products to rooftop as well as tower top applications by developing a portfolio of temporary and permanent solutions for mitigating external PIM at cell sites.
The test process:
The process begins with a traditional field PIM test. A PIM analyzer such as the Anritsu MW82119B PIM Master ™ is used to generate two high power CW test signals that are injected into the system under test. These signals travel through the antenna feed system and radiate any PIM sources in the RF path. PIM sources beyond the antenna act like point source radiators, sending IM product signals in all directions from the point of origin. These signals travel back in the direction of the antenna, through the antenna feed system and arrive at the PIM analyzer receiver. The magnitude of the third order intermodulation product frequency (IM3) is measured by the PIM analyzer and displayed.
If the PIM level is higher than desired, record this value, then estimate how far away the offending PIM source is from the antenna aperture. This can be determined by using the DTP / DTP overlay function of the PIM Master as described in application note 11410-00869. This function compares the distance between a PIM source placed on the antenna aperture to the actual PIM source(s) present at the site and reports the relative distance. Record this value. Armed with information you have an approximate starting distance to begin your external PIM hunt.
Since the PIM analyzer uses two CW test signals to excite PIM sources in the system, the IM3 signal generated is also a CW signal. Connect the Anritsu PIM Hunter probe and an appropriate band-pass filter to the RF IN port of the spectrum analyzer and tune the spectrum analyzer center frequency to match the same IM3 frequency measured by the PIM analyzer. Set a Limit Line at approximately 10 dB lower than the IM3 level you are looking for and turn ON the Limit Alarm. Start the PIM analyzer to generate the high power test tones and begin hunting for external PIM sources. When the IM3 value measured on the spectrum analyzer increases above the limit line, the audible alarm sounds, meaning you are near a PIM source.