Rooftop cell sites pose unique challenges for field technicians responsible for maintaining wireless network quality. This is particularly true when it comes to passive intermodulation (PIM), as it causes interference that degrades signals to the point where key performance indicators (KPIs) are not met. Given the multitude of possible PIM causes on roofs, implementing the proper testing process using the appropriate test kit is essential.

The first sign a site may be suffering from PIM is typically the network monitoring software sensing a rise in the uplinks’ noise floor. Once a technician arrives, tests a sector, and finds it contains PIM above the specified pass/fail limit, it must be determined if the PIM is internal or external to the antenna system.

If the strongest intermodulation (IM) source is external to the antenna system, a PIM hunt needs to be conducted. To do so efficiently, a Distance-to-PIM (DTP) plot is needed to note how far the IM source is from the antenna. Because the DTP plot can only estimate the distance of the IM source from the antenna and not the direction, the technician must look for PIM source(s) at that distance, in an arch, from the antenna. Due to the radiating patterns of the antenna, the further the IM source is from the antenna, the larger the search area and the more difficult it is to find.

PIM Hunting Steps

To conduct a successful PIM hunt, technicians will need a PIM hunting kit (figure 1). Before going out on a PIM hunt, a preliminary PIM mitigation should be done by:

1. 1.Picking up any obvious loose items around the antenna system.
   
   

2. 2.Checking to make sure the clamps holding the antennas to their mounting brackets are correctly torqued and not damaged. Often the prime source of PIM can be behind the antenna.
   
   

3. 3.Connecting the PIM test set to the antenna port that the carrier’s network monitoring software has identified as failing minimum uplink noise performance. It must be noted that some carriers require an initial swept PIM measurement to determine the IM condition across the frequency band under test.

Typically, a PIM analyzer, such as the PIM Master™ MW82119B, or a spectrum analyzer can measure the noise floor of the RF environment. If there is too much noise, usually due to uplink user equipment (UE) signals, it can limit the ability to find IM sources at the level of the pass/fail limit. If this happens, F1 and F2 frequency combinations should be selected to avoid interferers.

Rooftop PIM Procedure

Once on the rooftop, the technician needs to conduct a DTP measurement to determine an initial distance from the antenna for PIM hunting. A PIM probe is connected to the analyzer, and the PIM vs. time measurement on the instrument is activated for continuous transmission. A methodical sweep of the area is performed by the technician.

Anritsu’s Spectrum Master™ MS2720T and MS2712/13E spectrum analyzers have a specific PIM hunting mode (figure 3) that shows the results on the instrument display. The handheld analyzers also generate an audio tone whose pitch and volume match the strength of the PIM level detected.

As technicians perform the PIM hunt on the roof, it is important that they keep their body away from the antenna and the area being tested by the PIM probe. Since the human body will attenuate the RF signal by approximately 10 dB coming from the antenna, it lowers the RF power of the F1 and F2 signals hitting the IM source. The 10 dB attenuation can roughly lower the IM power by 25 dB to 30 dB. The IM power will drop another 10 dB by the technician’s body on the return trip to the antenna, for a total of 35 to 40 dB lower reading.

The PIM Search is On!

As the PIM vs. time measurement is transmitting the F1 and F2 frequencies, the technician will set up the spectrum analyzer to measure the IM3 (or IM5, depending on scenario) frequency shown on the PIM instrument screen. Then, the limit line is set to approximately 3 dB to 5 dB lower than measured PIM amplitude (dBm) of the IM product. The technician walks to an area based on how far the DTP measurement determines the location of the IM source.

After the audio volume on the spectrum analyzer is tuned up, the technician patrols the coverage area of the antenna pattern parallel to the PIM probe. As the probe nears an IM source, the RF power (dBm) will rise and the audio tone on the spectrum analyzer will go up in pitch. It is recommended that the technician walk past the IM source to see where the tone starts decreasing and then returns. This reduces the area of the IM source(s). Once the area is narrowed down to a single location, a PIM blanket is placed on top of the IM source. If the source masked by the PIM blanket is a significant PIM source, the interference level will drop.  

It is common for there to be multiple PIM sources on a rooftop, with the same amplitude level as the first IM source. Therefore, technicians should rerun DTP to determine the distance to the next IM source.

If DTP determines that the distance is beyond the edge of the roof, it is recommended to use a golf range finder and directional antenna to identify possible long-range PIM sources. If one is found, it is often caused by a fence, billboard, or similar obstacle that is not easily mitigated. In this case, the potential source should be noted in any closeout report. If the PIM problem is high enough, RF network engineering may choose to change the antenna coverage angle to avoid the PIM source beyond the roofline.