PIM is the undesire generation of interfering signals caused by nonlinearities in the mechanical components of a wireless system. Two signals mix together (amplitude modulation) to produce sum and difference signals and products within the same band, causing interference. PIM is a problem in almost any wireless system but is most noticeable in cellular base station antennas, and related components.

 

The interaction of mechanical components generally causes the nonlinear elements, especially anywhere that two different metals come together. Junctions of dissimilar materials are a prime cause. PIM occurs in antenna elements, coax connectors, coax cable, and grounds. It is caused by rust, corrosion, loose connections, dirt, oxidation, and any contamination of these factors. Even nearby metal objects such as guy wires and anchors, roof flashings, and pipes can cause PIM. The result is a diode-like nonlinearity that makes an excellent mixer. As nonlinearity increases, so does the amplitude of the PIM signals. PIM tends to increase as components age. Older systems are particularly susceptible. Environments where there are wide temperature variations, salt air or polluted air, or excessive vibrations exacerbate PIM.

 

Consider the two cellular frequencies of f1 = 869 MHz and f2 = 894 MHz. Mixing these two signals together produces 894 – 869 = 25 MHz and 894 + 869 = 1763 MHz. These two signals are way out of the cellular bands of interest, so typically they won’t cause PIM interference. However, the signals that mix with the second harmonic and higher harmonics are the problem. For instance, the second harmonics mixed with the base frequencies produce what we call third-order intermodulation products 2f1 – f2 and 2f2 – f1. These would be 844 MHz and 919 MHz, respectively, both in the same cellular band. These third-order PIM products are known as IM3 or IP3. Other harmful signals are the smaller fifth-order signals 3f1 – 2f2 and 3f2 – 2f1 called IM5. The signals to worry about are mf1 ± nf2 where m and n are the harmonic numbers. The figure below shows the spectrum generated by PIM.