Passive intermodulation near-field measurements on microstrip lines

A passive intermodulation (PIM) near-field measurement equipment has been earlier designed in order to localise PIM sources in antennas and open transmission lines. The sensitivity of the scanner is -110 dBm with P_Tx = 2 x 43 dBm and it works in the GSM 900 frequency band. The antenna structure is scanned with a near-field probe and the results will reveal whether there are any PIM sources within the scanning area. However, the interpretation of the results is not always obvious. This paper introduces circuit model based equations which explain how the PIM electric and magnetic fields behave along a microstrip line. The differences in the reverse and forward traveling PIM waves caused by the asymmetry of the loading impedances are considered. Theory is compared with measurements including one and two PIM sources.