Antennas come in many shapes and sizes, although printed-circuit-board (PCB) antennas provide the capability of packing a great deal of performance into miniature footprints. Of course, many antennas—including those based on PCBs—must be designed and fabricated for minimum passive intermodulation (PIM) levels for maximum effectiveness in today’s crowded signal environments. For PCB antennas, low PIM is a function of the antenna design but also of the choice of RF/microwave circuit material, since circuit materials can contribute a great deal to the overall PIM performance of a PCB antenna.
PIM is a nonlinear, diode-like effect that results in the creation of unwanted harmonic signals when two or more signals combine (such as from different transmitters). These extra signals can cause problems when they are at sufficient energy levels and when they fall within the frequency range of a receiver and can prevent the receiver from detecting its intended in-band signals. While PIM may not impact every application, it can disrupt the operation of wireless communications systems, especially those attempting to recover low-level signals.
PIM can occur at any junction or interface with two different metals, such as connectors and cable assemblies or antennas and antenna feeds. Loose connectors and connectors with internal rust or oxidation can cause PIM. PCB materials can also be sources of PIM, whether from the materials themselves or at feed points. But by understanding how different circuit material parameters relate to PIM, it is possible to select circuit laminates that are less likely to contribute to PIM problems in PCB antennas.