Power levels have been found to be critical to PIM testing: At low power levels, it is relatively easy to achieve good PIM levels for almost any circuit material under test. The results collected for Figure 3 were based on test-signal levels of +43 dBm at approximately 1900 MHz, which is considered a high power level for PIM testing. To illustrate this relationship of material thickness and test power level, the data of Table 3 were collected for three different thicknesses of the same circuit material. Since this is not considered an antenna-grade material and exhibits somewhat high levels of PIM, the dependency of average PIM level on test signal level and material thickness is apparent, with higher test levels resulting in poorer PIM performance.
It can also be seen from Table 3 that the PIM performance degrades as the substrate material grows thinner, with the best PIM performance (-145 dBc) measured with the highest test power (20 W) for the thickest (152-mm-thick) substrate material. Of course, remembering the PIM test tolerance of ±6 dBc, the values shown in Table 3 should be considered as trends rather than as precise values, although even as trends, the connections can be made between measured PIM, material thickness, and test power.
The growth of wireless services and devices appears relentless, especially with projections of billions of wireless devices connected to the Internet by means of Internet of Things (IoT) technology and an explosion of wireless networks and wireless devices to be produced as part of the coming Fifth Generation (5G) of cellular communications. As the number of wireless devices growths, the need to control and minimize PIM also grows and, for PCB antennas, the choice of circuit material can play a large role in keeping PIM in check.
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