Evaluate circuit material effects on PCB antenna PIM

October 17, 2016 //By John Coonrod, Rogers Corp.
Antennas are key components in modern communications systems, and printed-circuit-board (PCB) antennas are attractive for their capabilities of providing strong performance in small footprints. As PCB antennas are used over wider frequency ranges and in communications devices ranging from base stations to handsets, circuit designers are faced with understanding how different PCB material characteristics relate to antenna performance.

In particular, as the number of wireless applications, wireless signals, and antennas grows, passive intermodulation (PIM) becomes an important performance parameter for PCB antennas, requiring an understanding of how different circuit material attributes can affect printed antenna PIM performance. PIM is a form of interference and is a circuit or system issue, and it is cautious to be aware that circuit materials can contribute to PIM issues. An educated choice of circuit material can pave the way for fabricating wireless PCB antennas with low levels of PIM that also meet their other antenna performance goals.

Many of the circuit material parameters important for active or passive circuits used at RF/microwave frequencies, such as dielectric constant (Dk), dissipation factor (Df), thermal conductivity, coefficient of thermal expansion (CTE), and thermal coefficient of dielectric constant (TCDk), can also provide insights into how well a circuit material will perform when used for a PCB antenna. Understanding what each parameter means in terms of antenna performance, along with how a circuit material can affect the PIM performance of a PCB antenna, can help guide the selection of antenna-grade circuit materials that will deliver predictable, dependable antenna performance.

Any choice of antenna-grade circuit material will start with Dk, with the value for a given circuit material an indication of the size of the circuit dimensions required for a particular wavelength and frequency. In general, materials with higher Dk values will support smaller antenna structures for a given frequency, although materials with lower Dk values typically support stronger and more consistent radiation patterns. The Dk consistency across a PCB material is one factor in achieving consistent and repeatable radiation patterns with a given printed antenna design, as is TCDk one barometer of how consistent PCB antenna performance will be with changes in temperature.

A circuit material’s TCDk parameter is typically referenced to room temperature (+25°C) and by how much the Dk changes at other temperatures. In terms of antenna performance, since a PCB antenna is designed to resonate within a particular frequency range, and the dimensions of its resonant structure are based on Dk, even moderate changes in Dk with temperature can result in variations in antenna frequency and radiation pattern.

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