Different circuit materials behave differently with changes in temperature. To demonstrate, Figure 1 plots Dk as a function of temperature for a number of circuit materials commonly used for high-frequency circuits, including PCB antennas. The circuit materials include polytetrafluoroethylene (PTFE) circuit materials with woven glass (WG) and ceramic fillers, low-cost FR-4, and three high-frequency circuit materials from Rogers Corp. (www.rogerscorp.com): RO3003™, RO4003C™, and RO4835™ circuit materials.
The Dk values of these materials are low, typically from about 2.1 through the thickness (z axis) of the material for PTFE-based circuit materials to about 4.8 for FR-4 circuit materials. As the plots of Dk with temperature show, the Dk values can change a great deal with temperature, resulting in circuit and antenna performance that will also change with temperature. For the RO3003, RO4003, and RO4835 materials, with Dk values ranging from 3.00 to 3.48 and held to a tolerance of ±0.05 or better across the circuit board, the changes with temperature are minimal, indicating that the impedances of circuit transmission lines and antenna traces will be consistent with temperature for consistent antenna performance with temperature.
Although circuit materials are readily available with higher values of Dk, circuit materials for PCB antennas generally exhibit a Dk of 3.5 or less through the material thickness when measured at 10 GHz. The use of a circuit material with lower Dk value allows for the use of wider conductors for a design frequency and for efficient radiation. One design goal for any PCB antenna is to minimize loss exhibited by the antenna circuitry as well as any loss when transferring high-frequency energy from the feedlines to a PCB antenna’s radiating elements. The use of circuit materials with Dk of about 3.5 or less promotes the design of efficient conductors. By selecting materials with low Df and smooth copper conductor surfaces for a PCB antenna, insertion losses can be minimized.