What is in this article?:
Printed-circuit-board (PCB) materials with high dielectric constants can help miniaturize microwave circuits, but engineers using these materials must be aware of the impact of loss and dispersion on their designs.
Circuit materials for microwave printed-circuit-board (PCB) applications have appreciably evolved over the past few decades. Circuit designers can now choose from a wide variety of microwave circuit materials with many different dielectric constant (Dk) values. Historically, circuit materials with high Dk values have only been used in niche applications, but that is changing. High-Dk microwave circuit materials have been used for large-volume power-amplifier applications, and these materials are also drawing interest from antenna designers. High-Dk circuit materials can offer many benefits that designers often overlook.
Just what Dk value is considered high? For microwave applications, circuit materials are commonly available with Dk values from as low as about 2 to as high as just over 10, as typically measured at 10 GHz and through the z-axis of the circuit material. Classification of circuit materials considered as “high-Dk” materials can be somewhat objective, but it typically applies to a circuit material with a Dk value of 6 or higher.
Most circuit designers understand that by switching to a PCB material with higher Dk value, it is possible to miniaturize the dimensions of a microwave circuit for a given frequency. The size of a circuit for a particular wavelength/frequency decreases as the Dk value of the PCB material increases. Circuit materials with high Dk values can affect electromagnetic (EM) wave properties by operating with smaller circuits and shorter wavelengths and by slowing the phase velocities of the EM waves. Electric fields are also more condensed on higher-Dk circuit materials.
These simple attributes of high-Dk materials can bring numerous benefits to microwave circuits, including smaller sizes for those circuits, improved coupling, reduction in higher-order modes, reduced radiation loss, lower impedances for reasonably sized circuits, and slow-wave enhancements. They can also bring drawbacks, including increased problems with dispersion and limited high-impedance values.
The capability to reduce the size of a circuit for a particular wavelength and frequency by using circuit materials with high-Dk values can be appealing to designers for a variety of applications. Many microwave passive components and structures are designed with physical sizes related to a fraction of the wavelength for the operating frequency, such as one-quarter wavelength or one-half wavelength. For example, many antenna designs using microwave PCB technology are patch antennas or arrays of patch antenna elements. Radiating elements for microstrip patch antennas have a length of about one-half wavelength at the frequency of interest. To explore the effects of different PCB Dk values, simple computer models were developed for a patch antenna radiating element at a center frequency of 2.5 GHz. Models were run for 30-mil-thick (0.762-mm-thick) circuit materials with Dk values of 3.0, 6.5, and 10.8, using version 13.54 of the Sonnet Professional EM simulation software from Sonnet Software. Results of the simulations appear in the table.
The simulations show the size reductions for the patch antenna element with increasing Dk value, along with a narrowing of the 50-Ω feed line with increasing circuit material Dk value. The feed line will suffer higher insertion loss with higher Dk value, which may be a concern for some applications. It can be resolved by using a coaxial cable (rather than the PCB) as the feed line to the patch antenna element. Another way around high feed line loss is to use a hybrid multilayer PCB configuration, with different circuit material Dk values for different segments of the circuit.