How to choose PCB materials for high-frequency device power dividers and couplers

Many different circuits are used to design power dividers (which are combiners in reverse) and couplers, which come in a variety of different forms. Power dividers can be simple two-way power dividers or complex N-way power dividers, depending on the actual needs of the system. Many different directional couplers and other types of couplers have also been developed in recent years, including Wilkinson and resistive power dividers as well as Lange couplers and orthogonal hybrid bridges, which come in many different forms and sizes. Choosing the right PCB material in these circuit designs can help them achieve the best performance.

These different circuit types all compromise the design's structure and performance, helping designers select boards for different applications. The Wilkinson two-way power divider, which uses a single input signal to provide two output signals of equal amplitude and phase, is actually a "lossless" circuit designed to provide a pair of output signals that are 3dB (or half of the original signal) smaller than the original signal (the output power of each port of the power divider decreases as the number of output ports increases). In comparison, the resistive two-way power divider provides a pair of output signals that are 6dB smaller than the original signal. The added impedance in each branch of the resistive power divider increases the loss, but also increases the isolation between the two signals.

As with many circuit designs, dielectric constant (Dk) is often the starting point for selecting different PCB materials, and designers of power dividers/combiners generally prefer circuit materials with high dielectric constant (Dk) because these materials can provide effective electromagnetic coupling on smaller circuits than low dielectric constant materials. One problem with high dielectric constant circuits is that the dielectric constant in the circuit board is anisotropic, or the dielectric constant value of the circuit board material is different in the x, y, and z directions. It is also difficult to obtain a transmission line with uniform impedance when the dielectric constant varies greatly in the same direction.

Maintaining impedance invariance is very important when implementing power splitters/combiners. Changes in the dielectric constant (impedance) can lead to uneven distribution of electromagnetic energy and power. Fortunately, there are commercial PCB materials with excellent isotropy that can be used in these circuits, such as TMM 10i circuit materials. These materials have a relatively high dielectric constant value of 9.8 and remain at a level of 9.8+/-0.245 in the three coordinate axes (measured at 10GHz). This can also be understood as a uniform impedance characteristic in the transmission lines of power splitters/combiners and couplers, which can make the distribution of electromagnetic energy in the device constant and measurable. For PCB materials with higher dielectric constants, TMM 13i laminates have a dielectric constant of 12.85 and vary within +/-0.35 in the three axes (10GHz).

Of course, constant dielectric constant and impedance characteristics are only one of the PCB material parameters that need to be considered when designing power dividers/combiners and couplers. Minimizing insertion loss is usually an important goal when designing power divider/combiner or coupler circuits. Ideally, a two-way Wilkinson power divider can provide -3dB or half of the input electromagnetic energy to both output ports. In reality, every power divider/combiner (and coupler) circuit will have a certain insertion loss, which is usually frequency-dependent (loss increases as frequency increases), so for a power divider/combiner design, the choice of PCB material needs to consider how to control the insertion loss of the circuit to a minimum.

In passive high-frequency devices such as power splitters/combiners or couplers, insertion loss is actually the sum of many losses, including dielectric loss, conductor loss, radiation loss, and leakage loss. Some of these losses can be controlled through careful circuit design, and they may also depend on the characteristics of the PCB material and can be minimized by properly selecting the PCB material. Impedance mismatch (i.e., standing wave ratio loss) can cause losses, but can be reduced by selecting a PCB material with a constant dielectric constant.

Minimizing losses is critical in designing high-power splitters/combiners and couplers because at high powers the losses are converted into heat and dissipated in the device and PCB materials, which affects the dielectric constant value (and impedance value) of the material.

In summary, when designing and processing high-frequency power splitters/combiners and couplers, the selection of PCB materials should be based on many different key material properties, including the dielectric constant value, the continuity of the dielectric constant in the material, environmental factors such as temperature, reducing the material loss including dielectric loss and conductor loss, and power handling. Selecting PCB materials for specific applications can help to achieve success when designing high-frequency power splitters/combiners or couplers.