RT-Duroid® laminates are widely used in microwave and high-frequency circuit applications due to their stable dielectric properties and low loss tangent. Designing stripline devices with specific characteristic impedances involves determining the appropriate line width, which depends on the laminate’s dielectric constant (Dk), thickness, and copper cladding.

1. Factors Affecting Stripline Design

• Dielectric Constant (Dk):
  • RT-Duroid laminates have precise Dk values, such as 2.94 for RT-Duroid 6002 and 10.2 for RT-Duroid 6010. The Dk significantly impacts the line width required for a given impedance.
• Laminate Thickness (H):
  • The spacing between the center conductor and the ground planes directly influences the impedance. Thicker laminates require wider lines for the same impedance.
• Copper Thickness (T):
  • Conductor thickness affects the characteristic impedance due to changes in the effective width of the trace.
• Characteristic Impedance (Z0):
  • The desired impedance (e.g., 50 Ω or 75 Ω) is achieved by adjusting the line width and spacing.

2. General Design Formula

The characteristic impedance Z0Z_0Z0​ for a center stripline in a homogeneous dielectric can be approximated using:Z0=60ϵrln⁡(4Hw⋅(1+T/H))Z_0 = \frac{60}{\sqrt{\epsilon_r}} \ln \left( \frac{4H}{w \cdot (1 + T/H)} \right)Z0​=ϵr​​60​ln(w⋅(1+T/H)4H​)

Where:

• Z0Z_0Z0​ = Characteristic impedance (Ω)
• ϵr\epsilon_rϵr​ = Relative dielectric constant (Dk)
• HHH = Distance between the center conductor and ground planes (inches or mm)
• www = Line width of the stripline (inches or mm)
• TTT = Conductor thickness (inches or mm)

3. Example Line Widths for Common RT-Duroid Laminates

For standard configurations with typical laminate parameters, the following table provides approximate line widths for 50 Ω characteristic impedance:

Laminate	Dk	Thickness (H)	Line Width (w)
RT-Duroid 6002	2.94	0.020 inches	0.034 inches
RT-Duroid 6010	10.2	0.020 inches	0.015 inches
RT-Duroid 6006	6.15	0.030 inches	0.022 inches

4. Impedance Tuning

• Adjusting Width:
  Narrower traces increase impedance, while wider traces decrease it.
• Using Differential Pair Striplines:
  For differential signals, the spacing between paired lines also impacts impedance.
• Simulation and Tolerance:
  Perform EM simulations to account for manufacturing tolerances and ensure consistent performance.

5. Practical Considerations

• Fabrication Tolerances:
  Ensure precise control of line width, spacing, and laminate thickness during manufacturing.
• Copper Cladding Variations:
  Thicker cladding requires adjustments to line width due to skin effects at high frequencies.
• Environmental Stability:
  RT-Duroid laminates offer stable performance across temperature and frequency ranges, ensuring minimal impedance variation.

6. Applications

• Microwave Antennas:
  Precise impedance control enhances signal integrity in feedlines and phased array systems.
• Filters and Couplers:
  Accurate line widths ensure consistent signal transmission and minimal losses.
• High-Speed Digital Systems:
  Stripline designs support controlled impedance for reliable high-frequency data transfer.

By carefully selecting the line width and accounting for laminate properties, designers can achieve the desired characteristic impedance in RT-Duroid stripline devices. Utilizing simulation tools and adhering to fabrication guidelines ensures optimal performance and repeatability in high-frequency applications.