Microstrip
Understanding Microstrip
Microstrip is the foundation of modern RF circuit design. Nearly every RF PCB, hybrid module, and MMIC uses microstrip transmission lines to route signals between components. The characteristic impedance is determined by the strip width, substrate height, and dielectric constant.
Microstrip Design
- Impedance control: Width/height ratio determines Z0. For 50 ohms on Rogers 4350B (er=3.66), line width is approximately equal to substrate thickness.
- Effective dielectric constant: Because fields extend into both the substrate and air, the effective dielectric constant is lower than the substrate er.
- Dispersion: The effective dielectric constant varies with frequency, causing dispersion at higher frequencies.
- Loss: Conductor loss (proportional to sqrt(f)) and dielectric loss (proportional to f) both contribute. Loss tang of substrate material is critical above 10 GHz.
Substrate Materials
| Material | er | Loss Tangent | Use |
|---|---|---|---|
| FR-4 | 4.3-4.5 | 0.020 | Low-freq, cost-sensitive |
| Rogers 4350B | 3.66 | 0.004 | General RF to 30 GHz |
| Rogers 5880 | 2.20 | 0.0009 | Low-loss, mmWave |
| Alumina | 9.8 | 0.0001 | MMIC, hybrid modules |
Z0 = (87/sqrt(er+1.41)) x ln(5.98h/(0.8W+t))
Effective dielectric constant:
e_eff = (er+1)/2 + (er-1)/(2 x sqrt(1+12h/W))
50 ohm on Rogers 4350B (h=0.508mm):
W = 1.09 mm (approximately W/h = 2.15)
Frequently Asked Questions
What is microstrip?
Microstrip is a planar transmission line used on printed circuit boards. A conductor strip on the top surface, a dielectric substrate, and a ground plane on the bottom form the transmission line. The strip width, substrate height, and dielectric constant determine the characteristic impedance.
What substrate should I use for microstrip?
For frequencies below 6 GHz, FR-4 is acceptable for cost-sensitive applications. For 6-30 GHz, Rogers 4350B or similar PTFE-based laminates provide lower loss and more consistent dielectric properties. Above 30 GHz, low-loss substrates like Rogers 5880 or alumina are preferred.
Why not use microstrip at mmWave?
At millimeter-wave frequencies, microstrip loss becomes significant due to conductor (skin depth) and dielectric losses. Radiation increases because the substrate thickness approaches a significant fraction of wavelength. At W-band and above, waveguide or substrate integrated waveguide (SIW) is often preferred.