What microstrip is
A microstrip is a track on one side of a PCB with a solid ground plane on the other. Its impedance depends on the ratio of the track width to the board thickness and on the board's dielectric constant. For 50 Ω on standard 1.6 mm FR-4, the track is about 3 mm wide. On a thin four-layer board, with the ground only 0.2 mm below, it's only about 0.35 mm. The calculation uses the widely used Hammerstad equations, accurate to within about 1% for thin copper.
Practical tips
- Keep a solid, unbroken ground plane under every RF track.
- FR-4's dielectric constant varies between makers and falls a little at higher frequencies. It's fine up to a few GHz for amateur use.
- Copper thickness and solder mask lower the impedance slightly, by a few ohms.
- Add plenty of vias stitching the top ground pour to the ground plane near RF tracks.
For current-carrying tracks, use the PCB trace width calculator, and for splitters built from microstrip, the Wilkinson calculator.
Questions
How wide is a 50 Ω track on 1.6 mm FR-4?
About 3 mm, depending on the exact dielectric constant.
How wide is a 50 Ω track on a 4-layer board?
With about 0.2 mm to the ground layer, around 0.35 mm. Check the stack-up from your board maker.
Does copper thickness matter?
A little. Thicker copper lowers the impedance by a few ohms.
Is FR-4 OK for RF?
Yes, up to a few GHz for most amateur projects. Above that, low-loss boards such as Rogers are better.