Archive of the former Yahoo!Groups mailing list: Homebrew PCBs

There's another reason for choosing a particular trace width: transmission
line impedance.

If you're working with UHF or microwave frequencies, the traces on a PC
board can be a substantial fraction of a wavelength long, and you have to
treat them as circuit elements with inductance and capacitance, not just
"connections". One way of getting signals from one point to another is to
make the trace a proper transmission line with a known impedance, and drive
it with circuits matched to that impedance.

A simple transmission line can be made with "microstrip" construction. This
just uses a double-sided PC board, with the copper left unetched on one side
of the board. This large copper plane is connected to RF ground. On the
other side of the board, traces are made with a specific width, since width
controls impedance. The board itself needs to be epoxy-fiberglass or
special microwave board material, depending on the frequency.

For example, on 1/16 inch epoxy-glass board, a 50-ohm line is about 0.110
inch wide, while a 75-ohm line would be 0.054.

Dave