Microstrip vs Stripline vs CPW: When Each Wins

Three planar transmission lines carry almost all of modern RF design: microstrip, stripline and coplanar waveguide (CPW). We are named after one of them, so consider this our home turf.

Microstrip: the default for a reason

A trace over a ground plane, dielectric below, air above. Microstrip is the cheapest structure to fabricate, the easiest to probe and tune, and every stackup supports it.

Its weaknesses follow from the air interface. The field lives partly in air and partly in dielectric, so the effective permittivity — and the impedance — depends on etch tolerance, solder-mask coverage and even humidity in extreme cases. It radiates, which means it also couples: parallel microstrips need roughly three line-widths of separation before crosstalk stops being a design consideration.

Use it when: loss budgets are ordinary, you need to mount components on the line, or you need to trim after assembly.

Stripline: buried, quiet, predictable

Sandwich the trace between two ground planes and the field is fully contained in dielectric. Radiation is essentially zero, isolation between adjacent lines is dramatically better than microstrip, and the impedance is set by geometry the fab controls well.

The costs: you cannot mount components on a buried layer, every transition needs vias (and via stubs need attention above ~10 GHz), and the fully dielectric field means slightly higher loss per millimetre than microstrip on the same material.

Use it when: you need isolation — filter banks, LO distribution, board-to-board buses running past sensitive analogue — or when a line must hold its impedance over years and lots.

CPW: the RF probe's best friend

Grounded coplanar waveguide puts the return path beside the trace on the same layer. Ground is always a via away, which is why it dominates above 20 GHz: the parasitic inductance of a component's ground connection shrinks to almost nothing.

The catch is current crowding at the gap edges — conductor loss is the highest of the three for the same width — and its impedance is sensitive to the gap dimension, which is the parameter your fab controls least well.

Rules of thumb you can defend

Below 6 GHz, microstrip unless you have an isolation problem.
Choose stripline for anything that must not talk to its neighbours; our LNA module uses buried stripline for its bias distribution for exactly this reason.
Above 20 GHz, grounded CPW with a tight via fence.
Whatever you pick, model the transitions — the launches, not the lines, set the return loss of most real boards.

Microstrip: the default for a reason

A trace over a ground plane, dielectric below, air above. Microstrip is the cheapest structure to fabricate, the easiest to probe and tune, and every stackup supports it.

Use it when: loss budgets are ordinary, you need to mount components on the line, or you need to trim after assembly.

Stripline: buried, quiet, predictable

Use it when: you need isolation — filter banks, LO distribution, board-to-board buses running past sensitive analogue — or when a line must hold its impedance over years and lots.

CPW: the RF probe's best friend

Rules of thumb you can defend

Below 6 GHz, microstrip unless you have an isolation problem.

Choose stripline for anything that must not talk to its neighbours; our LNA module uses buried stripline for its bias distribution for exactly this reason.

Above 20 GHz, grounded CPW with a tight via fence.

Whatever you pick, model the transitions — the launches, not the lines, set the return loss of most real boards.