At microwave frequencies, transmission lines must be treated as distributed elements. Several planar and non-planar structures are used depending on frequency, power, loss, and fabrication constraints.

Microstrip

The most common planar line in PCB design. A conductor trace on top of a dielectric substrate with a ground plane below. Fields are partly in the substrate and partly in air, giving an effective dielectric constant \(\varepsilon_{eff} < \varepsilon_r\). Easy to fabricate but has higher radiation loss at millimetre-wave frequencies.

Stripline

Conductor buried between two ground planes in a homogeneous dielectric. All fields are in the dielectric so \(\varepsilon_{eff} = \varepsilon_r\) exactly. Lower radiation loss than microstrip but harder to access for shunt connections.

Coplanar Waveguide (CPW)

Signal conductor flanked by ground conductors on the same surface. Easy shunt connections (no via needed), suitable for MMIC (monolithic microwave integrated circuit) fabrication. Widely used above 10 GHz.

Coaxial Line

TEM mode, no cutoff frequency (down to DC). The dominant transmission medium for test equipment, cables, and connectors. Standard impedances: 50 Ω (minimum attenuation/maximum power tradeoff), 75 Ω (minimum attenuation in air-filled coax).

Waveguide

Hollow metallic tube supporting TE and TM modes. Has a cutoff frequency below which propagation does not occur. Very low loss at microwave/millimetre-wave frequencies. Used in high-power radar and satellite systems.