Coplanar waveguide

Coplanar waveguide is a type of electrical <a href="/facts/Planar_transmission_line/PtQSrIpB">planar transmission line</a> which can be fabricated using <a href="/facts/Printed_circuit_board/muSR2p43">printed circuit board</a> technology, and is used to convey microwave-frequency signals. On a smaller scale, coplanar waveguide <a href="/facts/Transmission_line/cYlqaPo3">transmission lines</a> are also built into <a href="/facts/Monolithic_microwave_integrated_circuit/Gorz1mH1">monolithic microwave integrated circuits</a>. 
Conventional coplanar waveguide (CPW) consists of a single conducting track printed onto a <a href="/facts/Dielectric/PnVt9P2G">dielectric</a> substrate, together with a pair of return conductors, one to either side of the track. All three conductors are on the same side of the substrate, and hence are coplanar. The return conductors are separated from the central track by a small gap, which has an unvarying width along the length of the line. Away from the central conductor, the return conductors usually extend to an indefinite but large distance, so that each is notionally a semi-infinite plane.
Conductor-backed coplanar waveguide (CBCPW), also known as coplanar waveguide with ground (CPWG), is a common variant which has a <a href="/facts/Ground_plane/quwvjQl0">ground plane</a> covering the entire back-face of the substrate. The ground-plane serves as a third return conductor.
Coplanar waveguide was invented in 1969 by Cheng P. Wen, primarily as a means by which non-<a href="/facts/Reciprocity_(electrical_networks)/cFe3yYLT">reciprocal</a> components such as <a href="/facts/Gyrator/YQFmuPbF">gyrators</a> and <a href="/facts/Isolator_(microwave)/dcaNBlRT">isolators</a> could be incorporated in planar transmission line circuits.
The electromagnetic wave carried by a coplanar waveguide exists partly in the <a href="/facts/Dielectric/PnVt9P2G">dielectric</a> substrate, and partly in the air above it. In general, the <a href="/facts/Dielectric_constant/vnKbPTn1">dielectric constant</a> of the substrate will be different (and greater) than that of the air, so that the wave is travelling in an inhomogeneous medium. In consequence CPW will not support a true <a href="/facts/Transverse_electric_and_magnetic_mode/nCIl5D6a">TEM</a> wave; at non-zero frequencies, both the <a href="/facts/Electric_field/tt4MooBs">E</a> and <a href="/facts/Magnetic_field/Ta8Ev588">H fields</a> will have longitudinal components (a <a href="/facts/Mode_(electromagnetism)/W8GuDxK1">hybrid mode</a>). However, these longitudinal components are usually small and the mode is better described as quasi-TEM.

Coplanar waveguide open-in-new

Coplanar waveguide