Re: [SI-LIST] : Coplanar Transmission Line

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From: HaroldLSJ@aol.com
Date: Thu Feb 03 2000 - 20:34:32 PST


Roy,

In millimeter frequencies, the coplanar waveguide can be equal to or better
than the microstrip when loss and dispersion are used as a basis for
comparison. Minimum loss for a given coplanar waveguide occurs at about 60
ohms whereas the minimum loss for microstrip occurs at about 25 ohms. With
manufacturing tolerances, they have about the same loss below 50 ohms but at
50 ohms and above the coplanar waveguide will have the edge. The physical
size at these minimum loss impedances are similar. At higher impedances the
coplanar waveguide can give much smaller loss but will take up more physical
space than the same impedance microstrip line.

A full wave analysis which includes space wave and surface wave radiation
shows that coplanar waveguide discontinuities radiate much less energy than
microstrip discontinuities.

The disadvantages of coplanar waveguides compared to microstrip are size,
even mode excitation and possible heat transfer.

The advantages of coplanar waveguides include easier construction, no via
holes, good grounding, less radiation at discontinuities and usually lower
conductor loss.

A good comparison can be found in "Coplanar Waveguide vs. Microstrip for
Millimeter Wave Integrated Circuits," by R. W. Jackson in IEEE MTT-S Digest,
p 699, 1986.

Harold L. Snyder, Jr.
Scientist & Consultant

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