# What are the advantages of CPW over microstrip?

I have to use a coplanar waveguide (CPW) for a project which is going to characterise small amounts of tissue. I have never used CPW before.

What are the advantages of using CPW? What would be some key words for searches to get me started and help me understand more?

Keep in mind that circuits are usually printed on double side full-metalized dielectric boards, so by using CPW technology :

1. You use one face of copper instead of two, which, in large scale production, is a non negligible saving on costs since you have to buy single-face metalized substrates instead of double.
2. You remove less copper when printing the circuit, because what would be removed on non coplanar microstrip is now a conductive ground plane. Which also lessen the tool fatigue if using mechanical engraving

Here is a good reference comparing the two technologies made by Rogers Corps, which is a company specialized in radiofrequency substrates (like RO3003, RO4003 ...). however they seem to focus a bit more on CBCPW (conductor backed coplanar waveguide, another variant)

Another one which is a simple table comparing the two from a functional point of view

Coplanar waveguides are used to reduce the trace width of the transmission line. They are also used on single-layer boards that lack a ground plane.

(This image shows CPW with a ground plane, but you can have CPW on a single-layer board)

As a rule of thumb, for standard 50-Ohm microstrip, the trace width (w) is approximately twice the distance between the trace and ground plane (h). If you have a 62-mil, 2-layer board, this makes for a very thick (~120mil) microstrip!

For comparison, for the same board, you could get close to 50-Ohm using a 40-mil wide trace with a 5-mil gap to the coplanar ground planes.

These numbers are only estimates. Make sure you do the calculations.

CPW geometries can be tricky to calculate. Many of the estimations out there require certain assumptions, such as that the gap (w) is significantly smaller than the board thickness (h). To paraphrase Kraig Mitzner, an attempted CPW design often results in an accidental microstrip.

I recommend using a field solver to avoid these pitfalls. If you don't have access to one, there is a good free (GPL) solver called TNT.