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Message: [QUOTE="John Larkin, post: 1963598"] A couple of things make pcb's lossy: the loss tangent of the material (and FR4 is pretty bad) and the copper losses. Copper loss gets bad on conventional FR4 boards because 1. FR4's Er is high, so for a given impedance traces are skinny. 2. The underside of the copper is treated to bond to the epoxy/glass, and the treatment (black oxide or something) greatly increases skin losses. Peel some up and look... it's gross. 3. In the case of microstrip, the current is concentrated on the underside (the dirty side) of the trace, so losses are that much worse... the shiny topside of the copper is underutilized. Stripline would be better, with balanced current density, except that the trace will be much thinner, which has its own penalty. A good microwave pcb has a low Er, low loss dielectric; is thick, for low current density and wide traces; has very smooth copper, which means traces and pads peel off easily. I don't think any simple geometry tricks (ie, emulating coax) will make FR4 any better, and would probably make it worse. For low losses, microstrip on a thick board is probably as good as it gets. John [/QUOTE]