ENGINEERING ANSWER
The cited RO4000 data sheet lists typical RO4350B dissipation factor of 0.0037 at 10 GHz and 0.0031 at 2.5 GHz. A loss tangent belongs to its measurement conditions and should not be treated as a broadband constant without validation.
What to decide before release
Build a loss budget that separates dielectric loss, copper loss, surface finish, mismatch and connector transitions. Short low-frequency lines may be dominated by other effects; long high-frequency routes can reveal differences that are hard to see on a small coupon.
Checks to put in your design brief
- Specify the operating band
- Compare equal line lengths and geometries
- Measure insertion and return loss together
Important distinction
Do not calculate total board insertion loss from Df alone. The geometry, conductor and fixture are part of the result.
Sources & basis
Material facts follow the cited source. Design checks are preliminary guidance, not supplier process instructions.