FR4 dieletric constant at 5.8Ghz

Do a site search for FR4. This topic has been discussed to death before. In general, since FR4 was not intended originally for RF, the dielectric constant does not have a specific value all vendors design to. It varies over a very wide relative range because of the different ratios of chemical components used by different vendors.

I have seen commercial applications to 4.2 GHz on plain old FR-4, that had good yield. Just don't try to build any tight filters on it.

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