Here's some information on high frequency values I believe to be fairly 
reliable.  Based on 4-ply 7628 glass (28-mil).  Still some variability (just 
like all things):

Freq.	FR-4	Getek
1 MHz	4.6-4.7	3.6-4.2
2 GHz	4.5-4.6	3.91
4 GHz	4.4-4.5	3.90
6 GHz	 n/a	3.88

Glenn Pelkey
Quality/Reliability Engineer
Maxtek Components Corp.

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