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You can look up the specs of the material at the PCB producer. The dielectric constant doesn't depend on the frequency but the loss in the material does. If the application has to be very precise go for Rogers material otherwise FR4 is good until several GHz (assume er=4.5). When working with microstrips also consider the tolerances of the trace widths. If you get close to the minimum trace width the manufacturing tolerances will screw things up.