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The RF-35A2 from Taconic is a Laminate with Dk (Dielectric Constant) 3.5 +/- 0.05, Df (Dissipation Factor) 0.0011. More details for RF-35A2 can be seen below.
Laminate for Microwave Printed Circuit Board with Enhanced Thermal Conductivity
You can now find similar products from multiple companies on everything RF.