My question is about the substrate FR4 , for 868 MHz and 1.5 GHz , is FR4 has the same relative permitivitty,
I want just to be sure when I calculate the dmemsion of a 50 ohms microstrip line at the thow frequencies
FR4 relative permittivity varies with frequency, but between 868 MHz and 1.5 GHz this don't vary too much.
Anyway would be less variation than other parasitics variation of components on your circuit.