Hello sir/ma'am
I have designed a metamaterial using FR4 substrate of relative permittivity of 4 for C, X, Ku band applications. Now I want to fabricate it for measurement. When I am sending it fabrication, they are providing the FR4 substrate of relative permittivity of 4 at 1GHz. Will it be constant for other frequency or it will vary?
it changes, but maybe not by much. Look at the manufacturers data sheets.
This is for Rogers material
https://www.clarke.com.au/pdf/Rogers_Design_Right_First_Time.pdf
It slightly varies over frequency, both for the real and the imaginary part. Be careful that "FR-4" is designating a family of materials, basically they are "Flame retardant". This means that their dielectric properties are not controlled at all. I would recommend to get the datasheet of the PCB material you'll use. If it contains data at the frequency of interest, then you can be confident that the substrate will perform as expected. You can also do a measurement of the permittivity of the substrate. Some companies do offer that service.
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