can we determine dielectric constant of material practically

I have manyfactured simple circular patch antenna for resonant frequency 2.45 GHz. But while testing it is showing that it is resonated at 2.48 GHz. Rest all parameters are as per specifications. I thnk problem will b in εr. i selected FR4 material and εr=4.4 for simulation.
Plz help regarding this.....

You see,
FR4 isnt a standard thing!
Its only a type of material & with an +/- εr value (eventually +/- 20%!) by all vendors....
You have to ask everytimes by impedance critical (PCB) designs your source_or PCB producer_ for the especially used/ possible exact Er-values.
Typicall has εr a range of 4.4...4.8 / .
K.

Thanks....
may i get information about practical calibration of dielectric material?

The common test method is IPC-TM-650 2.5.5.5

http://www.ipc.org/4.0_Knowledge/4.1..._2-5-5-5-1.pdf

Try for appl notes i.e. by HP VNAs...
K.

Added after 5 minutes:

Hi Volker,
besten Dank!
Its a good source ...
K.