Ejem.
Using the info you are providing its's more or less 46.5 billion light-years (the radius of the observable universe)
Seriously. Try to be be more precise: application, frequency, etc...
Good luck
Minimum 20dB is a good number in antenna separation. This isolation is fine for getting at least minimum performances for diversity or for a two-antenna duplex system.
suppose frequency of operation is 20GHz. then how to find the separation between antennas ex dipole antenna?
sum paper i reffered that it should be greater then lamda..is it true r not i don't know
Yes, distance greater than λ is fine, but is good to look also for isolation in dB at operating frequency to eliminate any kind of interaction between antennas.
above link gives the distance of separation between antenas
in this link it says that
The 2.4 GHz wavelength is approximately 4.92 inches. Therefore, to support diversity on a 2.4 GHz radio with two separate antennas, the antennas should be spaced approximately 5 inches apart. The antenna pair could also be spaced at multiples of 5 inches, but the distance between should not exceed 4 multiples: reflected waves farther apart than that are likely to be so distorted and different in delay spread that the radio could not work with them.