what is the maximum frequency that can be used in the design of smart antennas?

for designing a smart antenna using microstrip antennas ,what maximum frequency can be considered for the application of wireless adhoc networks

- - - Updated - - -

if antennas with defective ground structures are used for smart antennas,what will be the correct frequency that can be selected for the design?

smart antennas are multiple antennas that use phase and amplitude weightings when combining to form beam peaks and nulls. It seems that when the electrical length of the microstrip line becomes "unpredictable"...it will be hard to point the antenna anymore. For microstrip....probably somewhere in the 10-20 Ghz range it will get hard.