inter element spacing

In a multi-element antenna like a Yagi, elements (dipoles) have different lengths and distances between them. Both lengths should be adjusted to focus incoming wave front to the "main" dipole which is then connected to a receiver (or a transmitter).

There are tested and verified multi-element antennas, refer to ARRL Antenna Book, for example.

thank you jiripolivka...

i would like to ask you is it same for reflectarray antenna?

Yes, interelement spacing for reflectarrays is simply the center to center spacing of the elements.

:)

thank a lot tallface65.. :D

can anyone tell me how to determine the modes for the reflectarray antenna design?

The "reflectarray" is an usuall Yagi antenna with an added resonant reflector.
I would recommend you to find original papers by Mr. Ehrenspeck who invented the "backfire" antenna. He published empirically determined sizes and spacings.
There were other designs, you can always try to add a wider-area resonant reflector to anything starting with a dipole. Some trimming may be necessary, but the system works.

thank a lot jiripolivka..:D