Is it possible to have a 1/2 compression ratio with arithmetic coding and how

Need light on arithmetic compression. Currently I work on data compression more particular that arithmetic one. I would like to compress an integer vector between 1 and 255. My vector is of size 256 or 512 .... My problem is that after applying the arithmetic coding I find myself with a compression ratio almost zero. So my question is the following is it possible to have a 1/2 compression ratio with arithmetic coding and how. Thank you in advance and forgive my bad english. This is not my native language.