string in << >>

[Jakub Zawierucha]

Hello,

How can I put string into binary form ? Below you can find description 
on my problem.

49> f().
ok
50> X = "dupa".
"dupa"
51> <<"dupa", 3:2>>.
<<100,117,112,97,3:2>>
52> <<X, 3:2>>.
** exception error: bad argument

Thanks,
Jakub Zawierucha