Blackford (Finite Fields Appl. 24, 29–44 2013) introduced Type I constacyclic duadic codes over the finite field F_q, where q is an odd prime power, and obtained isodual codes from them. In this paper, we generalize this idea and present Type II q-splitting of some special natural numbers n over F_q . By using it, we construct isodual codes of length n + r over F_q for some r, where r is some divisor of n and q − 1, and provide some examples of optimal isodual codes.
