Recently, Yakubovich [Opuscula Math. 26 (2006) 161--172] and Passian et al.
[J. Math. Anal. Appl. doi:10.1016/j.jmaa.2009.06.067] have presented
alternative proofs of an orthogonality relation obeyed by the Macdonald
functions of imaginary order. In this note, we show that the validity of that
relation may be also proved in a simpler way by applying a technique
occasionally used in mathematical physics to normalize scattering wave
functions to the Dirac delta distribution.