A space-harmonic perturbation theory is presented for the multistrip coupler which does not require any adjustment parameters and which gives the dispersion relations separately for the symmetric and for the antisymmetric modes. The agreement with experimental results for different widths and spacings of the strips is very good when the number of strips per wavelength is greater than two, but is not as good for smaller values, because the effects of the reflected wave and of the radiated bulk waves have been neglected.