The origin of secondary peaks in the power spectrum of a launching structure consisting of multijunction sections is analysed generally by means of an approximate formula for the power spectrum. Results of the numerical optimization of a specific structure with three waveguide multijunction sections are given. It is shown that in the higher density regime the basic electrical length of the multijunction section cannot be chosen in such way that both a sufficient reduction of the secondary peak and a favourable power distribution among waveguides be achieved simultaneously. The difficulties are eliminated by the use of aperiodically-phased multijunction sections, resulting in equalization of the phases of the incident waves in the waveguides.