Using a p.c.m. system, a simple relationship is discussed for the limit length of a regeneration section. This reveals that delay equalization is required in the lower part of the transmission range between 35 and 80 Gc/s, if the regeneration sections are to coincide with the usable repeater spans. The number of delay-equalizer elements required is derived from a general relationship containing the field energy of the equalizer. A specific equalizer of the low-pass type would have to include about 145 elements for the equalization of a waveguide run of 30 km (50 mm guide diameter) with 35 Gc/s and 75 Mc/s separation from low-pass resonance, and about 49 elements when operated at an intermediate frequency of 4 Gc/s. The attenuation distortion narrows the usable bandwidth. It is investigated whether frequency modulation can be used, in view of the phase distortion caused by reflection and mode conversion. If the investigation is restricted to the H01reflection and H01/H02conversions at the diameter discontinuities of the partial lengths, it is found that the permissible distortion limit is already attained with a route 1 000 km long consisting of 5 m segments with diameter tolerances of 0.1 %. Since conversion is caused by effects other than those considered, frequency modulation shows promise only for minor route lengths.