The broad spectral coverage of radio sources including high frequency data are desirable for a number of reasons. Over the last decade, researchers have identified a new class of very high-redshift objects that are more closely related to normal galaxies than to quasars. Surveys with radio telescopes have been instrumental in finding these extremely distant galaxies, which are barely visible at optical wavelengths. The selection of candidates with steep radio spectra has proven a particularly effective means for finding galaxies with redshifts greater than z = 2. Secondly, they uncover flat spectrum sources which are likely to be compact and useful as radio astrometric standards and as well as candidates for further interferometric observations. Third, they provide information about physics and, in particular, about the high-energy particles responsible for radio emission. And last but not least, simultaneous observations at several wavelengths will exclude variability of sources in interpretation of their spectra. Here we present the preliminary results of an ongoing investigation of a complete sample of radio sources from the PMN Survey.