The spectra of discrete radio sources measured in the wavelength range of 12.6–25 MHz are analysed and classified. Besides the well-known spectral types with zero and negative curvature, the spectra include a type with positive curvature and also some with a complicated shape exhibiting curvature of variable sign. Preliminary results of measurements of the stability of the spectral characteristics show that most sources have spectra which are relatively stable with time, but that the sources 3C 84, 3C 144 and 3C 461 show variable spectra with a variation increasing at decreasing frequency in the decametric range. Some mechanisms of radio wave generation leading to various spectral types are discussed.