The spectra of radio sources are the main sources of information regarding the physical processes inside them. All the GMRT calibrator sources emit non-thermal radiation, which is due to synchrotron emission from high energy electrons spiraling in magnetic fields. Such sources often show anomalous behavior at low radio frequencies due to several effects, e.g., synchrotron self absorption, free-free absorption, Razin-Tsytovich effect, spectral steepening due to surrounding diffuse emission, etc.
In this chapter we will discuss the synchrotron emission mechanism, modification of power law spectrum, the radio spectra and their classification. We will also discuss the effects at low radio frequencies and try to investigate these effects using the observations done.