To exploit the wealth of information carried by the short transient radio pulses from air showers, the frequency spectra of the signals have to be investigated. Here, we study the spectral content of radio signals produced by inclined showers, with a focus on the 30-80 MHz frequency band, as measured, for example, by the antennas of the Pierre Auger Observatory. Two exponential models are investigated and used to describe the spectral shape of the Geomagnetic and Charge-excess components of the pulses. The spectral fitting procedure of the models is described in detail. For both components, a parameterization of the frequency slope as a function of the lateral distance to the shower axis and the geometrical distance between core and shower maximum is derived. For the Geomagnetic component, a quadratic correction to the frequency slope is needed to better describe the spectrum, and it has been parameterized, too. These pieces of information can be employed in event reconstruction to constrain the geometry, in particular the core position.