The well-known Zas-Halzen-Stanev (ZHS) Monte Carlo program, used to calculate coherent radio-pulses in dense media, has been extended to account for the deflections caused by the presence of a uniform magnetic field. This new development of the program can be used to simulate electromagnetic showers in constant density air. We compare results obtained to those of the ZHAireS program in the same conditions, and we explore the behavior of the frequency spectrum of pulses in air showers in a fairly generic way. By considering the effects in the generated pulses of different refractive indexes, densities and magnetic field intensities, it is possible to disentangle them and explain the main scaling properties of these showers, which are completely different from those in a dense medium such as ice. Finally the pulses obtained with ZHS for air at constant density are compared to those obtained with ZHAireS in a realistic atmosphere to demonstrate the validity and reach of the conclusions obtained.