 The radial extends as far as the transmitted signal can reach either by calculation or by measurement.Stations transmitting at low frequencies like the mediumwave and longwave AM broadcast bands, and some lower shortwave frequencies, have frequencies so low that any feasible antenna is necessarily short compared to the wavelength, the most common being a quarter wave vertical antenna.The radials at the antenna base provide a proper ground plane for the types of radio antennas used for long wavelengths.