As terrestrial audio broadcasting is moving from the analog to the digital era, there is an interest from the broadcasting industry to re-examine the existing Amplitude Modulation (AM) and Frequency Modulation (FM) hardware. The objective is to identify potential problems which may arise when using digital signals with the existing systems. In particular, this paper analyzes the effects that an antenna impedance mismatch has on the spectrum regrowth of a hybrid analog-digital AM signal at the passband. The interest in this research stems from the difficulty in controlling the antenna impedance as it may vary with weather conditions e.g., icing on the antenna.

Spectrum regrowth results are first presented in this paper when modelling the antenna load as a resonant circuit for both series and shunt load topologies when the inverse Chebychev reconstruction filter is deployed in the Envelope Elimination and Restoration (EER) amplifier. Different selectivity factors, Q, for antenna resonant circuit are considered. Following this, attempts to design more robust filters are offered that would be of the same order and would provide improved spectral performance when working with the antenna impedance deviations from the nominal resistive value of 50.