Radiation Efficiency Limits in Direct Antenna Modulation Transmitters

Relative bounds on radiation efficiency are established for time-modulated antenna systems, where radiation is generated by broadband conduction currents impressed using idealized nonradiating time-varying subsystems, such as those found in direct antenna modulation (DAM) transmitters. Analytical and numerical examples demonstrate that the condition of quasi-resonance, common in nearly all practical DAM transmitters, imposes severe restrictions on the otherwise unbounded gains in effective efficiency theoretically achievable by this class of time-modulated transmitters.