  Internal oscillations in switched antenna transmitters cause undesirable
fluctuations of the stored energy in the system, which is critical to the
concept of energy-synchronous direct antenna modulation. A detailed circuit
model for a direct antenna modulation system is proposed that includes the high
frequency parasitics that lead to these oscillations. Analytical and numerical
studies show that the proposed model accurately predicts the oscillations of
the system and the resulting signal degradation. To mitigate the parasitic
oscillations, a modified direct antenna modulation system with an auxiliary DC
source is introduced to stabilize the stored voltage on the antenna. Measured
phase shift keyed waveforms transmitted by the modified system show significant
increases in signal fidelity, including a 10-20 dB decrease in error vector
magnitude compared to an LTI system. Comparison to an equivalent, scalable
time-invariant antenna suggests that the switched transmitter behaves as though
it has 2-3 times lower Q factor bandwidth and 20% higher radiation efficiency.
