  Antennas operating at the high-frequency (HF) band (3-30 MHz) are frequently
electrically small due to the large wavelength of electromagnetic waves (10-100
m). However, the bandwidth-efficiency products of passively matched
electrically small antennas (ESAs) are fundamentally limited. Wideband HF
waveforms using bandwidths of 24 kHz or more have recently received significant
attention in military communications applications. Efficiently radiating such
signals from conventional passive ESAs is very challenging due to fundamental
physical limits on bandwidth-efficiency products of ESAs. However, active
antennas are not subject to the same constraints. In this work, we present the
design and experimental characterization of a high-power, active ESA with
enhanced bandwidth-efficiency product compared to {that of} passively matched
ESAs. Specifically, the proposed active ESA can radiate wideband HF signals
with banwidths of 24 kHz or more, with total efficiencies up to 80$\%$, and
radiated power levels approaching 100 W. Our approach uses a highly-efficient,
integrated class-E switching circuit specifically designed to drive an
electrically small, high-Q HF antenna over a bandwidth exceeding 24 kHz. Using
a high-Q RLC antenna model, we have successfully demonstrated wideband binary
ASK, PSK, and FSK modulations with the proposed class-E switching architecture.
Experimental results indicate that the bandwidth-efficiency product of this
class-E active antenna is 5.4-9.8 dB higher than that of an equivalent passive
design with the same data rate, and bit-error-rate (BER).
