Direct antenna modulation (DAM) methods using time-varying matching networks offer the possibility to reduce the detrimental effects on broadband signal transmission from a narrowband antenna. We propose time-and frequency-domain models for the effects of a DAM scheme for the transmission of ON-OFF-keyed RF signals and test the validity of the proposed models through over-the-air measurements at 27.12 MHz. Random bit sequences are transmitted separately using both a switched-mode DAM transmitter and a conventional transmitter and received in the far field. Comparison of measured data with our frequencydomain models shows that the DAM transmission method partially removes the effects of narrowband filtering due to the transmit antenna. Models and measurements also demonstrate that the removal of this filter improves the signal quality as observed in eye diagrams of the demodulated received signals. Finally, a simple time-domain model of the DAM transmitter is shown to predict the radiated transient signal under varying synchronization conditions. INDEX TERMS Antenna measurements, electrically small antennas, narrowband antennas, time varying circuits.

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A novel scheme for transmitting broadband phase shift keyed signals from electrically small antennas using energy-synchronous direct antenna modulation is described. We outline fundamental operating principles of the method and experimentally compare its performance to that of a conventional band-limited transmit antenna with the same electrical size and radiation efficiency. Transmitted waveforms are analyzed in the time domain both at RF and baseband. Results show significant increases in signal quality, suggesting a larger effective transmit bandwidth and greater potential throughput when the proposed direct antenna modulation scheme is used. INDEX TERMS Electrically small antennas, direct modulation, antenna measurements, phase shift keying, time-varying circuits.

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