Efficiency improvements of a Directly-Driven Antenna-based AM transmitter over the am frequency band

A wide frequency bandwidth and high power efficiency are important goals of radio transmitter design. Conventional radio transmitters have a narrow bandwidth, primarily limited by the bandwidth of the impedance matching network between the power amplifier and transmit antenna. The Directly-Driven Antenna (DDA) architecture has the potential to eliminate this limitation, as without the frequency constraints imposed by the conjugate impedance matching network, it is possible to couple power to a transmit antenna efficiently across a very wide bandwidth. Previous work demonstrates that the DDA architecture can produce an improvement by a factor of 10 in the radiated power for an electrically-small antenna when compared to conventional transmitter architecture. Results obtained over the past year, incorporating changes to the DDA circuit and measurement techniques reported previously, confirms that the radiated power improvement is close to a factor of 10 at ~1 MHz but changes with frequency. This letter discusses recent refinements made to the circuit design, measurement technique and results, and shows data demonstrating operation of a DDA transmitter across the AM broadcast band.