Analysis of the Efficiency Improvement of a Directly-Driven Antenna- Based AM Transmitter.

摘要

The operation of a traditional radio transmitter is limited by the frequency range over which the antenna input impedance can be conjugately matched to the power amplifier output impedance. This limitation is particularly strong when an electrically-small antenna is used. US patent number 5,402,133 'Synthesizer Radiating Systems and Methods' by Joseph Merenda suggested that this limitation can be overcome by driving the antenna directly with a digital version of the desired signal, such as that produced by a pulse- width modulator, through a pair of switching transistors, and using the antenna reactance to convert the digital input signal back into an analog radiated signal. The practicality of the Merenda method was demonstrated using an unmodulated carrier at 1 MHz by Palmer et al. in a paper presented at the 2008 Antenna Applications Symposium at Allerton. This work builds upon that result by demonstrating that an AM signal can in fact be encoded and transmitted with an electrically-small antenna by using this method, and by comparing the radiated power available from the directly-driven antenna (DDA) system to that of a traditional linear, conjugately matched transmitter.