Digitally Driven Antenna for HF Transmission

An electrically small antenna connected directly to a complementary pair of switching transistors is driven with a pulsewidth modulated HF signal, eliminating the requirement for a frequency-dependent impedance-matching network. The intrinsic reactance of the transmit and receive antennas acts as a filter to recover the HF signal from the digital pulse train. This is defined here as the digitally driven antenna architecture. A circuit simulator with broadband equivalent-circuit models for the transmit and receive antennas is used to predict the received signal in the time domain, and the expected received spectrum is calculated using Maxwell's equations and the fast Fourier transform. The simulated circuit is realized using a highly capacitive electrically small dipole antenna driven at 1 MHz with a 10-MHz reference signal on the pulsewidth modulator as the transmitter and a highly inductive 470- μH ferrite-loaded loop as the receive antenna. The 1-MHz signal is clearly evident in the time-domain received signal on an oscilloscope, and also in the received spectrum, as observed on a spectrum analyzer. This demonstrates that indeed it may be possible to produce efficient radiation across a wide bandwidth from an electrically small antenna by driving the antenna directly with a digital pulse train.