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.