Fundamental Limits, Bandwidth, and Information Rate of Electrically Small Antennas: Increasing the Throughput of an Antenna Without Violating the Thermodynamic Q-Factor

Abstract

The fundamental limits of antennas tie its electrical size to its quality factor (Q-factor) and radiation efficiency. The Q-factor is linearly proportional to the inverse of the instantaneous bandwidth for a narrow-band antenna. However, the thermodynamic definition of the Q-factor is equal to the ratio of stored energy to the dissipated power per cycle multiplied by 2p. In this definition, the frequency and type of stored energy are not considered; therefore, one can transmit information by modulating the frequency and/or type of stored energy, while maintaining the total stored energy with a rate higher than that which is restricted by fundamental limits. In this article, we show that instantaneous bandwidth does not necessarily define the maximum information rate of an antenna. Different techniques for transmitting a high-information-rate signal using a narrow-band (high-Qfactor) antenna are presented from an energy balance and thermodynamic point of view. A few examples are presented as proof of concept. To help with understanding this concept, we have applied some of the proposed modulation schemes to a pendulum system as a second-order mechanical resonator. Finally, the proposed modulation techniques are confirmed using simulations.