Direct antenna modulations for UWB pulse transmission and near field communications

By modulating the antenna directly in amplitude, a switched resonant antenna can radiate ultra-wide band (UWB) pulses efficiently with rich spectrum components far beyond the antenna's usually defined bandwidth. This is so-called direct antenna modulation (DAM) scheme which is based on the time-domain behavior and nonlinear dynamics of the antenna to obtain the radiation efficiency and bandwidth simultaneously to surpass the conventional performance bound of a narrow band, high Q antenna. The direct modulation to the antenna is through the integration of switch devices onto the antenna. The switches turn on and off the current flowing path in the antenna which is essential in forming the radiation. For a microstrip patch antenna, the switches can be placed along the radiating edges to either open or short-circuit the radiating slots. The antenna thus behaves as an integrated pulse generator, modulator and radiator. Through careful design of antenna modal structures in connecting with the switching position and timing, the energy loss due to the mode conversion can be avoided, resulting in higher radiation efficiency. Several interesting phenomena are observed including (1) the instantaneous radiated power of pulses can be much higher than the available source power at the antenna input due to the compression of energy in time; (2) In an idealistic setup, the radiation efficiency increases with the increase of the data rate and eventually saturates.;The study focuses on a switched microstrip patch antennas. The theoretical potential of such antennas is analyzed based on transmission line models and equivalent circuit models. A full-wave Finite Difference Time Domain (FDTD) code that incorporates non-linear devices in its mesh is then developed and applied to truthfully simulate the nonlinear dynamics occurring in the antenna. The simulation results have validated the proposed concept by demonstrating the capability of efficient UWB pulse transmission through the proposed antenna scheme.;The direct antenna modulation scheme can also be applied to near field communication (NFC) systems based on inductive coupling to improve its capacity performance. The capacity bound of a conventional inductively coupled near field communication system is first analyzed based on the equivalent circuit model of the system and the information theory. The analytical results provide good guidelines about the estimation of the capacity performance and the antenna design for an inductively coupled near field communication system. It is identified that in a traditional inductively coupled near field communication system, the power coupled through and the bandwidth of the system must be traded off for optimal capacity performance. In order to break the dilemma, the DAM technique can be used to break the efficiency bandwidth product limit of a high Q inductively coupled near field communication system through the nonlinear/time varying operation. The capacity performance of three inductively coupled near field communication systems operating at VLF/ULF: air to air link, air to ocean link and ocean to ocean link is evaluated based on the derived theory. Further electromagnetic simulations based on the commercial software COMSOL Multiphysics demonstrated great improvements of the capacity performance of NFC systems with DAM techniques.