Experimental Investigation of Time Reversal with Electromagnetic Waves in the Presence of Random Media

Wave propagation is a ubiquitous concept throughout physics. When the aim is to focus a wave at a given point in space or time, the resolution that can be achieved is usually limited by diffraction. However, time reversal is a method which takes advantage of multipath elements in a given channel and uses them to obtain resolution beyond the diffraction limit. This is known as super-resolution. This remarkable ability of time reversal systems to focus beyond conventional limits has found applications ranging from ultrasound, to underwater acoustics, and electromagnetics. Multipath elements of a wireless channel usually degrade the performance of such systems, but using time reversal, the multipath elements can actually lead to an advantage. When a time reversal system at RF frequencies is implemented in a waveguide, the important governing parameters that influence the effectiveness are bandwidth, number of antennas on the transmitting and receiving arrays, number of propagating modes, and reflections from terminations at the end of the waveguide. For single antennas on the transmitting and receiving ends, fine focusing resolution in space and time can be obtained if the channel is dispersive enough. However, for a homogeneous waveguide, if there are limited degrees of freedom due to bandwidth limitations, lack of multipath components due to matched open ends, or otherwise, the focusing capabilities are hindered. On the other hand, if a random medium is placed between the transmitter and receiver, the focusing capabilities can be recovered. It is this concept that we have set out to experimentally investigate. We have performed single antenna time reversal experiments in a heating ventilation and air conditioning duct (HVAC), which acts as a circular waveguide at RF frequencies. Using a center frequency of 8.1 GHz, and a bandwidth of 16 MHz, we have made a comparison between the focusing capabilities of a homogeneous waveguide and one partially filled with pieces of a random dielectric material (polyvinylchloride foam). The focusing abilities of the random waveguide are shown to improve with increases in the length of random medium through which the signal propagates. However, due to finite attenuation of the dielectric, the improvement in focusing over a homogeneous (empty) waveguide eventually reaches a minimum value as the length of random waveguide increases. From there, the loss associated with the dielectric becomes more dominant over the scattering introduced by the dielectric, and the focusing degrades.