Artificially engineered metamaterials have enabled the creation of electromagnetic materials with properties not found in nature. Recent work has demonstrated the feasibility of developing high performance, narrowband electromagnetic absorbers using such metamaterials. These metamaterials derive their absorption properties primarily through dielectric loss and impedance matching at resonance. This paper builds on that work by increasing the bandwidth through embedding resistors into the metamaterial structure in order to lower the Q factor and by using multiple elements with different resonances. This is done while maintaining an impedance-matched material at normal incidence. We thus present the design, simulation, and experimental verification of a broadband gigahertz region metamaterial absorber, with a maximum absorption of 99.9% at 2.4 GHz, and a full width at half maximum bandwidth of 700 MHz, all while maintaining low reflection inside and outside of resonance.

We report on the development of an easily deployable LF near-field interferometric-time of arrival (TOA) 3-D Lightning Mapping Array applied to imaging of entire lightning flashes. An interferometric cross-correlation technique is applied in our system to compute windowed two-sensor time differences with submicrosecond time resolution before TOA is used for source location. Compared to previously reported LF lightning location systems, our system captures many more LF sources. This is due mainly to the improved mapping of continuous lightning processes by using this type of hybrid interferometry/TOA processing method. We show with five station measurements that the array detects and maps different lightning processes, such as stepped and dart leaders, during both in-cloud and cloud-to-ground flashes. Lightning images mapped by our LF system are remarkably similar to those created by VHF mapping systems, which may suggest some special links between LF and VHF emission during lightning processes.

We present an experimental demonstration of phase conjugation using nonlinear metamaterial elements. Active split-ring resonators loaded with varactor diodes are demonstrated theoretically to act as phase-conjugating or time-reversing discrete elements when parametrically pumped and illuminated with appropriate frequencies. The metamaterial elements were fabricated and shown experimentally to produce a time-reversed signal. Measurements confirm that a discrete array of phase-conjugating elements act as a negatively refracting time-reversal rf lens only 0:12 thick. [7][8][9] gain for loss compensation and frequency tuning, respectively, as well as amplification [10]. Embedding a nonlinear response into metamaterials has also been used for powerdependent tuning [11][12][13] and coherent harmonic generation [14,15].In this Letter we introduce a new class of nonlinear active metamaterials (NAMMs). Combining metamaterials' nonlinear response with an active high-frequency external excitation in the simplest metamaterial building block, the split-ring resonator (SRR), we experimentally demonstrate phase conjugation of microwave signals. Using numerical simulations, we demonstrate that such NAMM can be used as a building block for a negatively refracting lens. Negative refraction [16] is one of the most discussed and intriguing phenomena exhibited by a special class of metamaterials with negative permittivity and permeability. It was shown theoretically by Pendry [17], however, that negative refraction could also be produced by a thin slab of a material exhibiting time reversal (or, equivalently, phase conjugation). Traditional thick slabs of phase-conjugating (PC) media have been used for the useful property of generating negative frequency waves that focus back on the source, sometimes called retrodirectivity [18]. Discrete arrays of PC elements have also been used for retrodirectivity by the microwave community [19,20]. Pendry showed that a thin slab would indeed generate these retrodirective waves but would also generate a wave on the other side of the material exhibiting negative refraction. A few recent works have examined using discrete PC elements for forward focusing [21,22], but these cannot be extended to create an effective medium. As an example, [21] used an array of patch antennas for receiving; a phase-conjugating circuit for each antenna composed of PLLs, filters, phase shifters, IQ modulators, and other components; and a separate array of patch antennas for retransmission.We demonstrate theoretically and experimentally that NAMMs, when driven with an active source, act as PC elements. The nonlinear characteristics of the metamaterials are obtained by embedding varactor diodes in each metamaterial element and exciting each element with a monochromatic rf signal, allowing easy extension to construction of a bulk medium of phase-conjugating SRRs with no external circuitry. We then demonstrate experimentally that nonlinear metamaterials do indeed act as phase-conjugating and thus negatively refracting elem...