Near and far field

About: Near and far field is a research topic. Over the lifetime, 15922 publications have been published within this topic receiving 220571 citations.

Near-field optical micromanipulation with cavity enhanced evanescent waves

Abstract: We show that the forces associated with near-field optical micromanipulation can be greatly increased through the use of cavity enhanced evanescent waves. This approach utilizes a resonant dielectric waveguide structure and a prism coupler to produce Fabry-Perot-like cavity modes at a dielectric-fluid interface. Fabricated structures show a ten times enhancement in the optical interaction and optical force for micrometer-sized colloids. In addition, stable accumulation and ordering of large scale arrays of colloids are demonstrated using two counter-propagating cavity enhanced evanescent waves.

Broadband Equivalent Circuit Models for Antenna Impedances and Fields Using Characteristic Modes

Abstract: An approach for modeling antenna impedances and radiation fields in terms of fundamental eigenmodes is presented. Our method utilizes the simple frequency behavior of the characteristic modes to develop fundamental building blocks that superimpose to create the total response. In this paper, we study the modes of a dipole, but the method may be applied to more complicated structures as the modes retain many of their characteristics. We show that the eigenmode-based approach results in a more accurate model for the same complexity compared to a typical series RLC resonator model. Higher order modes can be more accurately modeled with added circuit complexity, but we show that this may not always be necessary. Because this method is based on the physical behavior of the fundamental modes, it also accurately connects circuit models to radiation patterns and other field behavior. To demonstrate this, we show that far field patterns, gain, and beam width of a dipole can be accurately extrapolated over a decade of bandwidth using data at two frequency points.

Magnetic Near-Field Probes With High-Pass and Notch Filters for Electric Field Suppression

Abstract: Several new types of low-cost and robust magnetic near-field probes manufactured in low-temperature co-fired ceramics (LTCC) are presented in this paper. Parallel C-shaped strips and their variations are inserted into the loop area in the front end of probes to achieve common-mode high-pass and notch filters for electric-field noise suppression. These probes with this kind of filter have excellent wideband electric field suppression. They are called high electric field suppression probes type A ~ D. The size of loop aperture in all probes is 100 μm long and 400 μm wide. The signal received from the loop is routed to a measurement apparatus through a semi-rigid coaxial cable with an outer diameter of 0.047 in. The flip-chip junction with low loss and good shielding is used between the probe head in LTCC and the semi-rigid coaxial cable. We take the probes over a 2000-μm-wide microstrip line as device-under-test to measure the probe characteristics. The isolation between electric and magnetic fields for a reference probe based on an old design using the same LTCC process is better than 30 dB from 0.05 to 12.65 GHz. The type A probe has two parallel C-shaped strips, it has better isolation of 35 dB from 0.1 to 11.05 GHz. Type C has one end of its strip shorted to ground, its 30-dB isolation frequency range can be extended to 0.05 ~ 17.8 GHz. With additional layout variation in type D, isolation can be improved to 40 dB up to 10.9 GHz. The spatial resolution for these probes is 140 μm when the distance between the metal surface of the microstrip line and the nearest edge of the loop is held at 120 μm. The calibration factors of the proposed probes are only slightly increased as compared with reference probe.

Design of Wideband, FSS-Based MultiBeam Antennas Using the Effective Medium Approach

Abstract: We present the design, simulation, and measurement results of a broadband, low-profile, multibeam antenna. The antenna uses multiple feed elements placed on the focal plane of a planar microwave lens to achieve high-gain, multibeam operation with a wide field of view. The lens is based on a recently reported design employing the constituting unit cells of appropriately designed miniaturized-element frequency selective surfaces (MEFSSs) as its spatial time-delay units. A new technique for modeling such lenses is also presented that greatly simplifies the full-wave electromagnetic simulation of MEFSS-based lenses. This technique is based on treating the pixels of the lens as effective media with the same effective permittivity and permeability and significantly reduces the difficulty of modeling and optimizing the proposed multibeam antenna with its relatively large aperture size in a full-wave electromagnetic simulation tool. Using this procedure, a prototype multibeam antenna operating in the 8-10 GHz range is designed. The prototype is fabricated and characterized using a multiprobe, spherical near field system. The measurement results are in good agreement with the simulation results obtained using the proposed simplified modeling technique. Measurements demonstrate consistent radiation characteristics over the antenna's entire operational band with multiple beams in a field of view of ±45°.

Theory of weak scattering of stochastic electromagnetic fields from deterministic and random media

Abstract: The theory of scattering of scalar stochastic fields from deterministic and random media is generalized to the electromagnetic domain under the first-order Born approximation. The analysis allows for determining the changes in spectrum, coherence, and polarization of electromagnetic fields produced on their propagation from the source to the scattering volume, interaction with the scatterer, and propagation from the scatterer to the far field. An example of scattering of a field produced by a $\ensuremath{\delta}$-correlated partially polarized source and scattered from a $\ensuremath{\delta}$-correlated medium is provided.