**1. Introduction**

The concepts of traveling wave and standing wave can be used to categorize antenna types with corresponding dominant propagation mode for radiating. Traveling-wave antennas [1] often have long electrical length in the main propagation path, and the EM energy radiating proportion prevails over the reflected proportion in the propagation along the main path. In contrast, in standing-wave antennas, the EM energy reflects many times in the main propagation path. This causes the standing-wave or resonance phenomena and increases the EM field intensities with appropriate periodic excitation source, and this facilitates to increase the radiating proportion for a certain bandwidth (BW). The standing-wave antennas often have high Q-factor and with narrow BW, while these characteristics are opposite for the traveling-wave antennas. However, this categorization for the antennas is relative because the propagation mode for radiating depends on the

actual structure and is only dominant over a certain band of frequencies. Travelingwave antennas have been the research subject of many reports, and Vivaldi antennas [2] are the typical branch of traveling-wave antennas.

To model EM fields, characterize structural/operational features and optimize the performances for traveling-wave antennas, various approaches have been implemented. Transverse EM (TEM)-mode transmission line models have been used to describe the propagating and radiating mechanism of these antennas. For example, stepped-width transmission line slots connected end to end were used for the Vivaldi antenna, and the effect of the stepped discontinuities was solved by a power continuity criterion in [3], and in [4], a design process with least-square optimization was implemented for the calculation of input impedance and power division at the junctions of a stepped line by a transmission matrix chain. However, simplicity of the models constrained the accuracy and practical application of such methods.

Model accuracy improvements based on approximation to the conical slot lines [5, 6] yield electric field distributions and radiating fields with Green's functions. Diffraction at the end of the radiating slot and lateral edges was incorporated by a weight pattern for each edge. These improvements achieved the better predictions than the TEM-mode transmission line models.

While, in general, the EM propagation knowledge in a specific antenna structure is required for modeling, numerical three-dimensional EM solvers segment and discrete a structural space into a meshed volume of cells adapted to the material and geometric properties of the structure. Space, time and/or frequency distributions of EM energy in the volume are established, *a priori*, by solving Maxwell's equations numerically or approximately on a cell-by-cell basis for overall volume while taking account the boundary and excitation conditions. These EM distributions are the basic for derived total or global antenna characteristics, for example scattering parameters, far-field response.

Parallel improvements in EM wave theory, material characterization, numerical techniques, fast algorithms, and high-performance computing have realized faster, more accurate EM solvers for highly complex and real-life problems [7]. The improvements have even supported the design of complicated antipodal Vivaldi antennas from fractal fern leaf-shaped geometries [8]. By using EM solvers with feasible processes for design optimization, a variety of traveling-wave antennas with diverse EM responses have been proposed. Optimization of the conventional geometry and modifications/additions are the common methods to attain improved antenna performances such as in [9–12]. To explain the complex EM characteristics of the antenna geometries and the effects from the added elements in [10–12], direct observations of EM near-field vectors and of metal surface currents were implemented.

In a new near-field propagation research, an analysis method for the propagation of distributed near fields from a full-wave EM solver for the typical travelingwave antenna has been proposed [13]. With an adequately accurate data set of near field in the time and/or frequency domains for the antenna structure corresponding to a excitation condition, the EM fields in key regions can be evaluated and quantified to expose the relationships between the geometric properties and space, time and/or frequency EM energy distributions. The correlation characteristics reveal causal relationships of the geometric properties to the EM field propagation process.

Because the EM fields on the specific regions of the structure were observed and analysed based on time-domain impulse response analysis, the results were not be affected by superpositions of excited periodic cycles such as the conventional frequency-analysis method, it revealed the propagation processes of EM energy clusters and geometry-property influence details on the structure. Moreover, observations of EM responses at a consecutive point set along the dominant EM energy flows were implemented to analyse propagation progresses and the

*Near-Field Propagation Analysis for Traveling-Wave Antennas DOI: http://dx.doi.org/10.5772/intechopen.100856*

scattering components between the sections in the structure. This avoided locality in observation and permitted to overcome analysis bandwidth limits. Superresolution algorithms such as MUltiple SIgnal Classification (MUSIC) were also useful [14] to tackle these limits.

The rendered details in the space, time and/or frequency of the results are a powerful feature of the analysis method for the antenna design. The quantitative and qualitative analyses can be implemented to characterize for a subpart of the time and/or frequency EM energy response at each position in space and propagation mechanism dependency on a particular part of the structure. Design and optimization methods, built on such analysis, respond for refined adjustment of the structures. This approach reveals a new, deeper perspective in the hierarchy of antenna and related system design.
