Abstract
In this paper, a comparison is presented between Bessel beam launchers at millimeter waves based on either a cylindrical standing wave (CSW) or a cylindrical inward traveling wave (CITW) aperture distribution. It is theoretically shown that CITW launchers are better suited for the generation of electromagnetic short pulses because they maintain their performances over a larger bandwidth than those realizing a CSW aperture distribution. Moreover, the wavenumber dispersion of both the launchers is evaluated both theoretically and numerically. To this end, two planar Bessel beam launchers, one enforcing a CSW and the other enforcing a CITW aperture distribution, are designed at millimeter waves with a center operating frequency of f¯=60GHz and analyzed in the bandwidth 50 - 70 GHz by using an in-house developed numerical code to solve Maxwell's equations based on the method of moments. It is shown that a monochromatic Bessel beam can be efficiently generated by both the launchers over a wide fractional bandwidth. Finally, we investigate the generation of limited-diffractive electromagnetic pulses at millimeter waves, up to a certain non-diffractive range. Namely, it is shown that by feeding the launcher with a Gaussian short pulse, a spatially confined electromagnetic pulse can be efficiently generated in front of the launcher.
Highlights
In recent years, localized waves [1, 2] have gained increasing interest among researchers, because of their remarkable property of being non-diffractive, i.e. their transverse field profile theoretically does not spread out up to a certain distance called depth of field (DOF) or nondiffractive range [3]
We propose a comparative broadband analysis of two different approaches to generate EM pulses at millimeter waves by using a Radial Line Slot Array (RLSA); namely, by synthesizing either a cylindrical inward traveling wave (CITW) [9,25,26] or a cylindrical standing wave (CSW) [27] aperture distribution, whose respective mathematical description is an Hankel or a Bessel function, respectively
As outlined in [26], a proper slot arrangement shown in Fig. 1(b) allows for the synthesis of a RLSA Bessel beam launcher based on a CITW distribution of the form
Summary
In recent years, localized waves [1, 2] have gained increasing interest among researchers, because of their remarkable property of being non-diffractive, i.e. their transverse field profile theoretically does not spread out up to a certain distance called depth of field (DOF) or nondiffractive range [3]. Polychromatic solutions to Helmholtz equation can be obtained as a continuous superposition of monochromatic solutions on a certain frequency bandwidth They have been considered for the generation of localized pulses [10,11,12,13,14,15,16,17,18,19,20], i.e. efficiently spatially confined electromagnetic (EM) pulses. The spatial confinement of the generated EM pulses, both in the longitudinal and transverse directions with respect to the axis of propagation, is connected to the launcher dispersion characteristic and, it will be investigated in detail It has been shown in [29] that the bandwidth of the feeding pulse plays a key-role in the longitudinal confinement of the pulse, whereas Bessel beam beamwidth at the central frequency dictates the transverse confinement of the pulse. The proposed comparison clarifies strong points and intrinsic limitations of both RLSA CSW and CITW launchers for the efficient generation of EM pulses at millimeter waves
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