Experimental demonstration of highly localized pulses (X waves) at microwave frequencies

Abstract

A device that radiates transverse magnetic Bessel beams in the radiative near field is reported. The cone angle of the emitted radiation remains constant over a wide frequency range (18-30 GHz), allowing highly localized pulses (X Waves) to be generated under a broadband excitation. The design process, based on ray optics, is discussed. Both frequency and time domain experimental results from a prototype are presented. The measured fields show close agreement with simulation results and demonstrate the radiator's ability to emit X Waves within its nondiffracting range.

A new class of nondiffracting pulses based on focusing leaky waves

“…As a final comment, we should stress that our results are very similar to those experimentally obtained at microwaves with a broadband Bessel beam radiator [10]. Differently from [10], the XWs reported here exhibit a gradual broadening and decay of the energy as long as the pulse overcomes the theoretically predicted nondiffractive range. Furthermore, we should remark that XWs reported here are of subluminal type, as opposed to the superluminal ones reported in [10].…”

“…In fact, according to [7], the confinement ratios along the radial C ρ and the longitudinal C z axes (defined as the ratios between the − 3 dB widths of the main spot along the respective ρ, z directions) and the maximum nondiffracting extensions (i.e., 2ρ ap along the radial axis, and z ndr along the longitudinal axis) are given by the following two exact analytic expressions: where j 0,1 ≃ 2.405 is the first zero of J 0 ( • ), and Δf is the fractional bandwidth. Through equations (9) and (10), it is easily seen that, when Δf = 20% and ρ ap = 40λ = 20 cm, a good confinement of the pulse along both the transverse and the longitudinal axes (i.e., both C z ≪ 1 and C ρ ≪ 1) is obtained for axicon angles within the range 5°≤ θ 0 ≤ 30°. Such a cone dispersion is larger than that exhibited in Fig.…”

“…3(a) (viz., 10°≤ θ 0 ≤ 30°), thus confirming the consistency of the proposed LWA design. It should be stressed that equations (9) and (10) are exact only in the nondispersive case, i.e., θ 0 ( f) = θ 0 , but they still work as lower-bounds for the dispersive case. In general, the minimization of the cone dispersion is highly desirable since it determines the frequency-dependent character of z ndr [through equation 7], which is responsible of the spatio-temporal spreading of XWs: the higher the dispersion, the faster the pulse will broaden as it propagates [7,8].…”

“…Since we were interested in propagating the pulse over a distance z = z ndr (f max ) = 108cm, and the longitudinal group velocity v z of the pulse is approximately equal to v z = 0.696c 0 (c 0 being the speed of light in vacuum), the pulse has been observed over a period T=5 ns. To avoid fictitious replicas for 0 ≤ t ≤ T due to aliasing we have then numerically evaluated the integral of equation (10) using N⌈f 0 DfT⌉ = 60]] > frequency samples (where ⌈x⌉ returns the smallest integer ≥ x), according to the Nyquist-Shannon theorem.…”

“…therein). Just very recently, theoretical and numerical results have been reported in [7][8][9], whereas experimental results just appeared in [10].…”