Broadband comparison of standing and traveling wave aperture distribution {RLSA} {B}essel beam launchers for {EM} pulse generation

In recent years, increasing interest was gained by the so-called diffractionless beams, among which Bessel beams assume a key-role. Since their early in- troduction (J. Durnin, J. J. Miceli et alii, Phys. Rev. Lett., 58, 1499, 1987), several attempts have been made to practically realize Bessel beams launchers, both at optical frequencies and at microwaves/millimeter waves. An interest- ing application of such launchers can be the generation of electromagnetic (EM) pulses, when they are fed by suitable broadband pulses. Hence, the analysis of the broadband response of the designed Bessel beam launchers is required. In this paper, we propose a broadband comparison of two different approaches to generate electromagnetic pulses with Radial Line Slot Antennas (RLSA) at microwaves, namely by synthesizing an inward cylindrical traveling wave (S. C. Pavone, M. Ettorre, and M. Albani, IEEE Trans. Anten. Propag., acc. for publication, 2016) and a standing cylindrical wave (A. Mazzinghi, M. Balma, et alii, IEEE Trans. Anten. Propag., 62, 3911-3919, 2014) aperture distri- bution. Since for RLSA antennas the impedance bandwidth is usually very large, the band limitation of the device is then mainly caused by the aperture distribution deterioration out of the central design frequency. For a Bessel Beam launcher, unlike standard antennas, the Gain is not a useful parame- ter to characterize the performance and the bandwidth. On the other hand the spatial confinement of the generated EM pulses both in the longitudinal and the transverse directions w.r.t. the axis of propagation is connected to the launcher dispersion characteristic, and will be investigated in detail. It is shown that the bandwidth of the feeding pulse plays a key-role in the longi- tudinal confinement of the pulse, whereas the beamwidth of the Bessel beam at the central frequency dictates the transverse confinement of the pulse (W. Fuscaldo, S. C. Pavone et alii, sub. to Opt. Expr., 2015). To make a balanced comparison, both the inward cylindrical traveling wave and the standing wave aperture distributions are analyzed when fed by a Gaussian pulse, and the dis- persion of the generated Gaussian electromagnetic (EM) pulses are analyzed. The proposed comparison clarifies strong points and intrinsic limitations of both the Bessel and the Hankel aperture distribution RLSA launchers for the generation of EM pulses at microwaves/millimeter waves.