IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 62, NO. 8, AUGUST 2014 3911 Large Depth of Field Pseudo-Bessel Beam Generation With a RLSA Antenna Agnese Mazzinghi, Massimo Balma, Dorina Devona, Giacomo Guarnieri, Giuseppe Mauriello, Matteo Albani, Senior Member, IEEE, and Angelo Freni, Senior Member, IEEE Abstract—The paper discusses the possibility of generating a pseudo-Bessel beam, with a propagation distance of several hundreds of wavelengths in microwave and millimeter frequency band, by using a radial line slot array (RLSA). A specific appli- cation for non-contact microwave detection of buried mines has been considered as test case. The design benefits of a holographic approach to assure the required aperture field distribution and makes use of an ad hoc optimization tool to control the antenna slot layout. The predicted and measured antenna behaviors show that high efficiency and polarization purity can be obtained by such a compact and flat antenna, achieving at the same time both manufacturing and setup simplicity. Index Terms—Antenna arrays, Bessel beam, holography, local- ized waves, RLSA. I. INTRODUCTION G ENERATION of Bessel beams in the microwave and mil- limeter regime is a topic of very recent interest for the scientific community. Indeed, for these beams, the electromag- netic field remains confined in the vicinity of the propagation axis, and theoretically does not spread. Bessel beam is a partic- ular type of localized waves (LWs) [1], and it is a solution to the wave equations, as first shown by Stratton in 1941 [2]. However, such a theoretical solution would have an infinite aperture and energy. In actual fact, limited diffraction beams (pseudo-Bessel beams) can only be approximated over a limited depth of the field [3] by employing finite size antennas and finite energy [4]. Pseudo-Bessel beams are potentially an attractive alternative to using Gaussian beams in a number of scenarios. Moreover, the reconstruction properties of the beam enable new effects to be observed that cannot be seen with Gaussian beams [5]. In optics and acoustics, several techniques for launching Bessel beams exist [5, and references therein], [6]–[9]. A few Manuscript received April 30, 2013; revised February 20, 2014; accepted April 04, 2014. Date of publication June 02, 2014; date of current version July 31, 2014. This work was supported in part by the Italian Ministry of Defense within a PNRM project, Contract No. 060/09/0137 rep. 1197. A. Mazzinghi and A. Freni are with the DINFO, University of Florence, Flo- rence 50139, Italy (e-mail: agnese.mazzinghi@unifi.it; freni@unifi.it). M. Balma, D. Devona are with Selex ES S.p.A., Strada Privata Aeroporto, 10077 San Maurizio Canavese, Caselle, Turin 10077, Italy (e-mail: massimo. balma@selex-es.com; dorina.devona@selex-es.com). G. Guarnieri, G. Mauriello are with Selex ES S.p.A., Campi Bisenzio, Florence 50013, Italy (e-mail: giacomo.guarnieri@selex-es.com; giuseppe. mauriello@selex-es.com). M. Albani, is with the D.I.I.S.M., University of Siena, Siena 53100, Italy (e-mail: matteo.albani@dii.unisi.it). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TAP.2014.2324557 have been recently suggested in the millimeter-wave range [10]–[14]. In this paper, we prove that a radial line slot array (RLSA) [15], [16] can be effectively used for generation of Bessel beams with a propagation distance of several hundreds of wavelengths. The proposed antenna configuration presents very high radia- tion ( 95%) and focusing ( 80%) efficiencies. Further, the ad- vantages in using such a kind of antenna are manifold since it is planar, single-layer, low-profile, and simply fed in a single point at the center of the antenna. As a matter of fact, an RLSA consists of a radial parallel plate waveguide in which a radial propagating mode is excited to feed a slot distribution etched on the upper metallic plate. In Section III, we show a design relevant to a specific applica- tion for non-contact microwave detection of buried mines [17]. In this case, because of the relative low operating frequency (15 GHz) and for the operating environment requirements, we have chosen a low-cost, robust manufacturing technique. In par- ticular, the upper slotted plate of the RLSA is a laser cut steel metal plate, and a coaxial probe is used as feeder. Neverthe- less, the same configuration can be easily manufactured also at submillimeter and THz frequencies when printed technology is adopted and the coaxial probe is replaced by a slotted circular waveguide transition [18]. The design methodology, which has been detailed in [19] for axially symmetric far-field focusing antennas, has been here ex- tended to the near-field focusing and applied to synthesize the field distribution on the antenna aperture for the generation of the required pseudo-Bessel beam. This methodology can be in- terpreted as a holographic technique, as introduced in [12]–[14], followed by an optimization procedure that makes use of spe- cific fitness functions introduced on the basis of the physical pic- ture behind the working mechanism of the RLSA antenna. Dif- ferently from [13], here the antenna is not resonant. This permits to increase the antenna dimension (i.e., the depth of field) since the tolerance sensitivity is reduced. In addition, the absence of a metallic rim makes this solution scalable also at terahertz ap- plications. In [12], a nonresonant solution for near-field focused point is presented. It constitutes a real improvement of the rough solution introduced in [20], but the double slits designed to ra- diate do not give any control on the polarization, thus resulting in a quite high cross-polar component that is radiated outside the main beam, with a reduced efficiency. Moreover, since the RLSA configuration does not require a physical feeding network, it is possible to design very large ar- rays (60 free-space wavelength in our case) without resorting to 0018-926X © 2014 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.