[Paper Review] Proposte di Antenne generatrici di Fasci Non-diffrattivi per micro-onde (Proposal of apertures generating Nondiffracting Beams of microwaves)

Michel Zamboni Rached, Erasmo Recami+1 more|arXiv (Cornell University)|Aug 9, 2011

Antenna Design and Analysis3 citations

TL;DR

This paper proposes practical microwave antennas using annular slits to generate nondiffracting Bessel beams at 15 GHz with a 1-meter aperture, achieving a stable spot size of ~12 cm over 10 m of propagation despite finite aperture truncation. The method uses analytically derived amplitude and phase distributions across concentric slits to maintain beam integrity, demonstrating that even with realistic, small apertures, beam depth and spot stability can be significantly improved through optimized excitation profiles, enabling applications in remote sensing and high-precision microwave imaging.

ABSTRACT

We propose in detail Antennas for generating Nondiffracting Beams of Microwaves, for instance with frequencies of the order of few GHz, obtaining fair results even when having recourse to realistic apertures, with a quite reasonable diameter. The present proposal refers to sets of suitable annular slits. The possible applications are various, including remote sensing. The paper is in Italian. [Si propongono in dettaglio Antenne per la generazione di fasci non-diffrattivi di microonde, per frequenze ad esempio dell'ordine della decina di GHz, ottenendo discreti risultati pur ricorrendo ad antenne realistiche di diametro ridotto. La proposta e' quella di usare un set di opportuni Annular Slits. Le applicazioni possibili sono varie, includendo il remore sensing.]

Motivation & Objective

• To design practical microwave antennas capable of generating nondiffracting beams with finite apertures at frequencies around 15 GHz.
• To overcome the challenge of beam diffraction in small-aperture systems by using structured annular slits instead of ideal infinite apertures.
• To achieve a stable, narrow spot size (≈12 cm) over extended propagation distances (up to 10 m) despite aperture truncation.
• To demonstrate that beam quality can be significantly enhanced through precise amplitude and phase control across multiple annular slits.
• To enable practical applications such as remote sensing and high-resolution microwave imaging using compact, realizable antenna systems.

Proposed method

• The method employs a theoretical approach based on scalar diffraction theory to model truncated Bessel beams, replacing computationally intensive numerical integration with analytical expressions.
• Annular slits are used as the aperture structure, with the field amplitude at each slit set to the value of the Bessel function J₀(kρrₙ) at the radial position rₙ.
• The phase of the field is controlled by assigning alternating signs (±1) to successive slits, creating a π-phase shift between adjacent slits to enhance central beam concentration.
• Three prototypes are proposed with different excitation profiles: (1) standard Bessel amplitude, (2) amplitude scaled by √(n+1), and (3) uniform amplitude with alternating phase.
• The analytical method enables rapid computation of the far-field beam pattern, reducing simulation time from days to seconds compared to standard diffraction integral methods.
• The beam's transverse intensity profile is evaluated at z = 0 and z = 10 m to assess spot size stability and intensity decay.

Experimental results

Research questions

• RQ1 Can a nondiffracting microwave beam with a stable spot size of ~12 cm be generated using a finite-aperture antenna of only 1 meter diameter at 15 GHz?
• RQ2 How does beam stability and spot size evolve over 10 meters of propagation when the Bessel beam is truncated by a small aperture (R = 61 cm)?
• RQ3 To what extent can the beam's depth of field and spot quality be improved through optimized amplitude and phase excitation across multiple annular slits?
• RQ4 What is the impact of different excitation profiles (amplitude-only, amplitude+scaling, amplitude+alternating phase) on beam concentration and intensity at 10 m?
• RQ5 Can analytical methods replace full numerical diffraction integrals to accelerate the design and evaluation of such beam-generating antennas?

Key findings

• With a 1-meter aperture, the beam maintains a spot size of approximately 15 cm at 10 m of propagation, only slightly broadened from the initial 12 cm at the aperture.
• The intensity of the central spot decays by a factor of 1/3 over 10 m, but the spot radius remains nearly unchanged, indicating strong nondiffracting behavior.
• The third prototype, using uniform amplitude and alternating ±1 phase across slits, yields the most homogeneous intensity profile on-axis and the highest intensity at z = 10 m, outperforming the other two prototypes.
• The analytical method reduces computation time from days to seconds, enabling rapid evaluation of beam characteristics without relying on standard numerical diffraction simulations.
• Even with a small aperture (R = 61 cm), the beam shows delayed diffraction effects compared to the theoretical Z = 9.8 m cutoff, with significant degradation only starting at z ≈ 6 m.
• The beam's transverse intensity profile at z = 10 m shows that the field structure closely resembles a truncated Bessel beam, confirming the effectiveness of the excitation strategy.