Coronavirus-like all-angle all-polarization broadband scatterer

The study addresses how to design compact electromagnetic scatterers that exhibit large, broadband, angle- and polarization-independent radar cross-sections. Conventional analytical solutions exist only for simple shapes, and numerical optimization of wavelength-scale structures can be computationally intensive. Creeping waves circulating around curved conductors are known to strongly influence scattering (e.g., Mie resonances of spheres), typically either exploited or suppressed depending on application. At L-band/low-GHz radar frequencies, rounded aircraft features enhance returns via creeping waves, motivating suppression in stealth. Conversely, in applications such as radar chaff, deception, and RF beacons, increased radar visibility is desired. Existing solutions split into subwavelength resonant scatterers (often polarization-sensitive, narrowband) and large physical-optics structures (e.g., corner reflectors), which are very broadband and polarization-insensitive but bulky (~10 wavelengths per dimension). This work proposes a new class of “corona”-like, spiky metalized spheres that leverage both resonant effects and controlled creeping-wave interference to achieve broadband, all-angle, all-polarization strong scattering from compact, lightweight structures.

Prior work on electromagnetic scattering highlights Mie theory and the role of creeping waves for canonical shapes. For high radar cross-sections, two main approaches prevail: (1) subwavelength resonators enabling superscattering via multiple resonances, offering large cross-sections but typically with polarization bias and limited bandwidth (e.g., Refs. 4–17); and (2) large corner reflectors and other physical-optics designs that are polarization-independent and extremely broadband but require dimensions on the order of ~10 wavelengths, making them impractical at low-GHz frequencies. Surface-wave engineering via subwavelength texturing and corrugations is established in microwaves (Refs. 21–24), and pyramidal absorbers are used to suppress scattering in anechoic chambers. Topology optimization has been beneficial in photonics and electromagnetics (Refs. 18–20), guiding geometry evolution toward performance objectives. Building on these, the present work designs a spiky spherical surface to manage resonant creeping waves for broadband constructive interference with specular reflection.

Design and optimization: Starting from a smooth metallic sphere (broadband, all-angle, all-polarization baseline), the surface is functionalized with an array of pyramidal spikes arranged over the sphere (R = 20 mm bounding radius). The spikes effectively form gratings with varying local periods (via elevation), intended to support resonant creeping waves around the circumference. The cost function maximized is the total scattering cross-section (SCS) over 6–16 GHz, with a flatness constraint of ≤3 dB variation across the band. The geometry is constrained to fit within the R = 20 mm virtual sphere. Two top designs from the optimization have N = 98 spikes with (width, height) parameter pairs of (50, 50) and (80, 50) (in dimensionless units corresponding to 1.5, 11 and 2, 11 mm, respectively). Full-wave simulations evaluate total, forward, and backward SCS and angular/polarization dependence. Time-domain analysis of creeping waves: To reveal the mechanism, a 0.25 ns Gaussian pulse centered at 6 GHz is used in simulation. The field is monitored for 20 ns. The backscattered temporal response is compared between a smooth sphere and a corona scatterer. Separate peaks corresponding to specular reflection and delayed creeping-wave returns are identified, and delays are related to path differences (≈πr + 2r) around a sphere of radius r = 20 mm. Fabrication: Experimental samples are fabricated via additive manufacturing. A 3D-printed plastic skeleton is subsequently metalized with a micron-thick conductive layer, enabling electromagnetic behavior akin to solid metal while remaining lightweight and low-cost. Multiple samples (including smooth sphere, dipole reference, and two corona variants) were produced. Experimental setup and measurements: Experiments are performed in an anechoic chamber with broadband (2–20 GHz) NATO IDPH-2018 horn antennas connected to an N5232B PNA-L network analyzer (300 kHz–20 GHz). The sample under test is placed in the far field. Four types of measurements are conducted: (1) forward scattering to derive total SCS via the optical theorem; (2) backward and forward SCS; (3) time-domain reconstruction of the reflection using S11 measurements with calibration using a metal mirror at 1 m to observe pulse splitting and delays of creeping waves; and (4) near-field mapping at 8 GHz using a non-resonant dipole probe to visualize field concentration around spikes. Numerical multipole expansion (Cartesian multipoles) is also performed to dissect contributions from various orders across frequency (details in Supplementary Note S6).

• Two optimized corona scatterers with N = 98 spikes and (width, height) = (50, 50) and (80, 50) (corresponding to 1.5, 11 mm and 2, 11 mm) approximately double the total SCS relative to a smooth 20 mm radius metal sphere (factors of 1.98 and 2.03), and exceed a resonant half-wavelength dipole (16 mm length, 8.5 GHz resonance) by factors of 8 and 8.2, respectively.
• Bandwidth: The corona scatterers exhibit a scattering bandwidth about 3× broader than the dipole (half-width at maximum evaluated on smoothed spectra).
• Angle/polarization insensitivity: Simulations and experiments show variations of only a few percent across the band for different angles and polarizations, as expected due to the uniform distribution of many spikes.
• Time-domain evidence of creeping waves: In simulation with a 0.25 ns pulse at 6 GHz, two peaks are observed—specular return and a delayed creeping-wave return. The specular return for the corona scatterer is delayed by ~0.05 ns versus a smooth sphere due to the patterned surface. The creeping-wave amplitude for the corona scatterer is more than double that for the smooth sphere. The delay between the specular and creeping peaks is ~0.36 ns (simulation) and ~0.33 ns (experiment), corresponding to a path difference Δd ≈ 100–105 mm, consistent with πr + 2r (r = 20 mm).
• Scattering directionality: Measurements show backward scattering from the corona device is significantly higher than that of the smooth sphere. The observed spectral ripples (“wavy behavior”) correspond to resonant creeping waves.
• Multipole analysis: In the 6–8 GHz range, several multipoles spectrally overlap and constructively elevate the SCS beyond a single dipole. At higher frequencies (e.g., ~12 GHz), a truncated expansion with six basic multipoles underestimates total SCS (~2× mismatch), indicating the need for higher-order terms as the object becomes electrically larger.
• Near-field mapping at 8 GHz reveals strong field accumulation around the spikes, corroborating their role in enhancing scattering via local resonances and creeping-wave coupling.
• Spike height trade-off: Increasing spike height boosts low-frequency scattering but degrades high-frequency response and reduces overall bandwidth relative to a smooth sphere (Supplementary Note S5).

The results demonstrate that engineering a spiky, corona-like surface on a compact metalized sphere can harness and amplify creeping waves to achieve strong constructive interference with specular reflection across a broad frequency range. This approach addresses the challenge of achieving large, broadband, polarization-independent radar cross-section with compact size, which neither traditional subwavelength resonators (often narrowband and polarization-sensitive) nor large corner reflectors (bulky) fully satisfy. The combination of resonant cascading (dominant at lower frequencies) and physical-optics phenomena (creeping-wave management at higher frequencies) explains the extended bandwidth and improved SCS. Time-domain measurements validating increased creeping-wave amplitudes and delays consistent with geometry confirm the mechanism. Enhanced backward SCS and minimal dependence on incidence angle and polarization indicate suitability for practical applications such as radar deception (chaff), electromagnetic beacons, and calibration targets where compactness, broadband operation, and robustness to polarization are required. The observed trade-offs with spike height and the need for more complex surface structuring to tailor group velocity and dispersion suggest pathways to further optimize performance.

A broadband, all-angle, all-polarization corona scatterer was designed, fabricated, and validated. Topology-guided optimization of spikes on a spherical surface maximized total SCS and bandwidth within a compact 20 mm radius constraint. The final designs outperform a resonant dipole by nearly an order of magnitude in both peak cross-section and bandwidth, while doubling the SCS of a smooth sphere. The broadband response stems from a hybrid mechanism: cascaded resonant multipoles at lower frequencies and controlled creeping-wave interference at higher frequencies. Experiments in an anechoic chamber, time-domain analyses, multipole decomposition, and near-field scans all corroborate the mechanism and performance. Future work could explore more complex surface morphologies (e.g., spoof plasmon structures) to induce larger group delays and further enhance or tailor broadband scattering, as well as systematic optimization of spike geometries to balance low- and high-frequency performance.

• Design space and constraints: The optimization is constrained by an enclosing radius R = 20 mm and guided by an initial spherical geometry; topology optimization’s convergence can depend on the initial guess and may not guarantee a global optimum.
• Spike height trade-offs: Taller spikes improve low-frequency scattering but degrade high-frequency performance and reduce overall bandwidth (Supplementary Note S5), requiring compromise in geometry selection.
• Multipole model limits at high frequency: A truncated multipole expansion (six terms) does not capture total SCS at higher frequencies, indicating the object becomes electrically large and requires many higher-order terms or physical-optics descriptions.
• Group delay control: While a modest specular delay (~0.05 ns) is observed due to surface patterning, achieving substantially larger delays likely requires more complex surface morphologies (e.g., engineered spoof plasmons), which were not implemented here.
• Experimental estimation relies on the optical theorem from forward scattering and chamber calibration; while comparisons to simulations are good, uncertainties and chamber artifacts can influence absolute SCS values.