Magnetically Tuned 3D-Printed Antenna Arrays
Authors:   , Alhassoon Khaled A. , Daryoush Afshin S. 
Published: July 2021

3D additive printing technology has been employed recently for manufacturing of a wide variety of radio frequency (RF) circuits; particularly it is very attractive for composite manufactured structures in conformal (non-planar) shapes, such as magnetically tuned antenna arrays for both civilian and military applications. Low cost 3D additive printing of ferromagnetic materials and its tuning using applied external magnetic field eliminates the loading effects of electrical control on the antenna radiation pattern. Composite substrates using low-loss dielectric material filaments Acrylonitrile Butadiene Styrene (ABS) and Polylactic Acid (PLA) combined with ferromagnetic nanoparticle powders (FeCo, NiFe2O4, and ZnFe2O4) and metallic microstrip traces (copper and Ti₃C₂T_x as 2D MXene) is used to realize magnetically tuned radiating elements. Design modeling of 3D printed magnetically tuned antennas depends on accurate RF properties of 3D printed composite substrates in term of complex permittivity, complex permeability, and metal conductivity. Primary efforts are made by extracting RF characteristics of composite substrate in terms of filling factor (10% - 100%), printing patterns (rectilinear, triangular, ...), and metallic layer thicknesses. Accurate extractions are presented using curve fitting of simulated to measured scattering parameters of custom designed circuits. The extraction was performed over broadband (1-10GHz) using microstrip transmission line (TL) and narrowband using annular ring resonator (ARR) (2.4GHz and 5.4GHz) and metallic cavity resonator (2.4GHz). A statistical analysis of three samples of each category is performed to have an accurate extraction process. According to our extraction, the ABS filament had lower loss tangent than PLA. The complex permittivity of ABS filaments with triangular pattern and 10% infill had an average value of 1.36-j0.008, whereas with 100% infill it was around 2.50-j0.030 at 2.4GHz. The next extraction process is focused on extracting complex permeability of soft magnetic nanoparticles of NiFe2O4 using perturbation technique. The average extracted complex permittivity and permeability of NiFe2O4 were 3.3-j0.1 and 2.5-j0.6, respectively; while the complex permeability was 2.2-j0.08 with 4.3kG externally applied magnetic field over WiFi bands (IEEE802.11a (5.4GHz) and b (2.4GHz)). 3D printed copper metallic traces are extracted with achieved conductivity of 5.8x106 S/m using ARR. A contactless method of capacitively coupled TL is developed for extracting the conductivity of MXene films with the highest conductivity of 1.2x106 S/m for 4.3 [mu]m thick spray coated film. 3D printed magnetically tuned probe fed annular ring antennas (ARA) and arrays (1x2 and 2x2) were designed, manufactured, and evaluated against baseline performance using RT/Duroid for WiFi bands (2.4GHz and 5.5GHz). The 2x2 array resonates at 2.4GHz with a gain of 7.6dBi on the broadside and 100MHz tunability with an applied external magnetic field of 4.3kG. Improvement in antenna gain is demonstrated using singly stacked antennas. Furthermore, magnetic tuning of ARA is improved by 20% using 1-D electromagnetic band-gap (EBG) engineering technique with benefits of reduced volumetric losses since smaller magnetic nanoparticles are used. The characteristics of the tunable circuit is investigated using the B-H curve, which was measured using a vibrating-sample magnetometer (VSM). In addition, four spiral loops were designed and integrated to modulate the magnetic field intensity by external pre-biasing of 30kA/m with +/-10kA/m modulating step size using 750mA of induced current. Individual magnetically tuned ARA gain improvement of 4dB and 6dB is demonstrated using singly stacked passive elements of equal ring dimensions above the active ARA. Moreover, 3D printed frequency selective surface (FSS) with a stopband at 5GHz and passbands at 2.4 and 8-10 GHz was also demonstrated. Finally, 3D printed ARA and antenna arrays (1x2 and 2x2) on spherical surfaces of 15, 30 and 45mm curvatures radii are designed at 2.6GHz. Recommendations for future work are include: improvement in ferromagnetic materials' losses, aperture coupled antenna realization of and FSS structures above conformal arrays.