A transmitarray (TA) antenna consists of an illuminating feed source and a periodic transmitting planar surface, which transforms an incoming wave into a desired outgoing one. It’s a good candidate for a low-cost antenna option for operating satellites in the X, Ku, K and Ka-bands, as it can exploit the advantages of phased arrays and lenses. While it has a better tolerance to surface errors than a reflectarray, the two do have one similarity – they can be made up of multiple layers of printed elements in varying sizes, which helps transform the incident spherical wave in a planar one.
We’ve seen several different kinds of 3D printed antennae over the last few years, but not this type. A group of researchers based at Italy’s Polytechnic University of Turin (Politecnico di Torino) published a paper, titled “3D-Printable Dielectric Transmitarray with Enhanced Bandwidth at Millimeter-Waves,” about their novel, perforated dielectric TA configuration.
The abstract reads, “In this paper, a three-layer dielectric structure is presented as innovative unit-cell element for Transmitarray (TA) Antennas with enhanced bandwidth. It consists of a central layer, with a varying size square hole, used to compensate the phase of the incident field and located between two other identical layers with linearly tapered square holes, acting as matching circuits. The effectiveness of this unit-cell is demonstrated by the numerical and the experimental results here presented. As a first step, three different transmitarrays with increasing size are designed and simulated: their 1-dB gain bandwidth, centered at 30 GHz, varies from the 30.9% of the smallest configuration, having size of 10λ0×10λ0, to the 17.5% of the 20λ0×20λ0 TA. A slightly modified unit-cell is then designed, with the aim of realizing a prototype with an Additive Manufacturing (AM) technique. A 3D-printed dielectric Transmitarray with a size of 15.6λ0×15.6λ0 has been manufactured and experimentally characterized. The measured prototype shows excellent performances, achieving a 1-dB gain bandwidth of 21.5%: these results prove the enhanced features of the introduced unit-cell and demonstrate the TA feasibility with AM techniques.”
The team’s proposed unit-cell is made up of a triple-layer perforated dielectric element. A square hole, with a variable size W, in the middle layer compensates the incident field’s phase, and the external layers, as the researchers wrote, “present a truncated pyramid hole with the smaller basis having size W and simulating a tapered matching circuit.”
“The addition of these tapered structures allows to improve the performances of the TA for what concerns its bandwidth,” the team explained.
TAs have a limited bandwidth, because of the “intrinsic narrow band” of the antenna’s radiating element and the spatial phase delay’s frequency dependence of the paths from the feed to each element. By adopting perforated dielectric layers, like the university team did, it’s possible to control the incident field’s phase by changing up the size of the holes.
“From transmission line theory, it is well-known that the impedance transformer bandwidth limitation can be overcome using a tapered transmission line, i.e. a transmission line whose characteristic impedance varies continuously according to a predefined profile, that is in most cases linear or exponential. This concept is applied here to design a three-layer TA unit-cell,” the researchers explained.
In layman’s terms, the team was able to enhance the TA’s bandwidth by exploiting this tapered matching concept.
Three different sized TAs were designed and numerically analyzed in order to test how effective the team’s proposed unit-cell is. The researchers also used a commercial Objet30 by Stratasys to 3D print a prototype of their dielectric transmitarray in two pieces, which they then simulated with CST MW Studio and experimentally characterized.
“The best suited solution for the TA fabrication seems to be the use of an Additive Manufacturing (AM) technique, since the use of a conventional dielectric, as the one considered in the numerically analyzed configurations, would require to make the variable size holes and this is impracticable by conventional machining approaches, especially at millimeter-waves,” the researchers wrote.
However, the team did have to modify the unit-cell slightly because of the limited resolution of the 3D printer’s nozzle; additionally, the minimum size of the original holes had to be increased.
“A medium-size prototype working on a band centered at 30 GHz has been finally manufactured and experimentally characterized: the obtained results confirm the good performances of the proposed configuration, achieving a 1-dB bandwidth of 21.5% and an aperture efficiency of 38.6%,” the researchers concluded. “The antenna features could be further enhanced when dielectric material with higher value of r will be available for high resolution printers.”
Co-authors of the paper are Andrea Massaccesi, Paola Pirinoli,Valentina Bertana, Giorgio Scordo, Simone Luigi Marasso and Matteo Cocuzza from IMEM-CNR, and Gianluca Dassano.
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