RCS Reduction in Wideband Antennae: How I Replicated a Programmable Loaded Metamaterial
Stage two of my latest project complete! After researching into the applications of graded metamaterials for antireflection coatings on solar PV (check out my article below), I decided to simulate one myself.
Fundamentally, metamaterial coatings can be applied to a variety of use-cases, and still serve the purpose of optimizing energy conversion efficiencies through their included subwavelength structures. A wonderful research paper that showcased these applications in wideband antennae for radar cross section reduction and gain enhancement (coming up!), was able to load metamaterial heterostructures in a way that improves the energy radiating performance, as well as the scattering performance of wideband antennae. This had long been something I wanted to enhance in solar PV cells by substituting typical PN junction structures with new and improved metamaterial subwavelength structures that had rational designs (see article above).
Why Reduce RCS?
Radar Cross Section, or radar signature, is basically how detectable an object is on Radar. RCS is crucial for determining the positions and sizes of targets, making them important in military applications. In this scenario, wideband antennae are used in military vehicles and automobiles, so RCS reduction plays a crucial role in ensuring that such vehicles are not easily detected by guided weapons and other Radar-based projectiles.
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You like chess? I replicated the findings of the earlier linked research paper: Reducing the RCS of a wideband antennae using two artificial magnetic conductors that form a metamaterial surface array that looks like a chessboard, and yes it’s actually called that:
I simulated both Artificial Magnetic Conductor (AMC) patches with a metallic ground with dimension parameters specified by the research paper, using Ansys HFSS:
These metamaterial structures become the building blocks for the CLMS that eventually gets loaded onto a wideband antenna patch, fed by an antenna cable that transmits the absorbed gain. The entire diameter of this structure is smaller than a millimeter. These AMC unit cells were selected with the intention of reducing RCS in the wideband frequency range, such that they satisfy the condition that the phase difference of the incident waves is less than 180 degrees. Basically, light shouldn’t be allowed to completely change phase when it reflects off a surface that has a higher refractive index than air. So the AMC metamaterial surface that is loaded onto the antennae works perfectly in achieving this, because:
- they have negative refractive indices, and
- antennae designed for the wideband frequency range deal with wavelengths that are generally in the microwave region or lower, a majority of which is contained in the infrared spectrum. As I described in my previous article, optimizing the efficiency of solar panels for these subwavelengths was a must. So the possibility of using the principles in this replicate exercise in an actual solar PV remain open!
After programming each unit cell to perform the same function that it would in a CLMS loaded meta-surface, I got these results:
AMC Unit 1 — Wideband Square Patch Parameter Plot
AMC Unit 2 — Jerusalem Cross Patch Parameter Plot
These are the same findings obtained in the research paper, where it can be seen that both AMC units operate individually at different frequency bands. At 0 degrees, there’s a dual reflection phase seen for AMC Unit 2 at 9GHz and 18.9GHz, whereas in AMC Unit 1, a single phase at 12GHz can be seen. The craziest thing was that although all simulations were run in the exact same fashion, the values obtained for Unit 2 are not at all equal to those obtained in the paper, which could be due to software discrepancies since the margin of error is around ±1GHz. Ultimately, both cells’ performances were observed over the same wideband frequency range: 7–19 GHz, so these results provide amazing foresight into whether similar structures could be assembled as surfaces for solar irradiation which is the main goal.
How Do We Go About From Here?
Well, here’s a concept-replication of what the 3x3 loaded CLMS meta-surface would look like, if the middle array of unit cells was replaced with a radiation patch fed by the antennae cable. The surface’s substrate is made of a polymer with a permittivity of 2.65, which is again another specification prescribed by the research paper for optimizing reflectivity:
As I go on exploring these mechanisms, it becomes quite the challenge to understand the applications of RCS reduction in solar PV. RCS appears to be completely out of the question because solar panels are used for a completely distinct purpose. If we circle back to the main objective of finding metamaterial subwavelength structures, it is to maximize absorptive efficiency in a desired frequency range by enhancing absorption and scattering effects in a PV cell. There are more ways to design a solar cell that incorporate nanoimprinted metamaterial structures than just the typical ionic lattice array that I talked about in the previous article. With the upcoming Replicate 2, we’re going to explore those possibilities through silicon-based solar PV with moth-eye antireflection coatings, and see where parallels between these two applications of programmable metamaterials can be drawn.
Some things I took away from this replication are that metamaterial surfaces can be loaded using different orientations, geometries, and techniques onto uniquely purposed devices to achieve spectral selectivity . The research paper I cited even deduces that rotating AMC Unit 2’s radiation patch by 45 degrees produces similar phase difference results, meaning multiple metamaterial configurations can achieve what we really want — which is an improved radiation output. If we can apply that same loading logic to solar PV, then we move one step closer to the finish line.
Perhaps, RCS or not, these wideband antennae metamaterial structures shed some necessary light on how we can optimize solar PV for infrared applications using these brilliantly designed pieces of magic. In fact, in order to achieve a reflection phase difference of 0 degrees that we saw in the wideband antennae, a graded refractive index seems to be the way to go.
The only way is to keep exploring.
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References:
https://ietresearch.onlinelibrary.wiley.com/doi/full/10.1049/iet-map.2016.0746
https://ieeexplore.ieee.org/document/6374215