Kyocera Corp developed a small antenna whose transmission and reception performances do not lower even on metal, water surface, etc.
The antenna, "Amcenna," was exhibited at Ceatec Japan 2018, which took place from Oct 16 to 19, 2018, at Makuhari Messe, in Chiba City, Chiba Prefecture, Japan.
The exhibited antenna was targeted at 2.4GHz-band electromagnetic waves, but the Amcenna can support various frequencies. In addition, it has a large antenna gain for specific polarized waves. So, with appropriate angles, it becomes possible to closely place antenna devices, which has been difficult.
By exploiting the fact that the antenna can be installed in various locations with a wide range of antenna density, Kyocera plans to apply it to all kinds of IoT devices, MIMO (multi-input, multi-output) devices, healthcare devices and so forth.
'Right-handed' meta-material used
The performance of the Amcenna as an antenna does not lower even on metal, etc because its antenna element is made by using a "meta-material," Kyocera said. A meta-material is an artificial material whose effective dielectric constant (ε') and magnetic permeability (μ') were changed by periodically arranging "unit cells" (a kind of LC circuit).
Speaking of meta-materials, "left-handed meta-materials," which do not exist in nature and whose ε' and μ' values are both negative, are famous. However, the ε' and μ' values of the Amcenna's meta-material are both positive (right-handed).
The characteristic realized by using the meta-material this time is that its reflectance property for electromagnetic waves is "magnetic." Such a meta-material is called "artificial magnetic conductor (AMC)." The name Amcenna is a word created by combining "AMC" and "antenna."
When the reflectance property is magnetic, the phase of electromagnetic wave is not inverted at the time of reflection, Kyocera said. In general, when an electromagnetic wave is reflected from metal surface, there is almost no transmitted wave, and the electric field intensity on the metal surface is almost zero, inverting the phase of the wave at the time of reflection. It is so-called "reflection at a fixed end."
In this case, the intensity of electromagnetic wave near the metal surface is drastically lowered because the incident wave almost offsets the reflected wave. This is the reason why the performance of commonly-used antennas is drastically lowered on metal surface.
On the other hand, with an AMC, the reflection of electromagnetic wave is a "free-end" type, meaning that the phase of reflected wave is not inverted, Kyocera said. As a result, the incident wave does not offset the reflected wave, and the performance of antenna does not lower.
Size issue solved by using three-fold mirror as hint
In fact, such AMCs have been known for long, and, at least, a thesis was written about them in 2005. Antenna manufacturers including Kyocera have already developed some antennas that use an AMC as a substrate, etc. However, it had been difficult to commercialize them.
The size of antenna becomes large because it is necessary to periodically arrange many unit cells. For example, an antenna using an AMC that Kyocera developed in the past as a substrate measures 8 x 8cm and has a thickness of 3.8mm. It was made by arranging 13 x 13 unit cells like a lattice.
This time, Kyocera solved this problem by using a "three-fold mirror" as a hint, the company said. When a three-fold mirror is set at a certain angle and looked from the side, it looks that the face of the person is infinitely reflected.
Likewise, by making improvements to the boundary of 2 x 2 unit cells, for incident electromagnetic waves, it looks that many unit cells are spreading even outside the boundary. As a result, 2 x 2 unit cells became enough, enabling to reduce antenna area to 1/60.
Strictly speaking, the principle that enabled to realize the small size of the Amcenna is different from that of three-fold mirror. In the case of three-fold mirror, there look to be many faces because light is actually reflected from the mirror many times.
On the other hand, in the case of the Amcenna, electromagnetic waves are not repeatedly reflected in the antenna. By "producing a periodic boundary condition on the boundary of the AMC," Kyocera realized an effect similar to the effect of arranging many unit cells, the company said.
AMC functions as antenna element
Kyocera made one more improvement to an antenna prototyped by using a previous AMC. Specifically, the company used the new AMC itself as an antenna element. The previous AMC was used mainly for blocking the influence of metal.
This time, however, Kyocera made improvements to the design of the new AMC including its shape so that it functions as an antenna element (a kind of dielectric antenna). As a result, the thickness was reduced by half from 3.8mm to 1.9mm.
The Amcenna, which was developed in this way, has properties equivalent to those of existing antennas on the surface of nonmetal. On the other hand, while the properties of existing antennas drastically lower on metal surface, those of the Amcenna hardly lower, the company said.
High-density placement realized by high selectivity for polarized waves
The advantage of the Amcenna is not only that it can be placed on metal surface, etc but also that,even when multiple antenna elements are placed adjacent to one another, they do not affect one another much. Therefore, when it is applied to MIMO devices, which use many antenna elements, area occupied by antennas can be reduce.
This is because the Amcenna has a high selectivity for specific linearly-polarized waves. As a result, when two units of the Amcenna are located adjacent to each other and the orientation of one of them is 90° different from that of the other, they hardly affect each other.
In regard to the production cost of the Amcenna, Kyocera said, "It can be manufactured by using common substrate design technologies and processes, and, therefore, there is almost no factor that increases costs.