A world connected through cellular devices has brought about a technological explosion at all the levels. Starting with the world of semiconductors, with miniaturization and industrial scale-up to get lower production costs, following by mechanical design and finally, the design of the antennas. In this post we will analyze the state-of-art of the antennas, their evolution during these last years and their possible future.
Far away are those years in which a radio equipment had a huge antenna to connect with another. It was easy to know who was a radio ham because he had a huge mast on the roof of his home or in his car. It was more rare to see antennas in the cities, since television, radio or short and medium-range maritime communication services were located in repeaters on high
tops, far away the cities, to achieve the maximum coverage in a certain area. The broadcast repeaters, in addition, reprocessed the signal to be broadcast again, and were known for their high masts with many antennas.
When mobile telephony was extended, its cell-shaped design caused
that antenna infrastructures arrive at the urban areas. The emission
models also had to be modified considerably, because although for the
long-range repeaters the Friis free space model worked reasonably well,
in urban areas the model had difficulties due to the buildings, having to
develop more advanced emission models such as Okumura-Hata.
The design of the antennas also had to be optimized. In Europe there was already a lot of knowledge of antennas, especially in mountainous countries, such as Spain, Italy, the Balkan republics such as Serbia or Croatia, which are prestigious antennas manufacturing countries.
The United Kingdom, one of the pioneers in radio transmission, also contributed its knowledge, and this involved new designs, optimized for cellular networks.
Not only the antennas of the cells had to be optimized. The terminals were increasingly smaller and covered several bands, so the classic antenna technology, usually a monopole in the device, gave way to a new technology and an explosion of publications in impact magazines on antenna designs for these mobile apps. Multiband antennas printed on the PCBs were
occupying the space of the old antennas of the first cell phones and allowed their miniaturization.
This new generation of antennas was possible thanks to the three-dimensional design and calculation tools that allowed getting more reliable results than the old trial and error in the first antenna designs. The antennas were increasingly better parameterized, with notable
improvements in their efficiency.
From resonant antennas to multiband antennas.
As we have said, the first antennas of cell phones were resonant antennas, usually monopole. No other type of antenna was necessary since the cellular phones operated in a narrow band of frequencies, at 450 MHz the first analog terminals and at 950 MHz the digital terminals. By including the dual band (950 and 1800 MHz), it was necessary for the antennas to work on both frequencies, and this was not achieved with the resonant antennas. In addition, resonant antennas took up a lot of space and should be reduced.
The technology in the manufacture of PCBs has improved greatly since the 80s. PCBs are more stable, can be made multiple layers and can integrate the antennas. On the other hand, flexible structures have entered strongly into the market of electronic design, which allows the integration of antennas in small spaces.
On the other hand, the technology and research centers are publishing dozens of articles with innovative designs, mainly thinking about their use in mobile devices: multiband antennas, PIFA and MIFA resonant for WiFi and Bluetooth, NFC antennas for contactless connection, antennas for
charging wireless, etc.
In addition to the classical antennas, other types of antennas such as Vivaldi, fractals and those made on metamaterials are being studied. In this way, the world of classic antennas has evolved to techniques with great efficiencies and bandwidths.
These antennas have evolved, to a large extent, due to advances in the study of new materials.
In addition to the classic substrates, with ceramics that have low dielectric losses, and polyamides or polyesters, which allow the manufacture of antennas with very low profile, antennas are being made with metamaterials, which are artificial substrates where negative refractive indexes are achieved, achieving more efficient resonators. Graphene, a material with very high conductivity and where laminations with an atom thickness have been achieved, has also entered with force through nanotechnology. Everything seems to indicate that it could work well above THz, although it has not yet been possible to manufacture an antenna with
this material at those frequencies. This would allow getting very high connection speeds.
The new materials in the world of antennas
In spite of the great advances in the design of the antennas made by the research centers, it is clear that the next step is the research on new materials. As already said, fractal antennas and ring resonators using metamaterials are the next step in this evolution. But current materials still have electric limitations when we want to increase the operating frequency. Although ceramic materials have highly stable dielectric characteristics, they are still limited in frequency, and remain rigid.
Polyesters and polyamides also have a higher frequency limitation. They are currently used in the 2.4 GHz ISM band, but many antennas have not been developed in the 5.8 GHz band, and at higher frequencies they do not work well.
For this reason, progress is being made in the study of new materials, such as graphene and silicene, as materials for the immediate future. With these materials the profiles can become so small that they could easily be integrated into new generation terminals. And currently research projects are being created that focus on the study of liquid materials. Let us see how this new technology is.
Liquid materials for the antenna manufacturing
In the United Kingdom, a project for the development of liquid antennas has begun. The fundamental features of these antennas would be that they could be configurable regarding to the metallic antennas. They would also have the advantage of being transparent and small.
Despite these advantages, the biggest disadvantages of the liquid state are thermal stability, since the electrical and mechanical characteristics of liquids undergo considerable changes with respect to solid state devices. For example, water, when it reaches 0 degC, changes from liquid to solid state, changing its electrical and mechanical characteristics.
The liquid antennas must remain their status in very large thermal ranges, since they should be able to operate from -30 to 60 degC without changing the state and electrical characteristics. They must, therefore, be very stable so that their results do not vary at the operating frequencies.
This opens a new work line where material technologists are required, together with experts in Electromagnetism, working together in the research and development of this type of antennas.
The main research line of the new antennas will focus on the next stages:
1) Identify those liquid materials that show electrical, thermal and mechanical stability in a high range of temperatures and power levels. Something that liquid materials have is that they present higher losses than solid ceramic components. They are not normally good conductors of electromagnetic radiation and attenuate it easily.
2) Achieve the development of compact and flexible antennas with these new materials, being able to reconfigure the antennas according to the needs.
3) Achieve the industrial scaling so that they can be manufactured in series, lowering their manufacturing processes and the costs of raw materials.
This will be a long-term project, but we must also consider that the use of new materials is also in the development phase. Prototypes have been made, but the antennas developed with new materials are still under researching stage. But seeing the progress of recent years and with the current research centers, the future for liquid antennas is hopeful.