Andy Pye discovers when antennas are small enough to be integrated into an electronic chip, classical electromagnetism and quantum mechanics overlap.
One of the biggest problems in modern electronics is that antennas are still quite big and incompatible with electronic circuits – which are ultra-small and getting smaller all the time.
Now, a team of researchers from the University of Cambridge may have unravelled one of the mysteries of electromagnetism, which could enable the design of antennas small enough to be integrated into an electronic chip.
“Antennas are one of the limiting factors when trying to make smaller and smaller systems, since below a certain size, the losses become too great,” said Professor Gehan Amaratunga of Cambridge’s Department of Engineering. “The size of an aerial is determined by the wavelength associated with the transmission frequency of the application – and in most cases it’s a compromise between aerial size and the characteristics required for that application.”
Certain physical variables associated with radiation of energy are not well understood – there is still no well-defined mathematical model to describe the operation of a practical aerial. Most of what is known about electromagnetic radiation comes from theories first proposed by James Clerk Maxwell in the 19th century, which state that electromagnetic radiation is generated by accelerating electrons. This model has no counterpart in quantum mechanics, where electrons are assumed to jump from higher to lower energy states.
However, the classical theory fails when dealing with radio wave emission from a dielectric solid, a material which normally acts as an insulator, in which electrons are not free to move around. Despite this, dielectric resonators are already widely used as antennas.
“In dielectric aerials, the medium has high permittivity, meaning that the velocity of the radio wave decreases as it enters the medium,” says co-researcher Dr Dhiraj Sinha. “What hasn’t been known is how the dielectric medium results in emission of electromagnetic waves. This mystery has puzzled scientists and engineers for more than 60 years.”
The theory is that electromagnetic waves are generated not only from the acceleration of electrons, but also from a phenomenon known as symmetry breaking: radiation resulting from broken symmetry of the electric field. It is thought that this phenomenon may provide the missing link between classical and quantum mechanics.
There are three basic classes of piezoelectric materials used in microfabrication:
* piezoelectric substrates, such as quartz
* thin-film piezoelectrics, such as lithium niobate, gallium arsenide, zinc oxide, aluminium nitride and lead zirconate-titanate (PZT)
* polymer-film piezoelectrics, such as polyvinylidene fluoride (PVDF).
Gallium arsenide-based amplifiers and filters are already available on the market and this new discovery opens up new ways of integrating antennas on a chip along with other components.
Working with the National Physical Laboratory and Cambridge-based dielectric antenna company Antenova, the Cambridge team found that, at a certain frequency, thin films of piezoelectric materials not only become efficient resonators, but efficient radiators as well, meaning that they can be used as aerials.
The researchers believe that the reason for this phenomenon is due to symmetry breaking of the electric field associated with the electron acceleration. Symmetry is an indication of a constant feature of a particular aspect in a given system: when electronic charges are not in motion, there is symmetry of the electric field.
“In aerials, the symmetry of the electric field is broken explicitly, which leads to a pattern of electric field lines radiating out from a transmitter,” Sinha adds.
By subjecting the piezoelectric thin films to an asymmetric excitation, the symmetry of the system is similarly broken, resulting in a corresponding symmetry breaking of the electric field, and the generation of electromagnetic radiation.
“If you want to use these materials to transmit energy, you have to break the symmetry as well as have accelerating electrons – this is the missing piece of the puzzle of electromagnetic theory,” said Amaratunga. “These results will aid understanding of how electromagnetism and quantum mechanics cross over and join up. It opens up a whole set of possibilities to explore.”
The future applications for this discovery are important, not just for miniature antennas to be used in mobile phones, but also in implementation of the Internet of Things, where almost everything is connected to the internet. For these applications, billions of devices are required, and the ability to fit an ultra-small aerial on an electronic chip would be a massive leap forward.
Reference: Sinha, D, et al. Electromagnetic radiation under explicit symmetry breaking, Physical Review Letters.
Real-time near-field testing of PCBs and antennas
Being able to resolve even intermittent problems early in the design process helps optimise designs and avoid unexpected EMC compliance test results.
Designers of antennas and high speed printed circuit boards can cut testing times drastically by using scanners to provide real-time images of EMI and antenna emissions, enabling designs to be evaluated in seconds.
The EMSCAN EHX benchtop scanner from MDL Technologies, an independent UK company supplying a comprehensive range of electronic test and measurement equipment, can be used for testing PCBs at frequencies as high as 8GHz. This allows engineers to visualise the root causes of potential EMC and EMI problems, which are particularly acute in boards designed for high speed, high power and high density or complexity.
For antenna testing, the RFX scanner can show far-field patterns, bisections, Effective Isotropic Radiated Power (EIRP) and Total Radiated Power (TRP). Novel near-field results, including amplitude, polarity and phase, can give insights into the root causes of antenna performance problems and help troubleshoot far-field radiation patterns.
Real-time analysis of embedded antenna designs permits multiple design iterations. This provides wireless engineers with the freedom to carry out rapid prototyping and explore new designs.
Printing low cost antenna with graphene ink
One of the first commercial products manufactured from graphene was conductive ink, which can be used to print circuits and other electronic components. Graphene ink is generally low cost and mechanically flexible.
Conventionally, to make the ink, graphene flakes are mixed with a solvent, and a binder is added to help the ink stick. Graphene ink with insulating binders usually conducts electricity better than binder-free ink, but only after the binder material is broken down by high temperature annealing. Annealing, however, limits the surfaces onto which graphene ink can be printed because the high temperatures destroy materials like paper or plastic.
Now, researchers from the University of Manchester have printed a radio frequency antenna using compressed graphene ink. Together with graphene manufacturer BGT Materials, they have increased the conductivity of graphene ink without resorting to a binder. They accomplished this by first printing and drying the ink, and then compressing it with a roller. This process increases conductivity by more than 50 times, and the resulting “graphene laminate” was also almost twice as conductive.
Being able to print electronics onto cheap, flexible materials like paper and plastic could mean that wireless technology, such as RFID tags, could become more widely available. Currently, most commercial RFID tags are made from metals like aluminium and copper, expensive materials with complicated fabrication processes that increase the cost.