I noticed that throughout the series of articles relating to radio based neural interfaces that the question of shielding came up time-and-time again. I thought I should take some time out and explain some key concepts so that everyone understands how EMI shielding works.
This article will be quite comprehensive in terms of what needs to be considered and help people understand where they are going wrong in their shielding solutions.
Introducing the Particles & Fields
The first thing we need to understand is the particles involved. Everything in reality is composed of simple building blocks of sub-atomic particles. This includes matter and fields. These sub-atomic particles combine to provide us with the elements (or atoms), such as hydrogen, oxygen, etc. Elements combine to give us molecules, such as hydrogen and oxygen combining to make a molecule of water (H2O).
We have a very solid understanding of the particles involved and the rules for combining them. In the sub-atomic world, the list of particles is called the "Standard Model" and the rules for combining them and the interactions is known as "quantum theory". Quantum theory has a range of sub-disciplines, for examples Quantum Electrodynamics (QED), which describe particular areas of interaction. The current list of particles in the "Standard Model" can be found here:
https://en.wikipedia.org/wiki/Standard_Model
The "Standard Model" is an area of intense study and periodically new particles are added to it. The most recent particle that most will be familiar with is the Higgs Boson (or God particle) currently being investigated at the LHC. The Higgs boson has not been added to the above diagram at this point.
If we look at the diagram, the particles in red communicate forces. This is the function of a 'Boson'. If a force changes, a boson is involved. The 6 particles in the purple area are the basic particles that make up the Proton and Neutron and known as Quarks. These are held together by the gluon (the 'g' in the force area). The green area are "Leptons", the formal definition of this grouping is quite complex but it contains electrons and Neutrinos. Electrons are the basis of all Chemical reactions and Neutrinos rarely interact with anything. Countless trillions of Neutrinos pass straight through the Earth every second without bumping into anything.
That's it for the "Standard Model". Everything else in the Universe are composites of these basic particles. Current scientific knowledge does not specify how these particles came into existence, there are theories but science has no overriding opinion at present and certainly no evidence. As spacetime itself will eventually end up on this list, there will be a major problem. As spacetime defines cause-and-effect, thus the entire underpinning of logic itself, the ability to logically infer after this point collapses. This will restrict our view of the Universe until a new form of logic emerges.
"Elements" are composed of and interact using the sub-atomic particles in the "Standard Model". An Element is an atom composed of Protons and Neutrons. An Element is defined by the number of Protons (composed of Quarks) and there can be variations of this Element, called Isotopes, that changes the number of Neutrons (composed of Quarks). In the next diagram, we can see how the particles from the "Standard Model" come together to create the atom:
Most of us will be familiar with the list of these Elements known as the "Periodic Table". You can brush up on this here:
https://en.wikipedia.org/wiki/Periodic_table
When Elements combine we get what is known as a molecule. Molecules are the basic building blocks of substances such as water, plastics, alloys, wood, etc. At this point, science makes an arbitrary distinction in the classification of molecules. Some are defined as organic and others defined as inorganic. Inorganic compounds are created through natural processes of geology whereas organic compounds are created by biological systems. There is another accepted view that any molecule that lacks carbon is defined as inorganic. In this next diagram, we can see the electrons (see standard model) in blue and the atoms in yellow and how the molecule is held together.
You can read more about molecules here:
https://en.wikipedia.org/wiki/Molecule
As we can see, currently there are three basic layers to reality, sub-atomic, elements (atoms) and molecules (chains of atoms). The three main branches of science, physics, chemistry and biology investigate each of these areas with extensive overlaps.
In more formal terms, all forms of physical science are sub-disciplines of physics.
Electromagnetics And Radio Engineering
Electromagnetics is highly complex but we will try to keep this simple. Electromagnetics is from the "Standard Model", so we will be referring back to the above diagram when talking about particles.
We can first split the EM field into two basic categories, radiation and non-radiation. In radio engineering, this is referred to when speaking of near-field and far-field effects.
Near Field - Non-Radiation
Far Field - Radiation
The far-field is simple to understand, it is a photon with a specific energy travelling in open space. If you refer back to the "Standard Model", we can see the photon is in the list of particles (top right). It is this particle that we make use of when receiving/transmitting radio, TV, WIFI, mobile phones, fibre optics, etc., and it travels at the speed of light The only difference is the Energy of the photons and the patterns of photon emission. The Energy of the photon dictates the frequency and the patterns are measurable characteristics that we can use to convey information. We state what the patterns mean, that could be a small amount of Photons for a '0' and a large amount for a '1' within a specific time period. Do that 1 million times per second and we have a 1 Mbit digital broadcast. But we are free to make up what the patterns mean as long as it can be consistently detected and applied to a received/transmitted signal. This process of pattern manipulation is known as modulation/demodulation or encoding/decoding. There are a range of measurable aspects that lead to a variety of modulations (i.e AM, FM, etc).
The Energy/Frequency of a photon alters how we perceive it and how it interacts with matter. Lower frequency photons are radio waves and as the Energy per photon increases, we observe this as visible light, x-rays, gamma rays, etc. This is what we refer to as the "Electromagnetic Spectrum" and it is captured in the following diagram. Please note that the terms "frequency" or "wavelength" are very old terms, but are still functional in Engineering terms. In modern science, we now refer to the Energy of the photon, measured in Electron Volts (eV), rather than the rate of change of the direction of a charged particle. A common misconception is that the photon is a wave with a particular size, or that the wavelength is a characteristic of the photon. A photon has no physical dimensions whatsoever, this will be important when we come to discussing shielding later. If you are thinking of an imaging radar at this point and wavelength limiting the resolution, it is due to the rate of change in the current in the receiver, not the photon. Its quite possible to create a high resolution imaging radar in lower frequencies through acceleration curves of electron clusters. This type of radar is statistical and requires highly complex DSP and detection methods.
In terms of interaction, the ability of a photon to penetrate an object depends on the Energy level the particles capable of absorbing a photon can accept. Thus, for every mm of a given substance, it can absorb a certain percentage of photons at a range of Energy levels or frequencies. Further, there are two basic ways that Energy can be absorbed, either conductively or magnetically, and the best approach to use depends on which aspect absorbs the most energy from the photons of interest. At lower frequencies, under 1Mhz, the Energy of the photon is mainly absorbed magnetically. We will understand this better as we move along.
Now that we have a good understanding of the basics of radiation, we can look at the near-field or non-radiative portion. This area is the particles either creating, reflecting or receiving photons. A good example is a radio antenna. The near-field would be around the surface and the fields extends a portion into space. The main particles involved in this are charged particles.
Charged particles can be sub-atomic (i.e. Electron/Proton), atomic anion/cation (i.e. monoatomic ion) or molecular anion/cation (i.e. polyatomic ion). Charged particles move when they absorb photons. In addition to this, we also have the magnetic component, which comes from both the motion of charged particles and the spin magnetic moment. The spin magnetic moment is due to the interaction between an intrinsic property known as spin and the intrinsic property of charge. Explaining spin is quite complex, it is a form of angular momentum similar to force felt when on a roundabout. The charged particle is not actually spinning, it just has the energy of something spinning. When this energy interacts with Charge, it is the same as a Charge in motion, which produces a magnetic field.
There are two sources of Electric Fields, Charge distribution and time-varying magnetic fields. Charge distribution creates a static Electric field, or a build up of voltage between Charged surfaces. Unless provided a route to ground, or the Charges are balanced, the voltage will remain. The term static is a bit of misnomer and it all depends on your "frame of reference" or "how you look at it". From the perspective, or "frame of reference", of the Electric Field it is static. Its just sitting there, not changing. If I now move a conductor through it, such as a copper wire, the wire will 'see' a moving Electric field from its perspective or "frame of reference". That is, as it transitions through the Electric Field, it will go from no voltage to experiencing a voltage. This induces a current in the conductor. A time-varying field is one that is changing from the perspective of the Electric field. These two types of electric field have different characteristics, a time-varying field will cause the emission of photons, as will a conductor transitioning a static field, as the Charged particles change direction. A similar relationship exists in magnetism and fields produced by electric currents as opposed to time-varying electric fields. The latter of which will produce an EM wave.
As we can see, applying a voltage to an antenna causes all the various particles and fields to interact. They all have different masses, require different amounts of energy and respond differently to the applied voltage. This means we get an absolute mess of fields an interactions, thus the photons emitted are a mixture of Energies/Frequencies but mainly in the area of the resonant Energy/Frequency of the antenna. One critical thing that must be taken from this is that any object subjected to photons will have near-field effects and every object behaves like an antenna to some extent. That's not to say that you could extract a usable signal, because this requires a flow of charged particles (i.e. a current) of sufficient quantity to be detectable. In this interpretation, even a molecule that breaks apart due to a gamma ray would be considered an antenna.
In this next diagram, we can observe the near and far fields in action. The image shows a satellite transmitter with an 8GHz signal being applied. The signal comes through the pipe on the left hand side. This pipe is a waveguide and is made of metal. A radio signal propagates along the inside of the pipe much in the same way as light moves through a fibre optic cable. The photons are reflected by the metal and move along the pipe bouncing off each side. When it exits the pipe, it strikes the dish reflector which is like a mirror or magnifying glass that focuses the radio waves. We can see the near-field effects as the mess of colors on the dish and the photons streaming away from the dish heading to the right. Overlaid on the image we can see an outline of a shape in black. The shape shows us the magnitude of the photons released in a particular direction. Here we can see the "main lobe" which is the direction of the majority of photons, we can also see large releases of photons to the side and these are known as "side lobes". The "side lobes" are unintentional emissions and a product of the near-field effects on the dish's surface. In some systems, they are used to send information to receivers in another direction, so they can be useful.
You can read more about side lobes here:
https://en.wikipedia.org/wiki/Side_lobe
It should be obvious now that any professional antenna should be made from high quality, pure, materials. Further, for those really serious about their receivers/transmitters, they should be encased in a radio transparent shielding (Radome) and non-reactive atmosphere (i.e. Nobel Gas).
We could continue this analysis and provide a breakdown of every interaction throughout the transmit and receive stages, however, we have discussed enough to understand shielding.
This will be the topic of the next article.
