Electromagnetics and Photonics

Electromagnetics is the study of electric and magnetic fields and the waves they form. Photonics is what the same subject is called once the frequency is high enough that the waves are light and it becomes natural to speak of photons. They are one subject viewed at two scales, and the boundary between them is conventional rather than physical.

Fields Before Circuits

Circuit theory is a simplification. It assumes that voltage and current are well-defined at each node and that changes propagate instantly, which is a very good approximation whenever the physical size of a circuit is small compared with the wavelength of the signals in it.

When it is not — because the frequency is high, the structure is large, or the edges are fast — the approximation collapses and field behaviour reasserts itself. Voltage measured at one end of a conductor differs from voltage at the other end at the same instant. Signals reflect from discontinuities. Parallel traces couple energy into one another. Conductors radiate.

Engineers encounter this as the point where circuits stop obeying the schematic. It is not a mysterious effect; it is the moment the simplifying assumption expired. The practical rule is that when a structure's dimensions approach a tenth of a wavelength, it should be treated as a distributed system rather than a lumped one. Because the fastest edge determines the highest relevant frequency, a slow digital signal with sharp transitions can require this treatment while a smoothly varying faster one does not — a subtlety that regularly catches out designers of switching converters, whose deliberately fast edges radiate far more than the switching frequency alone would suggest.

Near Field and Far Field

Close to a source, electric and magnetic fields behave largely independently, store energy rather than carry it away, and fall off steeply with distance. This is the near field, and it is what makes near-field communication, inductive charging and most electromagnetic interference between adjacent components possible.

Beyond roughly a wavelength or two, the fields lock into a self-sustaining relationship: a changing electric field generates a magnetic field, which as it changes regenerates the electric field, and the pair propagates away at the speed of light. This is the far field — radiation proper — and it falls off much more gently, which is why radio works over distances at all.

The distinction matters practically because interference problems have completely different solutions in each regime. Near-field coupling is addressed by geometry: increase separation, reduce loop areas, orient conductors to minimise coupling. Far-field radiation is addressed by shielding and filtering. Applying the wrong remedy is a common and expensive mistake.

Antennas

An antenna is a deliberate transition between a guided wave on a conductor and a free-space wave. Everything about antenna design follows from doing that transition efficiently and in a chosen direction.

Two properties dominate. Gain is not amplification — an antenna is passive and adds no energy. It describes concentration: a high-gain antenna radiates more strongly in one direction by radiating less in others. Bandwidth is the frequency range over which the antenna works acceptably, and it trades against size and gain, which is why a physically small antenna covering many bands necessarily compromises somewhere.

Because the spectrum is shared and finite, the frequencies any system may use, and the emissions it must not produce outside them, are internationally coordinated through the International Telecommunication Union.

Impedance Matching

Whenever a wave meets a change in the medium's characteristic impedance, part of it reflects. On a transmission line this means power that fails to reach the load and returns toward the source, producing standing waves and, at high power, real damage.

Impedance matching is the practice of arranging for source, line and load to present the same characteristic impedance so that no reflection occurs. The ubiquitous 50-ohm convention in radio-frequency work exists precisely so that components from different sources can be connected without each junction becoming a reflection point. It is an engineering compromise between the impedance that minimises loss and the one that maximises power handling in a coaxial geometry — neither optimum, deliberately.

Photonics: Information on Light

Raise the frequency far enough and the wave is light, guided in glass rather than copper. The advantages are decisive for communication. Optical frequencies are enormous, so the available bandwidth is correspondingly enormous. Modern optical fibre is extraordinarily transparent, allowing signals to travel tens of kilometres before amplification. And because the signal is light in a dielectric, it is immune to the electromagnetic interference that afflicts copper.

Guiding works by total internal reflection: a fibre core with slightly higher refractive index than its cladding traps light striking the boundary beyond a critical angle, so it propagates along the fibre rather than escaping.

The dominant limitation is dispersion. Different wavelengths, and different propagation paths within the fibre, travel at slightly different speeds. A pulse launched sharply arrives spread out, and if it spreads far enough it overlaps its neighbours and the data are lost. This sets the fundamental relationship between data rate and distance, and managing it — through single-mode fibre, careful wavelength selection and dispersion-compensating elements — is much of the engineering in long-haul optical systems.

The same physics scaled down to chip dimensions becomes integrated photonics, where optical structures are fabricated using the techniques described on the nano devices page. Traceable measurement of both electromagnetic and optical quantities sits with metrology institutes such as the NIST Physical Measurement Laboratory.