We move outward from the near field into the radiating near field zone: the transition region between the near field and the far field. Here, electromagnetic energy begins to “break free” from its source’s influence, and the electric and magnetic fields start evolving into propagating waves.
I will explain how near-field energy develops into a far-field wave and why understanding this transition is important for measuring electromagnetic behavior in this second of our three-part blog series.
What Shapes the Transformation?
The journey from near-field energy to a propagating electromagnetic wave is not governed by distance alone. It begins with the source itself.
High-current, low-impedance circuits tend to produce strong magnetic fields, while high-voltage, high-impedance circuits are more likely to generate dominant electric fields. In a real product, these sources rarely exist in isolation: multiple circuit sections operate at the same time. Their fields overlap, creating a complex mix of electric and magnetic emissions across the frequency spectrum, from static fields (zero hertz) to tens of gigahertz and beyond.
Wave Impedance, an Important Parameter
Figure 1: Illustration of the near-field transition process, from source to far field.
A key parameter to understand is wave impedance. The term is the ratio between the electric field, illustrated in red in Figure 1 (above), and the perpendicular magnetic field, shown in turquoise. This is important because it is the fundamental parameter that changes as the wave emanates from the product.
The illustration (above) clarifies the two extremes of emission source types:
- The Dipole antenna (top left) begins with a strong electric field and high wave impedance. As the field travels away from the antenna, the electric field becomes weaker relative to the magnetic field until both form a far-field planar wave.
- The Loop antenna (bottom left) begins with a strong magnetic field and low wave impedance. As the field travels away from the antenna, the electric field grows relative to its magnetic field until it forms a far-field planar
So, fundamentally, no matter what impedances the source (your product) presents, the electric and magnetic fields eventually settle into the same type of far-field wave. Near a dipole antenna, wave impedance can be thousands of ohms before decreasing toward the approximately 377 Ω impedance of free space. Near a loop antenna, it can begin at only tens of ohms before increasing toward the same free-space impedance.
Frequency Dictates the Physical Distance of these Boundaries
Frequency strongly influences where this transformation takes place. As frequency rises and wavelength becomes shorter, the transition occurs much closer to the product. Size and geometry matter too. For electrically small sources, λ/2π is a useful rule of thumb for the reactive near-field boundary.
A Product is Often Made up of Many Antennas
A large enclosure, an extended circuit-board trace, or a long cable can behave like an efficient antenna, turning what began as a local disturbance into energy capable of radiating well beyond the product. Materials and shielding also affect how this energy travels. For example, a circuit-board trace may radiate, ferrite may concentrate a magnetic field, and a metal shield may contain or redirect energy. Together, these effects combine to shape the final electromagnetic wave. While we measure in either horizontal or vertical orientation (polarization), the maximum product emissions can happen at any angle.
What Surrounds Your Product Matters Too
A product’s surroundings can also affect how electromagnetic energy travels. Concrete, soil, water, and walls can shield, redirect, absorb, or otherwise change the fields, especially when mounted in a vehicle, for example. The shift from near field to far field can be gradual. Where it occurs depends on the wavelength, source size, and the distance. That is why EMC (Electromagnetic Compatibility) performance is sometimes measured in the final intended environment or a similar expected use environment.
Figure 2: Lighthouse in the fog
In Figure 2. Guess what? Light is part of the electromagnetic spectrum too, and, as such, its path can go through a similar change as it travels through its surroundings.
Summarizing That Important Transition
So, the ultimate destiny of both the electric and magnetic near fields is to either die off early or morph into a stable transverse electromagnetic wave. As the wave propagates, the source has less influence as it enters the radiating near-field transition phase before stabilizing as a far field. Their ratio approaches 377 Ω, the wave impedance of free space. Because the fields are still changing in the transition region, regulatory testing is much preferred in the far-field region, further out, although some standards involve measurements within the transition region.
Next, in the 3rd and last post of the miniseries, we will discuss far-field emissions and their importance.
Read all the blogs in the An Offending Electron series.