The Near-Field vs. Far-Field Regions of an Antenna
The near-field region is divided into the reactive and radiative near-field regions.
The region where the transition of the electromagnetic field from reactive to radiative begins is called the radiative near-field region or the Fresnel region.
The region furthest from the antenna is dominated by radiated electromagnetic fields and is called the far-field region or the Fraunhofer region.
Signals applied to antennas from transmitters radiate to free space. These signals are electromagnetic waves consisting of electric fields and magnetic fields. The characteristics of the electromagnetic field associated with these signals varies with the distance from the antenna.
Generally, the varying fields are bifurcated into near-field and far-field regions. The near-field and far-field regions correspond to the electromagnetic fields nearer and farther from the antenna, respectively. The most commonly-used parameter to define near-field vs. far-field regions is signal wavelength. Let’s take a closer look at the near-field vs. far-field regions associated with antennas.
Near-Field vs. Far-Field Regions
The voltage signal applied to an antenna from a transmitter generates an electric field, and the current in the antenna produces magnetic fields. The electric and magnetic fields interact with each other, forming the electromagnetic field associated with the antenna.
The electromagnetic field characteristics of the antenna’s radiation change with the distance from the antenna. For example, the electromagnetic field is spherical around the antenna. As it spreads out away from the antenna, the field becomes more planar. The radiation intensity is also different when measured nearer to the antenna as opposed to farther from the antenna. Electromagnetic field characteristics can be defined as a function of distance from the antenna, which is why it is imperative to have a sense of the near-field vs. far-field regions of the antenna.
The region next to the antenna is called the near-field region. The near-field region possesses an inductive effect, and is sometimes also referred to as the inductive field. In the near-field region, the antenna radiation pattern and strength of the fields vary with the distance from the antenna.
There is some disagreement among engineers on the distance up to which a field can be considered “near-field.” A generally accepted near-field region can be defined with the following equation.
In this equation ʎ, is the wavelength of the radiation or electromagnetic signal and D is the maximum linear dimension of the antenna.
Another equation for near-field distance acknowledged by many engineers is .
Let’s take a look at the two sections of near-field regions: the reactive and radiative near-field regions.
The Reactive Near-Field Region
The region closest to the antenna forms a reactive near-field region. In this region, the electric and magnetic fields are displaced by 90 degrees from each other, turning the field reactive. The radiative property of the electromagnetic fields in the reactive near-field region is comparatively less.
The electric and magnetic fields are strongest in the reactive fields and separate measurements of each field is possible. However, depending on the antenna type, one field dominates over the other in the near-field region. In a loop antenna, it is the magnetic field that dominates in the near-field region.
The Radiative Near-Field Region
Electromagnetic fields radiate when the electric and magnetic fields are perpendicular to each other with zero phase displacement. The region where the transition of the electromagnetic field from reactive to radiative begins is called the radiative near-field region or the Fresnel region. It lies between the reactive near-field and far-field regions.
The region furthest from the antenna is dominated by radiated electromagnetic fields and is called the far-field region or the Fraunhofer region. This region is immediate to the radiative near-field region.
The far-field region can be defined by the following equation:
In this region, the radiation pattern of an antenna is independent of the distance from the antenna. The radiation effect is greater in the far-field region, as the electric and magnetic fields are orthogonal to each other and the direction of propagation is similar to plane waves. In the far-field region, the strength of the fields can be calculated using the Friis formula.
When designing antennas or working on antenna interfaced systems, it is good to have some idea of the near-field vs. far-field regions. For your next project with antennas, Cadence software offers helpful tools to synthesize, optimize, and integrate antennas.
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