Field Regions

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The fields surrounding an antenna are divided into 3 principle regions:

  • Reactive Near Field

  • Radiating Near Field or Fresnel Region

  • Far Field or Fraunhofer Region

    The far field region is the most important, as this determines the antenna's radiation pattern. Also, antennas are used to communicate wirelessly from long distances, so this is the region of operation for most antennas. We will start with this region.

    Far Field (Fraunhofer) Region

    The far field is the region far from the antenna, as you might suspect. In this region, the radiation pattern does not change shape with distance (although the fields still die off as 1/R, the power density dies off as 1/R^2). Also, this region is dominated by radiated fields, with the E- and H-fields orthogonal to each other and the direction of propagation as with plane waves.

    If the maximum linear dimension of an antenna is D, then the following 3 conditions must all be satisfied to be in the far field region:

    far field distance for an antenna
    [Equation 1]

    far field distance for an antenna
    [Equation 2]

    far field distance for an antenna
    [Equation 3]

    The first and second equation above ensure that the power radiated in a given direction from distinct parts of the antenna are approximately parallel (see Figure 1). This helps ensure the fields in the far-field region behave like plane waves. Note that >> means "much much greater than" and is typically assumed satisfied if the left side is 10 times larger than the right side.

    parallel rays from arbitrary antennas

    Figure 1. The Rays from any Point on the Antenna are Approximately Parallel in the Far Field.

    Finally, where does the third far-field equation come from? Near a radiating antenna, there are reactive fields (see reactive near field region, below), that typically have the E-fields and H-fields die off with distance as and . The third equation above ensures that these near fields are gone, and we are left with the radiating fields, which fall off with distance as .

    The far-field region is sometimes referred to as the Fraunhofer region, a carryover term from optics.

    Reactive Near Field Region

    In the immediate vicinity of the antenna, we have the reactive near field. In this region, the fields are predominately reactive fields, which means the E- and H- fields are out of phase by 90 degrees to each other (recall that for propagating or radiating fields, the fields are orthogonal (perpendicular) but are in phase).

    The boundary of this region is commonly given as:

    near field for antenna

    Radiating Near Field (Fresnel) Region

    The radiating near field or Fresnel region is the region between the near and far fields. In this region, the reactive fields are not dominate; the radiating fields begin to emerge. However, unlike the Far Field region, here the shape of the radiation pattern may vary appreciably with distance.

    The region is commonly given by:

    fresnel region for antennas

    Note that depending on the values of R and the wavelength, this field may or may not exist.

    Finally, the above can be summarized via the following diagram:

    field regions for antennas, far field or near field

    Figure 2. Illustration of the Field Regions for an Antenna of Maximum Linear Dimension D.

    Next we'll look at numerically describing the directionality of an antenna's radiation pattern.

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