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Antenna Near Field. The maximum power density in front of an antenna (e.g., at the antenna surface) can be approximately by the following equation:
Extent of Near-Field Region.
In the near-field, or Fresnel region, of the main
beam, the power density can reach a maximum before it begins to decrease with
distance. The extent of the near-field can be described by the following
equation (D and in same
The magnitude of the on-axis (main beam) power density varies according to location in the near-field. However, the maximum value of the near-field, on-axis, power density can be expressed by the following equation:
Aperture efficiency can be estimated, or a reasonable approximation for circular apertures can be obtained from the ratio of the effective aperture area to the physical area as follows:
If the antenna gain is not known, it can be calculated from the following equation using the actual or estimated value for aperture efficiency:
Transition Region. Power density in the transition region decreases inversely with distance from the antenna, while power density in the far-field (Fraunhofer region) of the antenna decreases inversely with the square of the distance. For purposes of evaluating RF exposure, the distance to the beginning of the far-field region (farthest extent of the transition region) can be approximated by the following equation:
The transition region will then be the region extending from Rnf, calculated from Equation (12), to Rff. If the location of interest falls within this transition region, the on-axis power density can be determined from the following equation:
Far-Field Region: The power density in the far-field or Fraunhofer region of the antenna pattern decreases inversely as the square of the distance. The power density in the far-field region of the radiation pattern can be estimated by the general equation discussed earlier:
In the far-field region, power is distributed in a series of maxima and minima as a function of the off-axis angle (defined by the antenna axis, the center of the antenna and the specific point of interest.) For constant phase, or uniform illumination over the aperture, the main beam will be the location of the greatest of these maxima. The on-axis power densities calculated from the above formulas represent the maximum exposure levels that the system can produce. Off-axis power densities will be considerably less. For off-axis calculations in the near-field and in the transition region it can be assumed that, if the point of interest is at least one antenna diameter removed from the center of the main beam, the power density at the point would be at least a factor of 100 (20 dB) less than the value calculated for the equivalent distance in the main beam. For practical estimation of RF fields in the off-axis vicinity of aperture, use of the antenna radiation pattern envelope can be useful. For example, for the case of an earth station in the fixed-satellite service, the Commission's Rules specify maximum allowable gain for antenna sidelobes not within the plane of the geostationary satellite orbit, such as at ground level*. In such cases, the rules require that the gain of the antenna shall lie below the envelope defined by: 32 - {25log10(
Use of the gain obtained from these relationships in simple far-field calculations, such as Equation 18, will generally be sufficient for estimating RF field levels in the surrounding environment, since the apparent aperture of the antenna is typically very small compared to its frontal area. |
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"In God We Trust" Site Prayer "
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For cell phones not using RF Safe Approved antennas its highly recommended that you use an RF3 Air-tube Headset with a Wireguard and Pocket or Belt Clip Shield!
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| >> State Cell Phone Laws | ||
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| >> Cell Phone Radiation Levels | ||
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| >> Cell Phone Radiation Hazards | ||
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Please don't let your
KIDS use
cell phones except in emergencies! Children have much thinner skulls.
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units):
=
aperture efficiency, typically 0.5-0.75
)}
dBi for 1º
