5.3.9 Physical Dimensions
Antennas should be small and robust for ease of handling in mobile operations, but they also need to be of a physical size commensurate with their frequency of operation. For long-range (low-frequency), mobile operations, the choice of antenna is problematic and the antennas can sometimes be so large that the mobile unit must stop to erect a suitable antenna. Similarly, if high gain is required (such as in satellite communications where the received signal is very weak), the size of the antenna must be increased and the narrow beamwidth means that the antenna must be carefully aimed at the transmitter, which means that high-gain mobile communication is very difficult.
5.3.10 Field Regions: Reactive Near Field, Radiating Near Field, And Far Field
The electromagnetic field surrounding an antenna is conventionally divided into the reactive near field, radiating near field (Fresnel region), and far field (Fraunhofer region). These are engineering regions rather than sharply separated physical zones: the transition is gradual, and the approximate boundaries depend on wavelength λ, the antenna's largest dimension D, geometry, and the accuracy required for the task.
The reactive near field lies closest to the antenna. Stored electric and magnetic energy is exchanged with the antenna during each RF cycle, and the relative strength, phase, and spatial distribution of E and H depend strongly on antenna type and observation position. Electric or magnetic fields may dominate; E/H does not have the free-space value, the fields are not adequately described as a plane wave, and strength may change steeply with distance. For electrically large antennas, a commonly used estimate for the outer reactive boundary is R1 ≈ 0.62√(D³/λ). For electrically small antennas, reactive behavior is often significant out to a distance on the order of λ/(2π). These are approximate criteria, not exposure limits.
Beyond the reactive near field lies the radiating near field, or Fresnel region. Radiated energy dominates, but the wavefront remains curved, phase and amplitude vary across the observation plane, and the angular field pattern can still change with distance. For an electrically large antenna, this region extends approximately from R1 to the far-field boundary R2. A distinct radiating near field may be very small or absent for an electrically small antenna.
For an electrically large antenna, the far-field boundary is commonly estimated by:
where D is the largest antenna dimension and λ is the wavelength. The familiar R2 ≈ 2D²/λ criterion is principally an aperture-antenna criterion; it should not be used mechanically when D is small compared with λ. For electrically small antennas, several wavelengths of separation are commonly used to obtain a good far-field approximation. Measurement standards may prescribe additional distance or phase-error margins, which take precedence for compliance work.
In the far field the wavefront is locally planar, E and H are mutually perpendicular and perpendicular to the propagation direction, and their ratio approaches the free-space impedance η0 ≈ 377 Ω. The field amplitudes decrease approximately as 1/r, power density decreases approximately as 1/r² in free space, and the normalized radiation pattern becomes independent of distance. Under plane-wave conditions, the time-average power density may be expressed as S = EH = E²/η0 = η0H².
Most antenna parameters—including gain, directivity, beamwidth, sidelobes, and radiation pattern—are defined in the far field. For RF radiation safety, the field region determines how exposure is assessed: reactive near-field and radiating near-field situations may require separate electric-field and magnetic-field measurements, spatial sampling, or computational modeling, whereas a verified far-field plane wave can often be characterized by one field quantity or power density. Nearby conductors, reflections, and the body of a person can perturb any close-field distribution. Chapter 8 applies these principles to practical exposure measurement and calculation.