Characteristics of the Far Field

Some characteristics of the far field (which are, essentially, the opposite of those for the near field) include the following:

•In the far field the amplitude of the energy components attributable to the d2erʼ/dt2 portion of the field is stronger than the near field components that vary inversely as the square of the distance.

•The field strength varies inversely with the distance (1/r) and the influence of the inverse square field characteristics (1/r2) can be ignored. Essentially, if you measure field strength and then move twice as far away from the source the field strength will go down by half.

•Field power is represented by wave amplitude and the complicated interactions between electrical and magnetic fields need not be taken into consideration in real-world measurements. Consequently, many of the influences caused by metal objects (like the shielding inside a computer) are minimized. Metal objects can still block or reflect the RF signal but the influences resulting from the oscillating RF energy creating a magnetic field in a metal object, and then that magnetic field having an effect on the surrounding field pattern or strength is negligible.

•Field strength is not very sensitive to changes in the location of the receiving antenna relative to the phase center of the transmitted signal. When the receiving antenna is moved through a distance of 12 cm (roughly 1 wavelength in the 2.4 GHz 802.11 band) there is negligible change in the field strength.

•Coupling and multiple reflections between adjacent antennae are not significant. The complicated relationships between phase differences in signals transmitted at the same frequency are how many directional antennae obtain their directionality, as is the case with the Yagi, Patch, or Panel antennae (to be discussed later in this paper). These, and other, interactions between transmitters are negligible in the far field.

•The wavefront of the expanding radiation pattern emanating from the source consists of transverse waves. A transverse wave field has a flat (planar) front with the wavefront being perpendicular to the direction of wave motion. This means that the spherical propagation characteristics that give rise to the inverse square relationship in the near field are no longer predominant. Rather, the wavefront seen by the receiving antenna is close enough to being planar that the spherical expansion characteristics can be ignored. There was a time when people thought the spherical Earth was flat. A receiving antenna in the far field has a similar perception regarding the radiating wavefront.

Considerations Concerning Near Field Interaction

While most practical real-world RF analysis takes place relative to a transmitterʼs far field, there are some thoughts to be considered regarding the near field. Remember that the boundary between the near and far fields is given as 2D/λ from the source which, as weʼve discussed, is roughly 25 cm in the 2.4 GHz 802.11 band. When you mount an 802.11 access point on an exterior wall directly on top of a concealed steel supporting beam, youʼre putting a big mass of metal in the near field. If the access point is closer than 10 inches to ceiling light fixture, again youʼve managed to get magnetic material into the near field. Finally, when you place your notebook computer on a metal file cabinet you get a magnetic substance into your PCMCIA wireless cardʼs near field too.

When we discuss antenna coupling and re-radiation later on it will be explained how metal objects in an antennaʼs near field are energized by the signal power in a way that disrupts the radiated far field. Antenna designers use these disruptions in productive ways. Metal objects in your antennaʼs near field will probably result in degradation of transmission and reception capability.

Copyright 2003 - Joseph Bardwell