Thursday, August 20, 2009
Surrounding antennas are two spatial fields; the near field and far field. Near-field region contains reactive and oscillating electromagnetic field, while the far-field region contains only transverse-propagating electromagnetic fields.
The near-field is calculated by the formula on the right, where r is the furthest point in the near-field, D is the largest dimension of the antenna and lambda is the wavelength.
Reactive fields are formed by stationary charges or charges moving at uniform velocity. Examples are DC power sources as they cause a constant drift of charges resulting in a reactive field. AC sources also produce reactive fields in addition to radiating fields. Reactive fields store energy capacitively and/or inductively in the absence of a receiving circuit or antenna. If an antenna or receiving circuit is present, a reactive field will lead transfer energy through capacitive or inductive coupling.
Reactive fields characteristics are very dependent on the source of the energy, and is extinguished once the source is inactive. Reactive fields do not propagate and measuring these fields impacts the source voltage and currents. Reactive fields impedance and wave shape are dependent on the source and energy can be transported using transmission lines.
 Ron Schmitt, "Electromagnetics Explained: A Handbook for Wireless /RF, EMC and High Speed Electronics", Newnes, 2002.