3.3 Antenna Radiation Pattern and Polarization
An antenna is a physical device that radiates energy, almost always with some directional dependence. Even an omni-directional antenna will have one or two narrow directions where there is a reduction or a null in the radiated energy. The energy leaving an antenna is also polarized – meaning that the electric field is in a particular orientation.
Theoretically, for efficient transfer of energy between two antennas, their respective radiation patterns must be optimized in the correct direction and the antennas must be polarized with the same orientation. However, in the real world, where line of sight (LOS) between two antennas rarely exists, and at frequencies above about 500 MHz, objects in the path between the antennas (walls, structures, people, terrain, etc.) can substantially alter both the effective radiation patterns and the polarization. These scattering effects on the radiation pattern and polarization are random, uncontrollable, and typically change continuously with time, relative antenna position and orientation. Even if a device is located in a fixed position, polarization will typically change with time due to movement of nearby objects. As a result, while some general attention must be paid to radiation pattern shape and antenna polarization, these parameters can be optimized for a particular situation in only a general way.
For example, when a 2.4 GHz ISM band wireless link is operating indoors, antenna polarization can largely be ignored because reflections and multipath completely randomize the polarization at any given point in space. Once beyond LOS, even just a few meters from an antenna, the original polarization is completely lost. Most wireless devices (in the category under discussion in this paper) have small antennas mounted on small ground planes. The physics of antenna radiation stipulate that radiation from these structures will not have narrow directional lobes
Antenna Fundamentals – Technical Brief