Information Technology Reference
In-Depth Information
is packed on the field is preserved for recovery by the demodulation circuitry in the
receiving device. As we deal with very tiny signals in wireless networking, the anten-
nas must be as efficient as possible.
The typical wireless network interface card (W-NIC) contains a small antenna
that is usable in locations with adequate signal strength. However, if the distances, sur-
roundings, or line-of-sight attenuation dictates, you may need to add a supplementary
antenna (Fig. 13.3). External antennas often have a pattern that concentrates the sig-
nals from certain directions, effectively creating a “signal gain,” relative to an ordinary
antenna. Such gain antennas are often called
range extenders
, after their function.
Antennas may be either directional or
omnidirectional
. An omnidirectional
antenna is an antenna that transmits equally in all directions. The only truly three-
dimensional omni antenna is the theoretical point-source, or
isotropic
, antenna.
Such an antenna, if perfectly matched to its source, would convert all its applied
electrical energy to electromagnetic (EM) energy, and launch the EM (radio) waves
in all directions. The converse for receiving EM energy would be true, as well.
Bottom line: From any direction, in all three dimensions, the transmitted/received
energy would be equal. We rate real-world antennas in comparison to this theoret-
ical model, in decibels above or below the isotropic model, or dBi. Thus, a perfect
isotropic antenna would be rated as “unity gain,” 0 dBi.
FIGURE 13.3
Gain antenna (range extender) with wireless NICs.
