Allied Telesis AT-WL2411 - Manual (Page 5)

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PN 613-50537-00 Rev A
5
Basic Antenna Concepts
Antennas and Power
Antennas do not increase nor decrease the power applied to them. They can only transmit or receive the
amount of power that is applied to them. It is possible to have some power loss before or after the signal
leaves the antenna. An example of this is “line loss: which is a decrease in power due to imperfect
connections and imperfect conductivity to cabling materials. It is, however, possible to increase the power
output in a certain direction. But the total power emitted will always be the same as the amount applied to
the antenna minus the amount lost due to line loss, ohmic loss, reflection loss etc…
dBi – (decibels relative to an isotropic {spherical} radiation pattern)
An isotropic antenna is a theoretical antenna that radiates in the shape of a perfect sphere.
dBm – (decibels relative to one milliwatt)
dBm is a commonly used unit of measurement in the RF industry that expresses radio frequency power
relative to a 1 mW point of reference.
dBd – (decibels relative to a ½ wave dipole antenna)
dBd is gain with respect to a ½ wave dipole antenna. Some commercial antenna companies use dBd to rate
their antennas.
EIRP – (Effective isotropically radiated power)
The mathematical product of (1) the power supplied to the antenna and (2) its gain.
Gain
Gain is given in dB (decibels). If an “I” is added as in 3dBi, this rating is relative to an “isotropic” antenna.
An isotropic antenna is a theoretical antenna that radiates in the shape of a perfect sphere. If a “d” is added
as in 3dBd, this rating is relative to a “dipole” antenna. A dipole antenna with a rating of 2.14dB is
equivalent to a 0dBd antenna.
Line-of-sight
This refers to the fact that some electromagnetic wave frequencies require a clear line of sight between
transmitter and receiver. This is largely because higher frequency electromagnetic waves, such as those in
the 2.4 GHz range, do not bend around or penetrate objects as well as some lower frequency signals.
Multipath, Reflection, or Physical Interference
Because of the electromagnetic properties of waves used to transmit data, large metal objects in the
immediate transmission path of the antenna will likely cause distortion of the signal and should therefore be
avoided.
Omnidirectional
An omnidirectional antenna radiates evenly horizontally around the antenna in a plane parallel to the earth.
These antennas do not always radiate evenly vertically around the antenna in a plane perpendicular to the
earth. By giving up vertical coverage, above and or below the antenna and refocusing that signal around
the antenna, it is possible to achieve gain with an omnidirectional antenna.
Omni-gain
Wavelength and operating frequency determine the size of the antenna. By using a longer antenna, you do
not achieve greater overall coverage, but you can achieve greater distance in a focused direction. For
example, omnidirectional antennas with gain achieve some measure of gain in the horizontal plane.