On-Line Encyclopedia - N

The Maui Communications Networks
On-Line Encyclopedia

Alphabetical List Of Definitions For Technical Terms

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Near Field
The near field is the zone immediately surrounding an antenna in which the inductive and capacitive effects allow for energy to be returned to the antenna during the negative-going half cycle of each transmitted wave. It is in this zone that metal objects can dramatically distort the antenna pattern and cause problems (i.e. mounting an antenna on a steel I-beam). The Near Field extends out to a distance called the Fraunhofer which is defined as 2D^2/Lambda. At 5.8 GHz this zone extends roughly 1 foot from the antenna (called the Reactive Near-Field). You can use on-line calculators to determine exact distances for various frequencies and antenna types.

Noise Floor
The lowest input signal power level that can be recognized as a distinct, recoverable information signal by a radio receiver. The noise floor is determined by the thermal noise generated within the receiver itself and it ultimately limits the ultimate sensitivity to weak signals received through the antenna system since any signal below the noise floor will result in an detected signal with a signal-to-noise ratio of less than one and will, therefore, be difficult, if not impossible to recover. A typical Wi-Fi radio may have a noise floor as low as -120 dBm however this must not be confused with typical Wi-Fi radio receiver sensitivity which is usually between -95 dBm and -100 dBm. Receive sensitivity begins with the noise floor, but includes consideration for connector loss and circuit discrimination capabilities.
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