All about antennas

* Antenna basics

The subject of antennas is often relegated to a mere footnote in many enterprise wireless networking discussions. Yet it is the antenna - the conductor that converts electrical signals to radio waves for transmission and back again upon reception - that can optimize wireless application performance by altering how the signal propagates.

Omnidirectional antennas, for example, broadcast a signal in all directions. Certain types are used in outdoor point-to-multipoint bridging, while others are used in cellular and cordless phones, two-way radios, and for AM/FM radio broadcast reception.

Directional antennas, by contrast, are designed to focus their “beam” in a specific direction. Directional antennas have a number of sub-types. Here are a few and how they are often used:

* Yagi.

Point-to-point outdoor bridging across medium-range distances.

* Sector.

Point-to-multipoint outdoor broadcast to recipients in a certain direction.

* Parabolic.

For outdoor use, has an emitter mounted such that it is aimed into a bowl-shaped reflector, which focuses the signal into a tight beam. Used in terrestrial-to-satellite applications.

* Circularized polar.

Used indoors where a lot of reflective material, such as metal, shelves, and water, is present.

During antenna discussions, you might frequently hear the term “gain.” Gain refers to the ratio of the output signal strength of an amplifier to the input signal strength. This ratio is usually expressed in terms of decibels (dBi). (In wireless LANs, the higher an antenna's gain, the higher its price usually is.) Some systems let you adjust the signal strength of your access points to shrink your coverage cell for better throughput; some now also automatically adjust depending on network conditions (part of a function called “radio resource management”).

The term “antenna diversity” in WLANs traditionally refers to the use of dual antennas on a given access point to help relieve the effects of multipath interference. As you likely know by now, however, forthcoming 802.11n-standard WLANs use at least four antennas to actually capitalize on the effects of multipath to boost transmission speeds over 100Mbps.

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