Antenna Selection Criteria

 < Day Day Up > 



We will now look at some of the features you should consider during antenna selection. They include:

Gain. Antennae transmit and receive radio waves; the focused strength of this radiated energy is measured in terms of gain in decibels (dB). It is important to take gain into consideration when selecting network antennae. The gain in the antenna focuses the transmitted signal towards the targeted area of coverage. It also focuses incoming energy on the receive side. Gain is expressed relative to the performance of a theoretical isotropic antenna that radiates equally in all directions. By definition an isotropic reference antenna has a gain of 1 (0 dB). An antenna that has a gain of 2 (3 dB), compared to an isotropic antenna, would be written as 3 dBi. (Note that while power is expressed in dB, gain is expressed in dBi.) For every 3 dB increase, you double the emitted power out of the antenna. You can use higher gain antennae with lower powered radio cards to increase signal strength, link quality and distance, but be sure you don't go over the regulatory EIRP (Equivalent Isotropically Radiated Power) limit for your locale. The following table lists the power levels permitted in the three primary geographical regions. Operation in countries within Europe, and regions outside Japan or the United States, may be subject to additional or alternative national regulations. For example, in Australia the ACA or Australia Communications Authority defines an antenna's EIRP limit. Check your local regulatory agency for their specific wireless networking restrictions, which include antennae EIRP limitations—any deviation from the permissible settings is an infringement of national law and may be punished as such.

Transmit Power Levels for Different Regions

Maximum Output Power

Geographic Location

Compliance Document

1000 mW

United States

FCC 15.247

100 mW (EIRP)

Europe

ETS 300–328

10 mW/MHz

Japan

MPT ordinance 79

But also see Appendix III: Regulatory Specifics re Wi-Fi.

Note 

An antenna's power is equal to the total amount of power actually transmitted through the system's antenna, including the transmitter's power output, the cable's power loss, and the antenna's gain capability. This power is measured in Watts.

You'll want an antenna that provides enough gain on both the transmitting and receiving side to establish stable links while staying within regulatory limits. Antenna manufacturers typically include various gain models in their product offering, to accommodate differing access point gain requirements.

VSWR. Short for "Voltage Standing Wave Ratio," this is the ratio of the maximum/minimum values of the standing wave pattern along a transmission line to which a load is connected. VSWR value ranges from 1 (matched load) to infinity for a short or an open load. For most WLAN antennae, the maximum acceptable value of VSWR is 2.0, but try to find an antenna with a VSWR of 1.5 or less, which offers approximately a Return Loss of 14.5 dB. This means that most of the signal from the transmitter to the antenna is radiated (96% radiated and 4% reflected). A VSWR of 2.0 (return loss of 9.5 dB) means that 90% of the signal is radiated and 10% reflected.

Radiation pattern. An antenna's radiation pattern indicates how the transmitted radio wave energy is distributed in space. The vertical cut of a radiation pattern, also known as the elevation cut, is measured across the antenna's elevation plane. The horizontal cut of a radiation pattern, known as azimuth, is measured across the horizontal plane. Narrowing either the elevation or the azimuth pattern will increase the antenna's gain. Radiation patterns can also be altered according to the antenna polarization. This refers to the orientation of the antenna's radiated signal, which can be circular, horizontal, vertical, etc.

Note 

If you deploy a diversity antenna solution, keep in mind that since a pair of antennae must be within each other's radiation patterns in order to communicate; uniform radiation patterns across the frequency band are crucial to the network's performance and reliability.

Beamwidth. The beamwidth of an antenna describes how a signal spreads out from the antenna, as well as the range of the reception area. Beamwidth is measured between the points on the beam pattern at which the power density is half of the maximum power. A high gain antenna has a very narrow beamwidth and may be difficult to align.

Polarization. Radio waves are a combination of both an electric field and a magnetic field, which are perpendicular to each other. Together these fields are known as the electromagnetic field. The position and direction of the electric field with reference to the earth's surface determines the wave polarization. A horizontal polarized antenna is positioned to have the electric field parallel to the ground, whereas in the case of a vertical polarized antenna, the electric field is perpendicular to the ground. For a link to work, antennae at both ends must have the same polarization. And if the site has other APs in the area that are causing interference, you need to polarize your antennae differently to those other antennae so as to decrease the interference.

The gain, VSWR, and radiation pattern characteristics of an antenna design are closely related; therefore, if you alter one, the other two will be affected, which, in turn, will directly impact the antenna's performance.

Some other factors to consider when selecting an antenna include the following:

  • The range of the antenna.

  • The coverage requirements. For example, an antenna that serves a distribution center with numerous vertical aisles would have different coverage requirements than an antenna serving a corporate office.

  • The environment. When antennae are used indoors, the building construction, ceiling height, and internal obstructions must be considered. In outdoor environments, obstructions such as trees, vehicles, buildings, and hills must be considered, along with salt air, moisture, ice, high heat, etc.

  • The antenna's lifetime expectation. Antennae that are placed out-of-doors or in harsh environments will have a different life expectancy than an antenna in a library or corporate office.

  • The mounting options. Does the antenna need to be hidden, or does it need to be mounted at a height that allows its signals to avoid interference from, for example, trucks loading and unloading on a docking bay?

  • Whether the antenna's appearance should be a factor in the selection process. If the antenna can't be hidden under ceiling tiles, does it need to blend with its surroundings? Many times corporate offices, HotSpot venues, etc., require that the AP and its antenna be unobtrusive.

Check out the Webcats Wireless website, http://webcatswireless.com/toolbox/. It offers a number of very helpful antenna placement tools.

Note 

Cisco Systems provides some great charts on antenna gain and power limits for 802.11a and 802.11b. Their web site also contains a handy band plan. (Cisco also seems to update the site quite regularly.) Those documents can be found at http://www.cisco.com/univercd/cc/td/doc/product/wire-less/airo1200/accsspts/ap120scg/bkscgaxa.htm#1013020. Or, if the URL is just too troublesome to type in, go to Google and search for "Channels, Power Levels, and Antenna Gains"—be sure to use the quote marks.



 < Day Day Up > 



Going Wi-Fi. A Practical Guide to Planning and Building an 802.11 Network
Going Wi-Fi: A Practical Guide to Planning and Building an 802.11 Network
ISBN: 1578203015
EAN: 2147483647
Year: 2003
Pages: 273

flylib.com © 2008-2017.
If you may any questions please contact us: flylib@qtcs.net