Nortel 2330 Solution Manual

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Nortel 2330 Solution Manual Manual pdf 77 page image

Appendix 77Nortel WLAN 2300 Series Outdoor Solution GuideAntenna properties and considerationsIn the design and installation of a WLAN system, system installers need to have some fundamental knowledgeof antenna performance and RF propagation characteristics to ensure efficient performance of the network.This knowledge will assist these individuals with the proper selection of system antennas, as well as, guidingtheir decisions on antenna mounting location and orientation in an effort to ensure optimum system coverageand performance. A properly selected antenna system has the capability of improving overall systemperformance and may lead to a reduction in system cost if the overall number of access points can be reduced.Conversely, a poorly selected antenna system may degrade system performance and may lead to an increase inoverall system cost.Antenna propertiesThe external antennas supported by the Nortel WLAN 2300 system are passive devices. A key attributeincluded in their design is that they are DC-coupled to the output of the WLAN access point to enable thesoftware to detect their presence and direct the energy out their aperture rather than through the internalantennas mounted within the access point housing. Since they are passive devices, they do not process noramplify the RF signal. Even if they are 100% efficient, they cannot radiate more total power than is deliveredto their input terminal from the access point.The basic properties that are used to describe the performance of an antenna include impedance and VSWR(Voltage Standing Wave Ratio), radiation patterns, 3-dB beamwidth, directivity, gain, EIRP (EffectiveIsotropic Radiated Power), polarization and bandwidth. These properties and their impact on systemperformance are discussed in the following sections.Impedance and VSWRIn order to achieve maximum power transfer between an access point and an antenna, the input impedance ofthe antenna must identically match the characteristic impedance of the output of the access point. If the twoimpedances do not match, a reflected wave will be created at the antenna input and travel back towards theaccess point. This reflection is associated with a mismatch loss between the access point and the antenna andresults in an overall reduction of system efficiency and available transmission power. The WLAN supportedexternal antennas have been designed to match the output impedance of the WLAN 2300 series access pointsto ensure optimum system power transfer.The Voltage Standing Wave Ratio or VSWR is the ratio between the maximum voltage and the minimumvoltage that appears along the transmission line connecting the access point to the antenna. The VSWR, whichcan be derived from the level of reflected and forward waves, is also an indication of how closely or efficientlyan antenna's input impedance is matched to the characteristic impedance of the cabling and output impedanceof the access point. An increase in VSWR indicates an increase in the mismatch between the antenna and theaccess point. These increases can be a direct result of corrosion, damaged cables, loose connections or sealsand lengthy cable extensions from the access point. All of these concerns must be checked and optimizedduring the installation of the antenna system by the professional installer.To indicate how increased VSWR impacts overall system performance, the table below details the percentageof power reflected by the antenna, and the resultant overall transmission loss, for several typical VSWR values.For a 1.5:1 VSWR, the transmission loss is approximately 0.2 dB or a 4.0% reduction in efficiency.