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While directional antennas can be of great value for certain indoor applications, the vast majority of indoor
installations use omni−directional antennas. Directionality should be strictly determined by a correct and
proper site survey. Whether you use an omni−directional or patch antenna, indoor environments require
diversity antennas to mitigate multipath distortion. The Cisco Aironet Series Access Point radios allow for
diversity support.
Signal Attenuation
Signal attenuation or signal loss occurs even as the signal passes through air. The loss of signal strength is
more pronounced as the signal passes through different objects. A transmit power of 20 mW is equivalent to
13 dBm. Therefore, if the transmitted power at the entry point of a plasterboard wall is at 13 dBm, the signal
strength is reduced to 10 dBm when exiting that wall. This table shows the likely loss in signal strength
caused by various types of objects.
Signal Attenuation Caused By Various Types of Objects
Object in Signal Path
Signal Attenuation through Object
Plasterboard wall
3 dB
Glass wall with metal frame
6 dB
Cinder block wall
4 dB
Office window
3 dB
Metal door
6 dB
Metal door in brick wall
12 dB
Human body
3 dB
Each site surveyed has different levels of multipath distortion, signal loses, and signal noise. Hospitals are
typically the most challenging environment to survey due to high multipath distortion, signal losses and signal
noise. Hospitals take longer to survey, require a denser population of access points, and require higher
performance standards. Manufacturing and shop floors are the next hardest to survey. These sites generally
have metal siding and many metal objects on the floor, which result in reflected signals that recreate multipath
distortion. Office buildings and hospitality sites generally have high signal attenuation but a lesser degree of
multipath distortion.
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