AirborneM2M™ User Guide
24
ANTENNA LOCATION
The importance of this design choice cannot be over-stressed. In fact, it can be the determining factor
between success and failure of Wi-Fi implementation.
There are several factors that must to be considered when determining location:
Distance of antenna from radio
Location of host system:
− Proximity to RF blocking or absorbing materials
−
Proximity to potential noise or interference
−
Position relative to infrastructure (access points or laptops)
Orientation of host system relative to infrastructure:
− Is it known
−
Is it static
To minimize the impact of the factors above, the following things need to be considered during the
development process:
Minimize the distance between the radio and the location of the antenna. The coaxial cable
between the two impacts Transmit Power and Receive Sensitivity negatively. B+B SmartWorx
recommends using 1.32-1.37mm outer diameter U.FL coaxial cables.
Minimize the locations where metal surfaces come into contact or are close to the location of the
antenna.
Avoid locations where RF noise, close to or overlapping ISM bands, may occur. This includes
microwave ovens and wireless telephone systems in the 2.4GHz and 5.0GHz frequency range.
Mount the antenna as high on the equipment as possible.
Locate the antenna where there is a minimum obstruction between the antenna and the location
of the access points. Typically, access points are located in the ceiling or high on walls.
Keep the main antenna’s polarization vertical, or in-line with the antenna of the access points.
802.11 systems utilize vertical polarization and aligning both Transmit and Receive antenna
maximizes the link quality.
Even addressing all of the above factors does not guarantee a perfect connection. However, with
experimentation, an understanding of the best combination will help identify a preferred.
PERFORMANCE
Performance is difficult to define as the appropriate metric changes with each application or may indeed be
a combination of parameters and application requirements. The underlying characteristic that, in most
cases, needs to be observed is the link quality. This can be defined as the bandwidth available over which
communication between the two devices can be performed. The lower the link quality, the less likely the
devices can communicate.
Measurement of link quality can be made in several ways: Bit Error Rate (BER), Signal to Noise (SNR)
ratio, Signal Strength (SS), and may also include the addition of distortion. The link quality is used by the
radio to determine the link rate. Generally, as the link quality for a given link rate drops below a predefined
limit, the radio will drop to the next lowest link rate and try to communicate using it.
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