B&B Electronics Zlinx Xtreme ZXT9-RM, Руководство пользователя

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Document Number: ZXTxRM-5110m
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161
D.6
Remember Your Math
Contrary to popular opinion, no black art is required to make a reasonable prediction of the range of
a given radio signal. Several simple concepts must be understood first, and then we can apply
some simple rules of thumb.
The equation for successful radio reception is:
TX power + TX antenna gain – Path loss – Cabling loss + RX antenna gain – 10dB fade
margin > RX Radio sensitivity or (less commonly)
RF noise floor
Note that most of the equation’s parameters are easily gleaned from the manufacturer’s data. That
leaves only path loss and, in cases of heavy RF interference, RF noise floor as the two parameters
that you must established for your particular installation.
In a perfect world, you will measure your path loss and your RF noise conditions. For the majority
of us that don’t, there are rules of thumb to follow to help ensure a reliable radio connection.
D.7
RF Attenuation and Line of Sight
In a clear path through the air, radio signals attenuate with the square of distance. Doubling range
requires a four-fold increase in power, therefore:
Halving the distance decreases path loss by 6dB.
Doubling the distance increases path loss by 6dB.
When indoors, paths tend to be more complex, so use a more aggressive rule of thumb, as follows:
Halving the distance decreases path loss by 9dB.
Doubling the distance increases path loss by 9dB.
Radio manufacturers advertise “line of sight” range figures. Line of sight means that, from antenna
A, you can see antenna B. Being able to see the building that antenna B is in does not count as line
of sight. For every obstacle in the path, de-rate the “line of sight” figure specified for each obstacle
in the path. The type of obstacle, the location of the obstacle, and the number of obstacles will all
play a role in path loss.
Visualize the connection between antennas, picturing lines radiating in an elliptical path between
the antennas in the shape of a football. Directly in the center of the two antennas the RF path is
wide with many pathways. A single obstacle here will have minimal impact on path loss. As you
approach each antenna, the meaningful RF field is concentrated on the antenna itself. Obstructions
located close to the antennas cause dramatic path loss.
Be sure you know the distance between antennas. This is often underestimated. If it is a short-
range application, pace it off. If it is a long-range application, establish the actual distance with a
GPS or Google Maps.
The most effective way to reduce path loss is to elevate the antennas. At approximately 6 feet high
(2 m), line of sight due to the Earth’s curvature is about 3 miles (5 km), so anything taller than a
well-manicured lawn becomes an obstacle.
Weather conditions also play a large role. Increased moisture in the air increases path loss. The
higher the frequency is, the higher the path loss.