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12
Appendix A - Considerations for RF Communica-
tions
12.1
Building Materials
12.2
Distance
12.3
Obstructions
Siemens AB
Security Products
When considering how well RF signals might propagate through your facility, take
into account the distance between devices, the materials that the signal may en-
counter in the building, and how much those materials will either absorb and/or re-
flect the signal. There are three main effects: free space loss, absorption, and re-
flection. The better you understand these, the better you can determine how RF
signals might (or might not!) propagate within your specific facility.
Free space loss occurs as the RF field propagates away from the antenna. As the
receiving antenna is moved farther away from the transmitting antenna, the
strength of the received RF signal decreases.
Free space loss is almost independent of channel number (just 6% difference) but
channels with higher levels of interference require stronger signal at the receiver.
To ensure good communication over the wireless network, it is important to ensure
proper signal strength so the devices must not be too far apart.
Absorption of the RF signal occurs when the signal encounters material which
absorbs the signal at the working frequency. Such materials include glass, wood,
drywall and even foliage. Dampness increases the absorption
The angle between the incoming signal and the obstruction affects absorption. A
signal at an oblique angle to an absorbing slab has to go through more of the ma-
terial than if it were square on.
Reflection of the RF signal occurs when the signal encounters an obstruction of
material which is reflective at the working frequency. The proportion of signal re-
flected depends on the frequency of the signal, on the material and on the orienta-
tion, shape and size of the obstruction. Some materials, such as concrete and wa-
ter, reflect RF energy only moderately, while others, such as metals, reflect it com-
pletely. For materials that only partially reflect the signal, the remaining RF energy
can be completely or partially absorbed. If the signal is only partially absorbed, the
rest of the energy passes through the material.
Obstructions (both absorptive and reflective) are important and need to be ac-
counted for in the design of a network. It may be possible that the building con-
struction prohibits the use of wireless communications, or causes extra steps to be
taken or hardware to be installed. The right network layout minimizes the effect of
any obstructions.
Appendix A - Considerations for RF Communications
41
03.2014
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