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Anterus™ RFID Solution
Signal Strength Behavior of AMX RFID Tags and Readers
The AMX RFID system tracks assets and personnel by measuring the strength of the RF signal received
from a periodically transmitting badge or tag. Every reader within communication range measures the
signal strength and reports it to the master controller. Most often the asset or person will be located
closest to the reader reporting the strongest strength. There are, however, several environmental factors
that impact the strength of the received signal. The following subsections describe these factors, present
measured data, and describe the overall behavior to be expected from a system based on measurement of
signal strength of a transmitted UHF radio signal.
Environmental Factors
Several environmental phenomena have potential for changing the actual received level. They are each
discussed in the following sections. These effects are important to understand and keep in mind, as their
net effect will typically be to make the tag appear to be farther away than it actually is. Also be aware,
these factors can and do occur simultaneously and are additive in their effects.
Non-Ideal Antenna Gain
The ideal antenna would be one that transmits or receives with equal efficiency in all directions. In the
real world such an antenna, with a perfectly spherical gain pattern, does not exist. Commonly used
antenna types generally fall miserably short, with blind spot nulls in one or more directions. Both the
badge and asset tags employ a technique referred to as diversity antennas to achieve an omnidirectional
pattern that is much closer to the ideal, but still not quite the ideal perfect sphere. The reader antenna,
when oriented vertically, has a doughnut-shaped pattern, receiving equally well in all directions in a
horizontal plane. It has substantial blind spots when trying to receive from above or below, so it is
important to keep the reader antenna vertical if all tags to be read are on the same floor of a facility.
Antenna Elevation
RF signal strength loss is affected by the elevation of both the transmitting and receiving antennas above
ground. In general, if either or both ends are close to the floor the received signal will be weaker. This is
especially pronounced at greater distances. FIG. 8 illustrates this phenomenon. Thus, if maximum range
is required by a specific application the readers should be mounted at least a couple meters above the
ground.
FIG. 8
Antenna Height Influence on Received Signal Strength
8
Anterus RFID Solution
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