Appendix A - Radio Frequency Identification Defined
Wireless Communication and Air Interface
Page A-6 MDE-3664B TRIND® Start-up, Service, and Parts Manual · June 2013
Range and Power Levels
The range that can be achieved in an RFID system is essentially determined by:
• The power available at the reader/interrogator to communicate with the tag(s).
• The power available within the tag to respond.
• The environmental conditions and structures, the former being more significant at higher
frequencies including signal to noise ratio.
Although the level of available power is the primary determinant of range the manner and
efficiency in which that power is deployed also influences the range. The field or wave
delivered from an antenna extends into the space surrounding it and its strength diminishes
with respect to distance. The antenna design will determine the shape of the field or
propagation wave delivered, so that range will also be influenced by the angle subtended
between the tag and antenna.
In space free of any obstructions or absorption mechanisms the strength of the field reduces in
inverse proportion to the square of the distance. For a wave propagating through a region in
which reflections can arise from the ground and from obstacles, the reduction in strength can
vary quite considerable, in some cases as an inverse fourth power of the distance. Where
different paths arise in this way the phenomenon is known as “Multi-path attenuation”. At
higher frequencies absorption due to the presence of moisture can further influence range. It is
therefore important in many applications to determine how the environment, internal or
external, can influence the range of communication. Where a number of reflective metal
“obstacles” are to encountered within the application to be considered, and can vary in number
from time to time, it may also be required to establish the implications of such changes
through an appropriate environmental evaluation.
The power within the tag is generally speaking a lot less than from the reader, requiring
sensitive detection capability within the reader to handle the return signals. In some systems
the reader constitutes a receiver and is separate from the interrogation source or transmitter,
particularly if the “up-link” (from transmitter-to-tag) carrier is different from the “down-link”
(from tag-to-reader).
Although it is possible to choose power levels to suit different application needs is not possible
to exercise complete freedom of choice. Like the restrictions on carrier frequencies there are
also legislative constraints on power levels. While 100 - 500 mW are values often quoted for
RFID systems actual values should be confirmed with the appropriate regulatory authorities,
in the countries where the technology is to be applied. The authorities will also be able to
indicate the form in which the power is delivered, pulsed or continuous, and the associated
allowed values.
Having gained some grasp of the data communication parameters and their associated values it
is appropriate to consider, in a little more detail, the components of an RFID system.