
TAGS 63
Coupling
A tag’s coupling mechanism determines the way a circuit on the tag and a circuit on the
reader influence each other to send and receive information or power. The type of cou-
pling a tag uses directly affects the read range between the tag and reader. We can group
the different read ranges loosely into those systems where the read range is close (within 1
cm), remote (1 cm to 1 m), or long-range (more than 1m). A synonym for remote coupling
is “vicinity coupling.” In this section, we will discuss backscatter coupling, inductive cou-
pling, magnetic coupling, and capacitive coupling. Capacitive and magnetic coupling are
examples of close coupling, inductive coupling is a type of remote coupling, and backscat-
ter coupling may be remote to long-range.
Along with range, the choice of coupling mechanism strongly affects which frequency the
tag should use. Inductive coupling typically works best in either the LF or HF range. Back-
scatter coupling works better with higher frequencies. Magnetic coupling is most efficient
at 1–10 MHz, and capacitive coupling usually runs at about 10 MHz to provide a fast
enough clock for communications and processing.
RFID coupling, especially in the case of passive tags, may seem a bit mysterious at first.
How can a tiny tag with no battery participate in a radio conversation? Let’s find out.
Backscatter coupling
Backscatter coupling provides an elegant ...