User's Manual

MPR series User’s Manual: Draft version 0.95 11/4/04 page 23
operates in each channel for 50 to 400 milliseconds. During hops from one channel to another, the RF
output is turned off.
1.5.3.2 Antennas
Antennas are the intermediaries between the voltages sent and received by the reader, and the
electromagnetic waves used to provide power to and communicate with the tags. Three critical
characteristics of antennas used in RFID systems are their maximum directive gain, polarization, and
match.
Electromagnetic radiation consists of a traveling electric and magnetic field. The electric field has a
direction at any point in space, normally perpendicular to the direction of propagation of the wave; this
direction is the polarization of the wave. For linearly polarized radiation, the direction of the electric field
is constant as the wave propagates in space. Configurations can also be constructed in which the direction
of the electric field rotates in the plane perpendicular to the direction of propagation as the wave
propagates: this is known as circular polarization.
The best power transfer between antennas is obtained when their polarizations match. Thus the best read
range is obtained from e.g. a vertically polarized reader antenna transmitting to a vertically polarized tag
antenna. This is an excellent scheme to employ when the orientation of the tag during reading can be
controlled. However, if the orientation of the tag can vary, the tag could accidentally be perpendicular to
the polarization of the reader antenna – a horizontal tag with a vertically polarized signal in shown in the
diagram below – in which case very little power is received, and the tag will not be read. When the tag
orientation is unknown or uncontrollable, a circularly polarized reader antenna should be used. Vertical
tags, horizontal tags, and tags rotated to intermediate angles can then be read with equal facility. However,
this versatility is not without cost. A circularly polarized signal can be regarded as the combination of a
horizontal and vertical signal, each containing half of the transmitted power. A linearly polarized tag
antenna only receives its own polarization, and thus half the transmitted power, being of the wrong
polarization, is wasted. The read range of a circularly polarized antenna with a linearly polarized tag is
reduced from what could be obtained with a linearly polarized reader antenna, if the tag orientation is
known.