wilamowski-b-m-irwin-j-d-industrial-communication-systems-2011

8-10 Industrial Communication Systems
8.12 Semi-Active RFID
A wake-up chip is a passive device that wakes up under a strong incident field, yet the responses of a tag
are powered by a battery. Hence, it is called a semi-active RFID or a semi-passive RFID.
Dependent on applications, an incident electromagnetic wave as strong as 4.W EIRP (effective isotropic
radiation power) could be used to activate a wake-up module meters away from the source.
Compared with the milliwatt wave generated by an active RFID reader, it is a strong radiation. At the
UHF band, the radiation is a low-energy nonionizing one whose effects or lack of effects on our health
is a heated controversy in many industrialized nations. In order to excite a strong field on the one hand
and to protect our health on the other, high-gain directional antennas are needed. Due to physical constrains,
a directional antenna cannot be installed in a tag. As the last resort, a resonant half-wavelength
antenna is used. Hence, semi-active RFID systems are usually operated at one of the microwave ISM
frequencies; that is, at 2450 or 5800.MHz.
For an efficient linkage, a user is required to point the tag toward the reader. However, the orientation
of the dipole antenna in a tag may not align with the polarity of the antenna in a reader. Hence, circularly
polarized antennas are preferred in the reader and the tag. To this end, a patch antenna is preferred
in a tag because it is a low profile and the cost of production is low. At 2450.MHz, the length of a patch
antenna without a dielectric substrate is 60.mm. The length could be reduced to 20.mm by printing the
patch antenna on aluminum dioxide (ε r = 10) or any substrate with a high dielectric constant. With
or without a dense substrate, a 60.mm tag is small enough to serve as a handheld mobile unit in most
applications.
As a traveling plane wave, wave propagation is vulnerable to many obstacles because it is not
supposed to circumvent an RF-opaque object whose size is as large as its wavelength, and in this
case, 122.mm or 52.mm. Consequently, the effectiveness of a microwave RFID is restricted to the
light-of-sight tags.
In addition to the above blockage, microwave RFID is subject to higher losses. Besides a high rate
of attenuation in moist air, some of the RF-lucent materials at HF and UHF bands would become RF
absorbent at microwave frequencies. The notable examples are dry wood, wet wood, water, and some
liquids. In general, microwave RFID is designed to operate within a small cell, a terminology used in
cellular phones.
One of the exceptions is traffic control on a highway because the line-of-sight requirement is not
an issue of concern there. Hence, a semi-active microwave RFID is often used in toll payments, such
as the Autotoll system in Hong Kong (Figure 8.10). It is also used in electronic road pricing schemes,
such as 407 ETR in Toronto. Powered by a CR2025 button battery, an Autotoll tag could run for several
FIGURE 8.10
Semi-active RFID tag, Autotoll system for tariff payment in Hong Kong.
© 2011 by Taylor and Francis Group, LLC