Electric field coupled transponders generally provide vastly increased
ranges over their magnetic counterparts. Rather than being limited to the
ranges of the lines of force emitting from a magnetic field generator,
they use the electric field propogation properties of radio communication
which radiate out from the energising antenna, quartering in signal strength evey doubling of distance travelled,
to convey energy and data from the reader to the transponder and data from
the transponder to the reader.
Electric field propogation requires antenna systems that are typically
half a wavelength of the operating frequency in size (150cm at 100MHz,15
cm at 1GHz, 5 cm at 2.5Ghz and 2.5cm at 5.8Ghz). This causes practical
limits to how low a frequency to start using Efield propogation methods
due to the size of the antenna.
UHF tags refer to tags that operate in the 860 - 915 Mhz frequency range
Higher operating frequencies require more expensive components and loose
the ability to transfer energy at a rate of the inverse of the wavelength
squared. (A 2.45GHz system would need seven times the energising fields needed by a 915Mhz system)
In addition the energy density of a signal radiated using electric field
coupling, decreases as the inverse of the distance squared between the
source and the transponder. Whereas sensitive receivers can compensate
for this loss of energy for the data communications over long distances,
passive transponders which use the reader's energising field as a source
of power are practically limited to 10 to 15 meters.
Beyond that distance (which reduces drastically with increased frequency
to less than 1 meter at 2.5GHz) it is necessary for the tags to use an
external battery as a source of power (hence become active transponders).
Electric field tags are available in many different configurations and
price ranges, particularly dependant on the complexity of the transponder.
If the transponder is a read/write transponder and is required to operate
beyond the range of passive transponders, the receiver circuitry onboard
can be expensive and difficult to construct particularly if frequency stability
is needed with temperature.
However the invention of the backscatter modulation principle at Lawrence
Livermore Laboratories in the 1960s and the skills of semiconductor designers
to shrink all features into cheap integrated circuits, has meant that electric
field type tags in a read only mode can be made extremely cheaply, most
probably for less than 15 US cents in high volume. Such a tag would be
passive, have no onboard tuned circuits, be read only, consist of a single
integrated circuit and a simple antenna, would operate at any of a range
of frequencies, be temperature insensitive, and would broadcast a large
data value when illuminated by a reader's energising field. In such a system
the reader is complex because it provides the frequency stability, the
energy of the system, and the receiver selectivity to receive the weak
return communications, but the tags are very cheap. This is ideal for the
situations where there is one reader and many tags.
Electric field tags need to operate in an ordered spectrum management
system as their radiated energy (particularly from the reader) can be detected
by other sensitive receivers far away and cause possible interference.
Recent developments in passive tag technology see the amount of power
needed to power up the tag dropping dramatically. The reader radiates energy
from its transmit antenna, some of which is collected by the tag in an
area around its antenna called the "antenna's aperture". The
size of this area is dependant upon the characteristics of the tag antenna
and the operating frequency of the system, (e.g a 915MHz dipole has a 134cm2
aperture). Traditionally a 5 volt logic circuit on a half wave dipole in a transponder would
need 55 milliwatts of RF energy to operate while recent developments see
this amount of power dropping to less than 200 microwatts, thereby dramatically
reducing the power needed to be radiated by the reader for the energising field and increasing the range over which
passive transponders can operate effectively. (260 Times lower power giving 260 times battery life of reader or
16 times the operating range).
Electric field tags are used in transponder/smart card systems for toll road applications. Here the
tags are active (that is they have a battery) but only consume battery
power after the tag is "activated" by passing through a high
energy activation field. Thereafter the tag can send/receive data with
an overhead reader and can adjust the data representing the balance remaining
in the smart card after the toll fees are deducted. Such applications are
implimented in the 2.45GHz frequency band and more recently in the 5.8GHz
A seperate category also exists of "active" tags (battery
powered). These tags are "beacon" tags, that is they are not
interrogated by a reader, but wake themselves up from a low power "sleep
mode" periodically and broadcast their identity before returning to
"sleep mode". By broadcasting on a fixed frequency, a sensitive
receiver tuned to that frequency and within close proximity to the tag
will receive the identity message. This type of transponder offers ranges
up to hundreds of meters, but is not suited for situations where the location
of a tag is being determined to a couple of meters range, or where very
many tags are present in the reader zone. The range of the systems is dependant on the height of the transponder and the reader above the ground.
A common version of such a tag is the remote controlled operation of an electric gate or garage door.Encryption technology has also
been added to these systems to stop unwanted tags being accepted as valid
codes by the reader.
Despite the hurdles, the greater range, higher data rates and new technologies
make these transponders suitable for a great number of applications.