Theory and Experiments on Peano and Hilbert Curve RFID Tags

Recently, there has been considerable interest in the area of Radio Frequency Identification (RFID) and Radio Frequency Tagging (RFTAG). This emerging area of interest can be applied for inventory control (commercial) as well as friend/foe identification (military) to name but a few. The current technology can be broken down into two main groups, namely passive and active RFID tags. Utilization of Space-Filling Curve (SFC) geometries, such as the Peano and Hilbert curves, has been recently invest igated for use in completely passive RF ID applications [1, 2]. In this work, we give an overview of our work on the space-filling curves and the potential for utilizing the electrically small, resonant characteristics of these curves for use in RFID technologies with an emphasis on the challenging issues involved when attempting to tag conductive objects. In particular, we investigate the possible use of these tags in conjunction with high impedance ground-planes made of Hilbert or Peano curve inclusions [3, 4] to develop electrically small RFID tags that may also radiate efficiently, w ithin close proximity of large conductive objects [5]. 1.1. Passive RFID tags Passive RFID tags have no internal power supply. The elect rical current induced in the antenna by the incoming radio frequency signal provides just enough power for the integrated circuit (IC) in the tag to power up and transmit its response. Most passive tags signal by backscattering the carrie r signal from the reader. This means that the antenna has to be designed to collect its power from the incoming signal as well as transmit the response signal. The lack of an onboard power supply means that the device can be quite small. Passive tags have practical read distances ranging from about 2 mm up to a few meters depending on the radio frequency and antenna design, type and/or size. Passive RFID tags do not contain power sources (e.g. batteries), and theref ore can be much smaller then their semi-passive and active counterparts and can have an almost un limited life span. Because passive tags ar e cheaper to manufacture and have no battery, the majority of RFID tags in existence are of the passive variety [6]. 1.2. Semi-passive Semi-passive RFID tags are similar to passive tags except fo r the addition of a small battery. This battery allows the tag IC to be constantly powered. This removes the need for the aerial to be designed to collect power from the incoming signal. The antennas can therefore be optimized for the backscattering signal alone [6]. 1.3. Active Active RFID tags (also known as beacons) contain their own internal power source which is used to power the integrated circuit(s) and to generate the outgoing signal. They are often referred to as beacons because they broadcast their own signal. They generally have longer ranges than passi ve tags, as well as an ability to store information sent by the transceiver. Many active tags have practical ranges of tens of meters, and a battery life of up to 10 years [6]. 2. Peano and Hilbert Space-filling Curves The first few orders of Hilbert and Peano curves are shown in Figure 1 [7]. An interesting property of a space-filling curve is that as the higher iteration-orders of this curve are considered a long line can be compacted into a small surface area. From an electromagnetic point of view, such a curve may provide a structure that, although small in its footprint, it can be resonant at a wavelength much longer than its footprint. Such a compact resonator can be quite useful in various applications in radiation and scattering problems. For instance, this feature is of interest to antenna designers since it provides a planar resonant radiator that can have a very small footprint as one considers the higher order in iterative filling of a 2-D region [8-11]. These curves also have been used in design of inclusions informing high-impedance surfaces (artific ial magnetic conductors) [3, 4].