What is Near-Field UHF?
All antennas, regardless of the frequency, generate electromagnetic fields that have both magnetic and electric components. Part of the electromagnetic field produced by an antenna is a radiating field which will propagate over long distances covering many wavelengths. This is known as the 'far field.' Examples include broadcast radio station antennas, and also cell phone antennas. At distances near the antenna, however, there exist other electromagnetic field components that decay much more quickly with distance and, as a result, are only active near the antenna. Depending on the resonant properties of the antenna and its geometric design, it is possible to design antennas (such as loop antennas or coils) which have a strong magnetic near-field component. UHF tags are now comercially available with loop antennas that can couple to this near-field. Although these 'near-field' UHF tags can only be used near the antenna (within a half-wavelength or so = 50 cm for UHF) such tags are of interest for applications which require penetration through liquids or dielectrics. There are also other tag designs which contain a 'hybrid antenna' which can operate with both the near-field and far-field components produced by the reader antenna. With few exceptions, most lower-frequency RFID tags operating at 13.56 MHz and 125 KHz are all near-field RFID systems, with the reading range roughly limited by the diameter of the reader antenna.

How small can you make an RFID Tag?
Although the electronic chip used in most RFID tags can be smaller than a grain of sand, a tag requires an antenna. The size of an RFID tag is generaly contrained by this antenna design. At higher frequencies the antenna is usually designed to be some small fraction of a wavelength, such as quarter-wavelength (roughly 4 inches at 900 MHz and 1 inch at 2.4 GHz); if the antenna is made smaller, then the reading distance of the tag will be greatly reduced. If long reading distance is not necessary, it is also possible to use inductive coupling for the UHF tags as well (near-field UHF), which can enable resonably small (1 cm) tags with loop antennas. In lower-frequency RFID systems, such as 13.56 MHz or 125 KHz, inductive coupling is used almost exclusively, and requires antennas in the form of coils, however at these lower frequencies the coils require a large number of turns. Small antenna coils wound on ferrite (to increase magnetic flux and range) are commonly used for tagging animals; these tags are the size of a grain of rice. Smaller tags can be made by electroforming the antenna directly onto the silicon chip, but the reading range is necessarily small.

What determines the reading range of a passive RFID Tag?
It is important to understand that there are many types of RFID technologies and each type has its own set of factors which determine range. At lower frequencies (125 KHz, 13.56 MHz), the tag antenna is electromagnetically coupled to the reader antenna, so the reading range of the RFID tag is determined by the sensitivity of the reader, the transmit power of the reader, the Q-factor of the tag, the power consumption of the tag IC chip, the size of the reader antenna, and the ratio of the tag antenna size to the reader antenna size. A crude rule of thumb is to say that the reading range is limited to 1.5 times the diameter of the reader antenna. At higher frequencies, such as UHF or microwave, the tag communicates through backscatter modulation, so the reading range is primarily determined by the reader sensitivity, the reader power, the power consumption of the tag IC chip, the radar scattering cross section of the antenna, the efficiency of the antenna, and the gain of the reader antenna and tag antenna. Some of these factors are regulated by the local governments. Present-day commercial UHF RFID systems have a range of 5-8 meters in free space using US power regulations, which allow 1 Watt of RF power and 4 Watts radiated power (EIRP).

What is chipless RFID?
It is important to understand chipless RFID is not one single technology, but rather it is a category of RFID that does not require the use of an IC chip. Some chipless RFID tags are designed to function as sensors (e.g. temperature or humidity) but others are dedicated to ID-only. Because no chip is used, the ID or sensor information is encoded in the analog electromagnetic signal of the tag, such as the resonant freuqency or harmonics. There are dozens of different chipless RFID technologies (too many to discuss here) ranging from very low frequency magnetic materials (such as library book security systems) to HF, UHF and microwave resonant, harmonic or backscatter systems (such as those used in retail anti-theft systems). Time-domain chipless tags are now also available, which echo back pulses transmitted from the reader. Although chipless RFID tags have less functionality than chip-based tags, the chipless technologies are generally 10X-100X lower in cost and can function at extreme temperatures, high radiation levels and other environments where chip-based technologies cannot operate.

What is the lowest cost RFID Tag?
We must first define what is an RFID tag. If we define a tag as a chip with an antenna on a paper or plastic substrate (with no adhesive or topcoat or printing), then this is known as an "inlay." Simple 96-bit UHF-frequency RFID tag inlays can be purchased today (2006) at a cost of approximately 10 cents (US $) at quantities over 50,000. The antennas for lower-frequency tags are a bit more expensive since the electromagnetic penetration depth is longer whcih requires a thicker metal layer; also, most tags at lower frequencies require a coil antenna, which generally requires a 2-layer metal process a cross-over linkfor the coil. If more memory or functionality is required, such as encryption, then the cost of the RFID chip itself can be significantly higher. RFID tags used for payment cards, for example, can store several kilobits of memory and support encryption and JAVA applications. There also exists other forms of RFID, such as chipless RFID which do not use a chip or can use special printed inks. Other families of RFID technologies are emerging as well which will have other cost structures. Some of these are shown in the figure below.

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