How to Select a Correct Tag – Frequency

HF tag antennas are usually made of a copper, aluminum, or silver coil with three to seven turns. They are easy to manufacture, and the tags are usually very thin, almost two-dimensional. HF tags are lower in cost due to simpler antenna design than LF tags. They can be made in different sizes, some less than a half inch in diameter. HF tags can be easily read while attached to objects containing water, tissues, metal, wood, and liquids. They are affected by metal objects in the close vicinity, however. HF interrogators are less complex than UHF interrogators and usually cost less than the latter.

Radiative coupling used by the UHF interrogators uses radio waves to power and communicate with the tags. Reflection, diffraction, and refraction of radio waves gives rise to a multipath effect, where radio signals arrive at the receiver using multiple paths. Some of the signals from multipath attenuate the original signal. This creates an IZ with varying signal strength. The tags may not be readable in low signal spots, causing random tag readability problems. The UHF antennas are directional, which helps create an IZ with well-defined boundaries, though the zone may have holes in it. UHF tags are getting a big boost from the mandates by large organizations for their use in the supply chain. This and the creation of the Gen 2 protocol have created a tremendous momentum in RFID industry to manufacture low-cost UHF tags in high volume. UHF frequency regulations are not as uniform as HF frequency regulations. Governments in various parts of the world had assigned the UHF frequencies at around 900 MHz, long before RFID, for uses other than for RFID. Therefore, there is no common frequency range around 900 MHz available for RFID use. Different countries have different bands available with varying allowable maximum power levels and duty cycles. To overcome this problem, the Gen 2 protocol was designed to work with any frequency band within the 860–960 MHz range and with different maximum power levels. The government regulations divide the allocated frequency range into a number of narrower frequency bands. These narrower bands are called channels. Different countries have different numbers of channels available within their allocated bandwidths. The regulations also require that the interrogators do not use a single channel all the time but pseudo-randomly hop among the available channels.

Microwave antennas are directional, which helps define the IZ for passive and semi-passive tags. Due to their shorter wavelength, they are easier to design to work with metallic objects. A wider band of frequencies is available to use and more hop channels are available. However, many commonly used devices such as cordless phones and microwave ovens use this frequency. Therefore, interference at microwave frequencies is possible. Government regulations regarding use of microwave frequencies for RFID are almost nonexistent. The semi-passive microwave RFID tags are used in long-range access control for vehicles, fleet identification, and highway toll collection. Active microwave tags are used for real time location systems (RTLS).