RFID Frequency bands
2006年5月26日 Raghu Das
RFID frequency options for passive chip tags
Chip tags are usually made to work at specific frequencies which are licence free. These are:
- Low Frequency (LF) 125-135 KHz
- High Frequency (HF) 13.56 MHz
- Ultra High Frequency (UHF) 868-930 MHz
- Microwave 2.45 GHz
- Microwave 5.8 GHz
All have various advantages and disadvantages, and affect not only the performance and size of the tag but also the price of the tags and readers. Further, the tolerated power levels and regulations for these vary from country to country. For example, the maximum permitted legal power level (the power level at which the interrogator is set at) for 2.45 GHz in the USA is higher than in the Europe. This creates a difference in read range. Therefore work still needs to be done to balance these regulatory issues worldwide, which is progressing quickly at UHF.
The following figure shows some of the tag format options at each frequency:
Lower frequencies have low energy, which means they transmit data more slowly and range is limited. Tag antenna size is typically quite large for bext range. However, even though they have a smaller range than higher frequencies they are more tolerant of obstacles, even moderately tolerant of small amounts of ferrous metal in the way. One can flood an area with radiowaves from one or two antennas, avoiding blind spots.
High frequencies have more energy and therefore can be used for long range applications. A beam is involved, so it can be used for locating a smart label in three dimensions. Power drops off as the cube of distance for low frequency but only as the square of distance at high frequency. However, these high frequency beams are more easily stopped. They can give problems with reflections, irradiation of humans, aiming beams (flooding an area is less easy), inability to see round corners and problems with blocking of the beam even by some things the human eye can see through. High frequencies, having more energy, have faster data transfers.