High Frequency RFID vs Low Frequency RFID: The Ranges Explained

Radio Frequency Identification (RFID) is a technology that allows for the automatic identification and tracking of assets, people, and animals. This technology operates across multiple frequencies, including Low Frequency (LF), High Frequency (HF), and Ultra High Frequency (UHF). Different frequencies have different properties and applications. Understanding why high frequency RFID can have longer ranges compared to low frequency RFID, despite the general rule that lower frequencies offer better penetration and range for other wireless technologies, is crucial for optimizing RFID system design and implementation.

Understanding RFID Frequencies

There are three primary frequencies used in RFID technology:

LF (Low Frequency): 125 or 134 KHz HF (High Frequency): 13.56 MHz UHF (Ultra High Frequency): 860–930 MHz

The key to understanding why high frequency RFID can have longer ranges lies in the dimensions of the antennas and the way the energy is transmitted and received.

Why High Frequency RFID Has Longer Ranges

For an RFID system to have a wide range, the antenna dimensions should be approximately half the wavelength of the frequency. The wavelength for the low frequency (LF) of 125 KHz is approximately 2400 meters, while for high frequency (HF) of 13.56 MHz, the wavelength is about 22 meters. At these wavelengths, it is practically impossible to use such large antennas in RFID tags, making them unsuitable for practical applications.

The Magnetic Near Field

The solution lies in the use of the magnetic near field. In the near field, the energy is not radiated but oscillates in the vicinity of the sender. This is particularly effective for low frequency RFID, where large antennas are feasible. However, in the magnetic near field, the fields drop by the third power of the distance (1/d^3). Therefore, doubling the distance results in an 8-fold decrease in field strength. Because the response of the tag is also reduced by the same factor, the overall field strength drops by the sixth power of the distance (1/d^6).

Increasing Reader Output Power

While increasing the output power of the reader can help, it is only a partial solution. The most effective way to increase read range is to use large size reader antennas and as large as possible tag antennas. All antennas in RFID are essentially coils. Under these conditions, the maximum read range is approximately 1 meter.

UHF RFID and Half-Wave Dipole Antennas

Ultra High Frequency (UHF) RFID operates in the far field, where the energy is radiated and the wavelength is much more manageable. For UHF tags, it is possible to create efficient half-wave dipole antennas, as the wavelength is around 33 cm. In the far field, the read range can easily exceed 10 meters.

Unfortunately, UHF RFID technologies often do not provide the frequency options that would allow for ideal read ranges. A frequency close to 433 MHz would provide read distances over 20 meters with tags that are only twice as large as normal UHF tags. Despite this, such frequency options are not available, making them practical for many applications.

Low Frequency RFID and the Near Field

Low frequency RFID uses what is called the ‘near field’. The energy transmitted by the transmitter is not radiated but oscillates in the vicinity of the transmitter. This is because low frequencies can only be effectively transmitted by large antennas, up to tens to hundreds of meters.

Conclusion

The superior read range of high frequency RFID can be attributed to the use of efficient antennas and the operation in the far field, unlike low frequency RFID which operates in the near field. Understanding these differences is crucial for selecting the most appropriate RFID frequency for a given application, whether it is for increased read range or better penetration and coverage.