Learn how near-field vs. far field work for RFID and wireless sensors and the implications for the different frequency bands used in them.
These series of posts are not intended to be a technical paper. Radio frequency is in itself a very complicated topic so bear with me in this post as I will generalize some concepts and assumptions for simplicity’s sake.
Limiting this post to RFID and wireless sensors, you will generally hear about three different bands: Low Frequency (LF), High Frequency (HF) and Ultra High Frequency (UHF).
As shown in the picture below, wireless power transfer and communication can happen in two different environments.
The near-field produces classical electromagnetic induction and electric charge effects on the EM field. Typically near-field effects are not important farther away than a few wavelengths of the antenna. This is the same principle that applies in induction coupled devices, such as a transformer, which draws more power at the primary circuit, if power is drawn from the secondary circuit. It is important to note that power decreases drastically as you move away from the power source in the near field.
This is different with the far-field, which constantly draws the same energy from the transmitter, whether it is immediately received, or not. The power received decreases in relation with the distance to the source but not as dramatically as in the near field.
To complete the concepts of near-field and far field associated to the different frequencies we need to understand what their wavelengths are. Please note that a wavelength is calculated as:
c = 3e8
υ = frequency used
The following table shows approximate wavelength values for the main RFID:
This may suggest working in the near field using LF and HF frequencies could be a good solution for battery-free sensor tags.
Lower frequencies have longer wavelengths so their near-field will be longer in distance. You will be able to harvest larger amounts of power in the near-field and that sounds great for your sensor tags.
However, there is a reason why wireless sensor manufacturers tend to use UHF for their solutions and also a reason why the passive sensor tags in LF and HF have very short communication ranges: antennas.
In general, antenna size must be proportional to the wavelength. This means antennas for LF and HF solutions should be a lot longer than those for UHF products. We will dig deeper into this in the next post of these series.