Understanding UHF “Near Field”
There are two components of the RF (Radio Frequency) wave: magnetic and electric. Generally speaking, HF RFID (13.56 MHz) relies on the “near-field” magnetic aspect of the field, while long range UHF RFID (860–960 MHz) exploits “far-field” radiation (which consists of both electric and magnetic components). But just which part of the RF wave a UHF tag responds to depends on two things: the tag antenna and its distance from the reader.

Because the magnetic component of the wave diminishes greatly in strength over a relatively short distance. This magnetic component is characterized as near field; that is, its effective range is limited by antenna geometry to about one or two wavelengths. And because HF tags rely on inductive coupling to this magnetic field in order to receive power, the HF tag antenna is made up of an inductive, coil-like structure that requires more conductive material and more complex manufacturing processes than the equivalent UHF tag antenna. Fortunately HF tags have no corner on the near-field magnetic field; with the right antenna, UHF tags
can just as easily harvest the same near-field energy—and do so more efficiently and cost-effectively. How? By virtue of the physics.

Maxwell’s four equations are the basis of electromagnetic analysis and design. Faraday’s law is one of these equations: “Voltage induced on a coil in a magnetic field is proportional to the intensity and frequency of the field.” This translates to a very simple concept: higher frequency = greater efficiency. UHF is ~60x the frequency of HF, meaning that UHF is ~60x more efficient in coupling its energy between tag and reader antennas. You can’t argue with the physics!

The conventional wisdom is that UHF is not suitable for item-level tagging—UHF tags are too big, UHF won’t work on liquids, metals, or small items packed in close proximity to one another, UHF has too great a range—ignores the fact that Gen 2 can exploit UHF’s near field just as easily and much more effectively than HF. That means anything that can be read by an HF system can also be read by a UHF
system—and that includes items high in liquid or metallic content. More importantly this means that item-level applications can now leverage all the benefits that the UHF Gen 2 standard brings to the supply chain at large.

Antennas
There is big confusion in the market regarding UHF Near Field and Far Field.
Every antenna has near field Near Field features – the magnetic field near the antenna surface. The magnetic field of a Near Field antenna is slightly stronger but Near Field antennas can read Far Field tags from a few meter distance. As explained before all depends on the form factor and design of the tag antennas. People expect that Near Field antennas will read only in close proximity to the antenna, this is truth only when using Near Field tags. Near Field antennas have a stronger magnetic field and are designed to read Near Field tags all around the antenna’s surface. Near Field antennas can also read Far Field tags at long distances.
Reducing the read range of an antenna when using Far Field tags is achieved simply by reducing the reader output power. The antenna read range is a function of the readers output power (input to the antenna) and the tag form factor, dimension and design.