Radio frequency waves (RF) are generated when an alternating current goes through a conductive material. Waves are characterized by their frequency and length. Frequency is measured in hertz (or cycles per second) and wavelength is measured in meters (or centimeters).Radio waves are electromagnetic waves and they travel at the speed of light in free space.The equation that join frequency and length is the following:

speed of light (c) = frequency x wavelength.

In the equation you can see that, if the RF frequency increases, its wavelength decreases.The RFID technology uses four frequency bands: low, high, ultra high, and microwaves . Low frequency uses the band of 120-140 kilohertz. High frequency uses RFID technology in 13,56 MHz. Ultra high frequency RFID uses the frequency range 860 to 960 mega hertz. Microwave RFID generally uses 2,45 Giga Hertz and superior. For the four frequency bands used in RFID, microwave frequencies have the shorter wavelength.

Electromagnetic waves consist of two different (but related) fields: an electric field (known as “E” field), and a magnetic field (known as “H” field). The electronic field is generated by voltage differences. Since a radio frequency signal is an alternation, constant tension change creates an electric field that increases and decreases them with the frequency of radio frequency signals. The electronic field irradiates from an area of increased tension to one of less voltage.

In RFID

It is important to be conscious of both fields that make up electromagnetic waves. This is because RFID tags are going to use as much electric field as magnetic field to communicate their information, depending on the frequency they are using. RFID tags in LF and HF frequency bands use the magnetic field, while RFID UHF and microwave tags use the electric field.

When a reader emits radiofrequency signals , it causes variations in the electric and magnetic fields. When a conductive material, as a tag´s antenna, is inside the same variable field, a current is produced in its antenna.When a tag is near a reader´s field, the coupling of the tag´s antenna with a reader´s magnetic field creates a current. This coupling is known as inductive coupling. Inductive coupling is the communication process used by LF and HF passive tags.

In the case of UHF and microwave tags, tags modulate and reflect the reader´s sign to communicate with the reader. This is called backscatter passive communication (or backscatter modulation.).
The term «energy» refers to the radio frequency signal strength. It can be considered as the sum of RF transmitted, or the receiver’s signal strength. The basic unit of energy is the watt. However, in the RF world, we talk about power in terms of milliwatts, shortened to mW. One mW = .001 Watt.

Performing calculations using the decimal form of milliwatts can be tedious, so the standard is to calculate in decibels, or in powers of ten. The abbreviation “dB” is used when decibels are used. In the case of RF calculations, energy levels are generally mentioned as 1mW decibels, and the abbreviation "dBm" is used.

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