RFID systems use radio waves to exchange information between RFID transponders, or tags, and interrogators or readers. How radio waves behave under various conditions in the RFID interrogation zone (IZ) affects the performance of the RFID system. Radio waves propagate from their source and reach the receiver. During their travel, they pass through different materials, encounter interference from their own reflection and from other signals, and may be absorbed or blocked by various objects in their path. The material of the object to which the tag is attached may change the property of the tag. To understand how all these phenomena affect the performance of the RFID system, you need to understand how radio waves are used and propagated.
Radio frequencies constitute a small portion of a larger electromagnetic spectrum. The entire electromagnetic spectrum includes gamma-rays, X-rays, ultraviolet light, visible light, infrared light, microwaves, and radio waves. The difference between these various parts of the electromagnetic spectrum is their wavelength, or frequency. The energy radiated by radio waves is called electromagnetic radiation.
A wave is a distortion in a material or medium. While the wave passes through the material or medium, the individual parts of the material cycle back and forth or up and down. All the waves have similar characteristics and follow the same laws and principles. The characteristics of a waveform are wavelength, amplitude, velocity, and frequency. All periodic waveforms have these common characteristics. A Sinusoidal wave is a continuous, uniform wave with a constant frequency and amplitude whose waveform is the same as that of a trigonometric sine function.
The relationship between velocity, frequency, and wavelength is shown by the following equation:
Using this equation, you can calculate frequency from wavelength or vice versa.
RFID technology uses the radio wave portion of the electromagnetic spectrum; therefore, our discussion of electromagnetic spectrum is limited to the radio wave portion of the spectrum, specifically the radio waves in the range of 100 kHz to 5.8 GHz.
Radio signals propagate in many different ways, including free space propagation, ground wave propagation, ionospheric propagation, and tropospheric propagation. These relate to the effects of the media through which the signals propagate. RFID applications occur at very small distances; therefore, free space propagation is the most important type. In free space propagation, the major factor affecting the signal strength is the distance between the transmitter and the receiver. RFID systems need to have their radio propagation models generated for factory, warehouse, office, or urban situations. Under these circumstances, the free space propagation is modified by multiple reflections, refractions, and diffractions. Despite these complications, it is still possible to generate rough guidelines and models for these radio propagation scenarios.