An electromagnetic wave moves or propagates in a direction that is at right angles to the vibrations of both the electric and magnetic oscillating field vectors, carrying energy from its radiation source to an undetermined final destination. The electric and magnetic fields are mutually perpendicular.
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 are affected in many ways by objects in their path and by the media through which they travel. This means that radio signal propagation is of vital importance to anyone designing or operating a radio system. The properties of the path through which the radio signals propagate govern the level and quality of the received signal. Reflection, refraction, and diffraction of the original signal can occur and may add constructively or destructively to the original signal. The resultant signal is a combination of several signals that have traveled by different paths due to reflection, refraction, and diffraction. In addition, the signals traveling via different paths can be delayed and can distort the resultant signal.
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.
Within the radio spectrum is an enormous range of frequencies. To categorize and manage the different areas of the spectrum, the radio spectrum is split into many different segments, but RFID technology uses only four of these segments.