Exam Essentials

Explain the difference between near field and far field. The near field and the far field are two regions in electromagnetic radiation. The near field is the region near the antenna and extending approximately λ/(2π) from the antenna. The near field is an energy storage field primarily characterized by the magnetic field. The far field exists beyond the near field. The far field is an energy propagation field primarily characterized by electromagnetic waves.

Explain nulls and standing waves. Just like nulls, standing waves are caused by the interference of waves, but, unlike nulls, standing waves maintain a measurable amplitude.

Explain the dielectric constant. The dielectric constant is a dimensionless quantity that characterizes the relative electrical permittivity of a material, which is to say the ability of the material to store electrical energy in an electric field. For most materials under most conditions, the dielectric constant has a constant value independent of both the magnitude and direction of the electric field.

Identify RF propagation techniques. Radio frequency energy propagates through a medium in a manner that may be modeled as an electromagnetic wave. An electromagnetic wave consists of two perpendicular components, an electric field and a magnetic field, that are both orthogonal to the direction of travel of the wave. Electromagnetic waves exhibit linearity and may be refracted, deflected, reflected, and attenuated as they propagate through an environment.

Describe antenna field performance as it relates to reflective and absorptive materials. An antenna within an unobstructed free-space environment radiates energy in a pattern specific to that antenna. For antennas commonly used in UHF RFID systems, this pattern is similar in shape to an egg. When RF reflective and RF absorptive materials are present in the environment, the antenna's radiation pattern no longer resembles the simple free-space pattern for the antenna. Reflective material redirects the antenna's radiated energy, thereby distorting the field in a manner specific to the location of the reflective material relative to the antenna. A large sheet of metal, for example, that is placed at a 45-degree angle to the antenna approximately half-way into the antenna's pattern will cause the antenna's radiation pattern to extend 90 degrees to either the left or the right (depending on which of the two 45-degree angles is chosen). In this way, the radiation pattern can be made to extend around corners. Shielding, such as a cage, can also be used to trap the energy in a confined space. This has the effect of increasing the available energy in that space. RF absorptive materials absorb the energy emitted by the antenna. In this way, they reduce the size of the radiation pattern, limiting its reach in the directions where it must pass through the absorptive materials. Highly absorptive materials can be used to completely absorb energy in certain areas where the amount of RF energy is desired to be reduced.