Reader antennas are the most sensitive component of an RFID system. Typical antenna set-ups for packaging line and dock door reading are shown in Figure 2.14 and 2.15. Most reader antennas are housed in enclosures that are easy to mount, and tend to look like plain, shallow boxes.

Figure 2.16 shows a portal designed to detect directionality. This configuration might be used at a doorway between the stockroom and the retail floor. Forklifts carrying RFID tagged cases going through the doorway will be read by antennas on one side before the other. Software in the inventory system then notes the two reads within a close time window, and interprets it as a direction. This allows the system to note that cases were carried from the stockroom to the retail floor. When the cases are detected going back to the stockroom, the system interprets that the cases have been emptied. This might be confirmed by another portal at the trash compactor.

ree characteristics of antennas contribute to tag readability:

• Pattern or footprint - The three-dimensional energy field created by the antenna. This is also known as the reading area.
• Power and attenuation - The maximum power of a reader antenna is fixed in order to meet FCC and other regulatory requirements. The signal can be decreased or attenuated, however, to limit the tag read window or aim it only at tags you want to read.
• Polarization - The orientation of the transmitted electromagnetic field. Linear antennas typically provide the longest range, but are sensitive to the orientation of a tag (Fig. 2.17). They can be used in instances such as an RF reading curtain mounted over a conveyor. The tags would be affixed to packages in a consistent orientation to maximize readability.

UHF antennas are nearly always externally mounted and connected to a reader via shielded and impedance matched coaxial cable.

One or more antennas can be connected to a signal reader, depending on your application requirements. An antenna is first selected for the operating frequency and application (omni-directional, directional, etc.). De-tuning, or signal weakening can occur due to the following:

• RF variations
• Skin-effects
• Losses due to metal proximity
• Antenna cabling losses
• Signal fading
• Proximity of other reader antennas
• Environmental variations
• Harmonic effects
• Interference from other RF sources
• Eddy Fields
• Signal reflections
• Cross talk