The
loss factors which affect antenna??™s ef?¬?ciency can be attributed to the effects of the mismatch
(re?¬‚ection) losses at the connection of the antenna and the source. Therefore, the total
power delivered to the antenna terminal is equal to the ohmic (Pohmic) losses plus the
radiated power (Prad) by the antenna, Pin??Prad??Pohmic.
The gain [G(u,f)] is de?¬?ned as the ratio of the antenna radiated power density at a
distant point to the total antenna input power (Pin) radiated isotropically. Thus, the
antenna gain represents the actual ef?¬?ciency of the antenna, where the gain and directivity
are related with the ef?¬?ciency factor that is applied to the directivity value to calculate
the gain of the antenna. The graphical representation of the antenna gain is accomplished
with the radiation patterns, which are typically drawn for the vertical and horizontal
polarizations or three-dimensional polar plots.
The antennas can be classi?¬?ed into two principal groups: directional and omnidirectional.
The directional antennas have a high gain, but a narrow ?¬?eld view (i.e., patch). The
omnidirectional antennas have a low gain, but a wide ?¬?eld view (i.e., dipole).
In a typical RFID system, the tags are designed to use either a dipole, omnidirectional
type antenna or a combination of two-dipole antennas to eliminate the problem of nulls in
the radiation pattern of a single dipole (Figure 1.
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