If the load impedance is open (1),
Gopen??1, and if the two impedances are matched, that is, complex conjugates, Gmatch??0.
Thus, all other variations of matching the antenna and load impedances result in a G
somewhere between ??1 and 1, which is the value used in the Friis equation of Figure 1.1.
There is a GT for the transmitting device and a GR for the receiving device.
1.3 Antenna Directivity and Gain
Two critical antenna characteristics are the directivity and the gain. The de?¬?nitions for
antenna directivity and antenna gain are essentially identical except for the power terms
used in the de?¬?nitions.
The directivity of the antenna [D(u,f)] is de?¬?ned as the ratio of the antenna radiated
power density at a distant point to the total antenna radiated power density isotropically.
Figure 1.18 demonstrates the coordinate system on which the directivity and the gain
are based. The directivity is typically expressed in decibels (dB) above a reference, where
D(u,F)[dB]??10 log10 D(u,f). The directivity of an isotropic radiator is equal to one. An
isotropic antenna with the spherical pattern is the typical reference antenna, which is
speci?¬?ed in the dBi units (Figure 1.19).
Antenna
impedance
(Z A)
Chip (load)
impedance
(Z L) FIGURE 1.17
Antenna and chip (load) connection.
FIGURE 1.18
Coordinate system.
Physics and Geometry of RFID 13
In real world applications, an antenna is driven by a source (power generator), where the
total radiated power of the antenna is not the total power available from the generator.
Pages:
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54