68
3 Physical Theory of RFID System Physical Anti-Collision
3.1.3 Experimental Method and Result Analysis
(1) Influence of the thickness of the plastic box
The transmission coefficient of electromagnetic waves in a material has a great
relationship with the dielectric constant of material and the larger the dielectric
constant, the lower the transmission coefficient. Moreover, the attenuation speed
of an electromagnetic wave depends on the attenuation constant (α) in conductive
medium:
α = ω
με
2
1 +
σ
ωε
2 − 1
α ≈
σ
2
μ
ε
1+
σ
ωε
2 ≈ 1 +
1
2
σ
ωε
2
(3.20)
where ω is the angular frequency of electromagnetic wave, μ is medium permeability,
ε is the dielectric constant of the medium. If
σ
ωε
1:
1+
σ
ωε
2 ≈ 1 +
1
2
σ
ωε
2
(3.21)
Hence, Eq. (3.20) could be rewritten as
α ≈
σ
2
μ
ε
(3.22)
Therefore, we use the polytetrafluoroethylene (PTEE) to make a plastic box, which
has a small dielectric constant. Supposing the relative dielectric constant ε r = 2.1, the
relative magnetic permeability μ r = 1, and bulk conductivity σ V = 2.5×10
−17 S/cm,
we could obtain the effects of medium thickness on attenuation constant, as shown
in Fig. 3.5.
From Fig. 3.5, the attenuation constant increase gradually with the increase of the
medium’s thickness, but the index is 10
–15 , so the attenuation of the electromagnetic
wave in the medium can be ignored.
In order to verify the influence of the plastic box’s thickness on the reading
performance of the RFID tag, we test the plastic boxes at room temperature, whose
thickness is 0.3, 0.5, 1, 2, and 3 mm respectively, as shown in Fig. 3.6.
The thickness of the plastic box has little effect on the performance of tags in the
range of 0.2–3 mm and the maximum range is 0.02 m, which could be considered
within the limit of measurement error in practical applications. Simultaneously, the
error is equal for the reading distance at different temperatures, so the following
3 Physical Theory of RFID System Physical Anti-Collision
3.1.3 Experimental Method and Result Analysis
(1) Influence of the thickness of the plastic box
The transmission coefficient of electromagnetic waves in a material has a great
relationship with the dielectric constant of material and the larger the dielectric
constant, the lower the transmission coefficient. Moreover, the attenuation speed
of an electromagnetic wave depends on the attenuation constant (α) in conductive
medium:
α = ω
με
2
1 +
σ
ωε
2 − 1
α ≈
σ
2
μ
ε
1+
σ
ωε
2 ≈ 1 +
1
2
σ
ωε
2
(3.20)
where ω is the angular frequency of electromagnetic wave, μ is medium permeability,
ε is the dielectric constant of the medium. If
σ
ωε
1:
1+
σ
ωε
2 ≈ 1 +
1
2
σ
ωε
2
(3.21)
Hence, Eq. (3.20) could be rewritten as
α ≈
σ
2
μ
ε
(3.22)
Therefore, we use the polytetrafluoroethylene (PTEE) to make a plastic box, which
has a small dielectric constant. Supposing the relative dielectric constant ε r = 2.1, the
relative magnetic permeability μ r = 1, and bulk conductivity σ V = 2.5×10
−17 S/cm,
we could obtain the effects of medium thickness on attenuation constant, as shown
in Fig. 3.5.
From Fig. 3.5, the attenuation constant increase gradually with the increase of the
medium’s thickness, but the index is 10
–15 , so the attenuation of the electromagnetic
wave in the medium can be ignored.
In order to verify the influence of the plastic box’s thickness on the reading
performance of the RFID tag, we test the plastic boxes at room temperature, whose
thickness is 0.3, 0.5, 1, 2, and 3 mm respectively, as shown in Fig. 3.6.
The thickness of the plastic box has little effect on the performance of tags in the
range of 0.2–3 mm and the maximum range is 0.02 m, which could be considered
within the limit of measurement error in practical applications. Simultaneously, the
error is equal for the reading distance at different temperatures, so the following