3.2 Electromagnetism Analysis of Physical Anti-Collision …
85
E
= E
0 e
i
k
·· x−ωt
= E
0 e
−αz e
i(βz−ωt)
(3.33)
where E
0 is the electric field amplitude in salt mist environment,ω is angular
frequency, α is attenuation coefficient, β is phase coefficient, given by
α = β =
ωμσ
2
(3.34)
In the RFID testing system, free charge movement in the salt mist environment
forms the eddy current, which generates Joule heat. The current density j in the salt
mist environment can be given as:
j = σ E
(3.35)
The average power consumption per unit volume of the salt mist environmentis:
p =
1
2
R E
j · E
∗
=
1
2
R E
σ E
· E
∗
=
1
2
R E
σ E
0 e
−αz e
i(βz−ωt)
· E
0 e
−αz e
- i(βz−ωt)
=
1
2
σ E
2
0 e
−2αz
(3.36)
The height of the salt mist environment is set as a, the power per unit surface area
is:
P =
a
0
pds =
1
2
σ E
2
0
a
0
e
−2αz dz
(3.37)
where
a
0
e
−2αz dz = −
1
2α
e
−2αa
− 1
≈
1
2α
(3.38)
Therefore, the power of per unit surface area can be deduced as:
P =
σ E
2
0
4α
(3.39)