Radio-wave transmitting cover and method for setting thickness of radio-wave transmitting cover
A radio-wave transmitting cover is configured to be arranged in the path of the radio wave of a radio-wave radar device. The thickness of the cover is set to a reference thickness at a reference position where the incident angle of the radio wave from the radio-wave radar device is 0°. The reference thickness is set to a value obtained by multiplying half of the wavelength of the radio wave by an integer. In the area around the reference position, the thickness of the cover is gradually changed such that the greater the distance from the reference position, the greater becomes the difference by which the thickness of the cover is larger than the reference thickness.
1. A radio-wave transmitting cover, which is configured to be arranged in a path of a radio wave of a radio-wave radar device, wherein a thickness of the cover is set to a reference thickness at a reference position where an incident angle of the radio wave from the radio-wave radar device is 0° , the reference thickness is set to a value obtained by multiplying half of a wavelength of the radio wave by an integer so that a radio wave attenuation, which changes periodically according to the thickness of the cover, has a local minimum, and in an area around the reference position, the thickness of the cover is gradually changed such that the greater a distance from the reference position, the greater becomes a difference by which the thickness of the cover is larger than the reference thickness, such that the radio wave attenuation, which changes periodically according to the thickness of the cover, has a local minimum, wherein the cover includes a base, which has a back on which the radio wave from the radio-wave radar device is incident, and a front-side member, which is provided on a surface on a side opposite to the back of the base, and a small curvature portion in which a curvature is partially reduced is provided at an interface between the base and the front-side member.
2. A method for setting a thickness of a radio-wave transmitting cover, which is configured to be arranged in a path of a radio wave of a radio-wave radar device, the method comprising:
setting the thickness at each position of the cover based on the following (Expression 1) to (Expression 8) that are satisfied among an incident angle at the position of a radio wave from the radio-wave radar device, the thickness of the cover, and an attenuation of the radio wave, such that the radio wave attenuation, which changes periodically according to the thickness of the cover, has a local minimum
attenuation=|2×transmittance| (Expression 1)
transmittance=−20 log τ (Expression 2)
τ
=
(
1
-
γ
2
)
exp
(
-
φ
L
0
-
j
φ
)
1
-
γ
2
exp
(
-
2
φ
L
0
-
2
j
φ
)
(
Expression
3
)
φ
=
2
π
d
λ
(
ɛ
ɛ
0
-
sin
2
θ
)
1
/
2
radians
(
Expression
4
)
L
0
=
(
ɛ
/
ɛ
0
)
tan
δ
2
(
ɛ
/
ɛ
0
-
sin
2
θ
)
(
Expression
5
)
γ
=
1
-
ɛ
e
+
jL
1
1
+
ɛ
e
-
jL
1
(
Expression
6
)
ɛ
e
=
{
(
ɛ
/
ɛ
0
)
-
sin
2
θ
cos
2
θ
(
perpendicular
polarization
)
(
ɛ
/
ɛ
0
)
2
cos
2
θ
(
ɛ
/
ɛ
0
)
-
sin
2
θ
(
parallel
polarization
)
(
Expression
7
)
L
1
=
{
(
ɛ
/
ɛ
0
)
tan
δ
2
cos
θ
[
(
ɛ
/
ɛ
0
)
-
sin
2
θ
]
1
/
2
(
perpendicular
polarization
)
(
ɛ
/
ɛ
0
)
tan
δ
[
(
ɛ
/
ɛ
0
)
-
2
sin
2
θ
]
cos
θ
2
[
(
ɛ
/
ɛ
0
)
-
sin
2
θ
]
3
/
2
(
parallel
polarization
)
(
Expression
8
)
(Expression 8)
in (Expression 1) to (Expression 8),
τ: transmission coefficient of cover
φ: phase
L 0 : attenuation by loss
λ: wavelength of radio wave
d: thickness of cover
ε: permittivity of cover
ε 0 : permittivity of vacuum
θ: incident angle of radio wave
tan δ: dielectric loss tangent of cover
ε e : transmission relative permittivity
γ: Fresnel reflection coefficient
L 1 : reluctance due to loss of vertical polarization.
3. The method for setting a thickness of a radio-wave transmitting cover according to claim 2 , wherein
the thickness of the cover at a reference position where the incident angle of the radio wave is 0° is defined as a reference thickness, and
the method further comprises setting the thickness of the cover in an area around the reference position to a thickness that is closest to the reference thickness among thicknesses of the cover corresponding to local minimums of the radio wave attenuation.