1. Description of the casing
1. 1 Enclosure Requirements
Millimeter-wave sensor antennas generally require a protective casing to protect them from environmental influences. To minimize the impact of the casing on
electromagnetic signals, the following principles should be followed when selecting or designing the casing:
(1) The distance between the casing and the millimeter-wave sensor antenna should be ≥6.2mm (if the casing design principle is not met, please verify by actual
measurement);
(2) When the casing material is PC or ABS, a casing thickness of approximately 3~4mm is most suitable;
(3) The casing material should not contain metal materials, metal paint, carbon powder, particles, etc.;
(4) The casing should be a flat plate structure with uniform thickness;
(5) Avoid any vibration of the casing relative to the millimeter-wave sensor antenna；
If the existing housing selected by the user does not meet the design principles, actual testing and verification should be conducted to verify the impact of the selected "housing-antenna distance," "housing thickness," "housing material," and "housing with slight variations in thickness" on sensor performance. Even if there is a minor impact, if the performance and functionality still meet the requirements, the selected housing can still be used.
If users need to design housings or understand the relationship between housings and millimeter-wave sensors in more detail, they can refer to “Housing Design Principles" chapter.
1. 2 Enclosure Design Principles
Substandard housings may increase electromagnetic wave reflection and increase electromagnetic wave attenuation when penetrating the housing and distort the antenna pattern due to uneven surfaces. These issues can significantly impact the performance of millimeter-wave sensors, even causing malfunctions. Housing design requires a uniform material of consistent thickness and is influenced by three key factors: thickness, distance, and material composition.
1.2.1 Enclosure thickness
The wavelength λₘ of millimeter waves in materials has a significant impact on the thickness of the outer enclosure. λₘ is shorter than the wavelength λ₀ of millimeter waves in air. The calculation steps for λₘ are as follows:
Note: 𝑓𝑐 is the electromagnetic wave frequency 24.125 GHz, 𝑐₀ is the speed of light 3 ∗ 10⁸ m/s, and λ₀ is the wavelength of the 24.125 GHz electromagnetic wave in air
Note: εᵣ is the dielectric constant of the shell material at a frequency of 24.125 GHz.
When the thickness Tᴅ of the outer shell satisfies formula (4), the outer shell can be regarded as "transparent" to millimeter waves; that is, there is no attenuation of electromagnetic wave signals.
Assuming PC material is used, with a dielectric constant of 2.9, and n is typically taken as 1, the calculated shell thickness Tᴅ is
1.2.2 Distance between the casing and the antenna
A proper distance between the housing and the millimeter-wave sensor antenna can significantly reduce the reflection effects caused by the housing. If the echo reflected from the outer casing is in phase with the transmitted wave at the antenna, the reflection effect is minimized; that is, the distance between the outer casing and the antenna, dₘ = 0.5 ∗ λ₀
Note: A distance less than 6.2mm should be avoided; the distance must be greater than 6.2mm.
1.2.3 Enclosure Material
The housing material must not contain metal, metallic paint, carbon powder particles, etc. To calculate the optimal thickness, the relative dielectric constant (εr) must be known. To reduce electromagnetic wave attenuation, select a material with a low loss tangent tan δ. The material εr and tan δ vary with frequency; at higher frequencies, the value of εr is typically slightly lower. Table 1.1 shows the parameters at 24 GHz for materials commonly used in millimeter-wave sensor housings.
