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Important note: The development and construction of radomes are complex. The data mentioned in the article are only approximate values. This information can only be used as a preliminary understanding of the subject and cannot replace the necessary evaluation and testing.
Radar sensors consist of the front end (RFE) (the microwave part with the antenna structure) and components for signal processing. The front end is the actual heart of the radar, as it is the part of the antenna that transmits and receives electromagnetic signals. In order to interpret the collected information, the front end forwards it to the signal processing unit (Figure 1).
Figure 1: Basic components of a radar system (iSYS-4004 shown). (Image source: InnoSenT)
To protect the radar antenna and electronics, the sensor is usually enclosed in a housing. This protects the RFE from external influences that could cause damage or degrade performance. Radars are often also appreciated for their aesthetic appearance, due to their ability to penetrate materials. This aspect is particularly appreciated by product designers.
When talking about this protective housing for the antenna structure, radar technicians refer to a "radome". This word is a combination of the words "radar" and "dome". Dome-shaped covers, like those on the iSYS-6003, are mainly used for large, fixed-mounted radar systems, such as those on aircraft or ships.
However, sensors and systems used in industrial or commercial applications also need protection from mechanical or chemical influences that could impair the antenna function. These are designed to be suitable for the antenna and to match the characteristics of the radar waves. It is
also crucial to use the right material when designing the radome. If the electromagnetic wave encounters an object or a person during propagation, the properties of the material can affect the propagation. In order to find suitable materials for radomes, it is important to consider their effects upon radar waves.
Table 1 summarizes the absorption and reflection properties of various materials for microwaves, as well as the penetration of microwaves into these materials.
Table 1: Effects of various materials on radar waves
The radar waves must be able to penetrate the radome. Metals shield the radar sensor. Due to their highly reflective properties, metal is not suitable for placement in front of the antenna. Wood paneling (which usually has a certain amount of residual moisture) is also not suitable, as electromagnetic waves have limited penetration through it.
Foams such as polystyrene are very suitable as covering materials and can even be wrapped directly on the antenna without processing. However, due to their lower stability and sensitivity to chemicals, foams are often not an option when choosing materials.
Therefore, plastics are the most common alternative material for producing protective covers or housings. However, when designing the radome, designers must take the properties of plastics into account. The thicker the material and the closer it is to the antenna, the less electromagnetic waves can penetrate.
In the case of black plastics, losses may occur during measurement, as these plastics usually contain carbon. In addition, water that cannot be drained away can have a negative impact on the information acquisition at the front. Subsequent processing of the plastic radome, such as painting, can also have a negative impact on the data acquisition of the radar antenna.
Size and position of the radome
When constructing the radome, not only the material selection of the radome is very important, but also the precise fixing and its shape. In order not to restrict its functionality, the following aspects must be taken into account:
The distance between the bottom of the radome and the antenna
The thickness of the radome material
The shape of the radome (as uniform as possible)
These factors determine whether a large part of the radar waves will be reflected or absorbed by the constructed radome.
Proper distance
The uniformity of the distance from the antenna to the radome is extremely important. Even slight deviations, such as a small notch in the bottom of the protective cover, can change the propagation of electromagnetic waves. For this reason, tilted radomes are also unfavorable, as they may affect the normal reflection. The same applies to rounded ends, lugs, reinforcements or grooves in the material (Figure 2).
Figure 2: The right picture shows a "wrong placement": the surface of the radome is not flat and is not placed parallel to the antenna. The left picture shows a "correct placement": the distance to the radome is uniform, the position and size are correct. (Image source: InnoSenT)
To determine the correct, uniform distance, the following conditions apply:
If the distance to the radome is exactly half a wavelength (or a multiple thereof), the propagation of the wave is only slightly disturbed.
This means that the antenna surface (wave center) must be placed parallel to the radome at a distance of λ/2 (or a multiple thereof). The
optimum distance is approximately 6.2 mm at a center frequency of 24.125 GHz (half a wavelength is approximately 6.2 mm).
Appropriate material thickness
In this regard, the same principle applies as for determining the appropriate distance: To minimize disturbance of wave propagation, the radome should be placed at half a wavelength. Likewise, the appropriate radome material thickness must be selected based on half a wavelength.
However, the way in which the radome material changes the wave (by penetrating the material) must also be considered. This change corresponds to the conductivity (dielectric function ε) of the material used and shortens the wavelength by a factor of √(εr).
For example, for plastics this dielectric constant is between 3 and 4, but in practice it varies greatly. To get an approximation, an average value of 1.5 can be used for the calculation. The thickness of the material can then be calculated using the formula λ/2√(εr). Using these initial values gives a thickness of 4 mm.
Figure 3: Example of calculating the appropriate thickness of a radome material. (Image source: InnoSenT)
To construct a radome, it is necessary to have a good understanding of the composition of the materials used and the propagation of electromagnetic waves. The information provided in this article is intended only as a guide to highlight the things that are absolutely necessary to consider when constructing a radome.
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