Basics for the Construction of a Radome

By Marion Henneberger, Content Manager, InnoSenT

Important note: The development and building of a radome is very complex. The mentioned data are only approximate values. This information gives only a first insight into this topic and does not replace necessary evaluations and tests.

Radar sensors consist of a front end (RFE) (microwave part with antenna structure) and components for signal processing. The actual core of the radar is the front end, because it is here that the antenna transmits and receives electromagnetic signals. In order to interpret the information collected, the front end then forwards it to the signal processing (Figure 1).

Image of basic components of an InnoSenT iSYS-4004 radar systemFigure 1: Basic components of a radar system (iSYS-4004 shown here). (Image source: InnoSenT)

In order to protect the radar antenna and electronic components, the sensor is usually enclosed with a casing. This protects the RFE from external influences that cause damage or impact performance. Thanks to its ability to penetrate through materials, radar is often preferred for aesthetic reasons as well. This is a particular aspect that product designers very much appreciate.

When speaking of such a protective casing for the antenna structure, radar technicians refer to a ‘radome’. The word is a combination of the words ‘radar’ and ‘dome’. The dome-shaped cover, like the one on the iSYS-6003, is primarily utilized with large radar systems installed fixed in place, such as the radars of aircraft or ships.

However, sensors and systems for industrial or commercial applications also require protection from mechanical or chemical impacts in order not to impair antenna function. These are adapted to the antenna and the properties of the radar waves.

In designing a radome, it is also crucial to use the correct material. If electromagnetic waves hit objects or persons, the material’s properties influence their spread. In order to find out which materials are suited for a radome, it is important to take into account the ensuing effect when they are hit by radar waves.

Table 1 is an overview that assesses various materials in terms of the absorption and reflection of as well as the ability to be penetrated by microwaves.

Material Absorption Reflection Wave penetration
Metal None Straight-on incidence: complete; Diagonal angle of incidence: Refraction and partial reflection possible Virtually impossible, only millimetre fractions penetrate into the surface (skin effect)
Wood (depending on humidity) Medium to high Low Low
Water Very high Depending on the angle of incidence: Partial or complete reflection possible None, due to absorption
Foams (e.g. polysty-rene, Roofmate) Low None Very good
Plastics Low to high (depending on material and thickness) Low to high (depending on material, thickness, and distance) Low to high (depending on material, thickness, and distance)

Table 1: Influences of various materials on radar waves

Radar waves must be able to penetrate the radome. Metals block the sensor. Because of their highly reflective properties, they are not suitable for positioning in front of an antenna. Wooden panelling (usually with a certain degree of residual humidity) is not suitable either, due to its limited capability to be passed through by electromagnetic waves.

Foams such as polystyrene are very well suited to be used as cover material. They can even be applied directly to the antenna in a very rough structure. However, due to their low stability and sensitivity to chemicals, foams often do not make the cut when it comes to material selection.

Plastics are therefore the most common alternative for producing a protective cover or housing. In the planning of a radome, however, the designer must take into account the properties of the plastic. The thicker and closer the material is to the antenna, the less the electromagnetic waves penetrate it.

In the case of black plastics, losses may occur in the measurement since these often contain carbon. Accumulating water that does not drain can also impair the information acquisition of the front end. Subsequent treatment of the plastic radome, for example by painting it, also negatively impacts data collection by the radar antenna.

Dimensioning and positioning of the radome

In constructing a radome, not only the selected material but also the precise fixation and shape of the radome are very important. In order not to restrict its functionality, the following aspects must be taken into account:

  • The distance between the underside of the radome and the antenna
  • The thickness of the radome’s material
  • The shape of the radome (as homogeneous as possible)

These factors determine whether the constructed radome reflects or absorbs most of the radar waves.

The right distance

The uniformity of the individual distances of the radome to the antenna is of tremendous importance. Even slight deviations, e.g. a little notch on the underside of the protective cover, can alter the spread of electromagnetic waves. For this reason, sloping radomes also have an adverse impact as they can prove detrimental to proper reflection. The same applies to round ends, lugs, reinforcements, or grooves in the material (Figure 2).

Diagram of right vs wrong positioning of an antennaFigure 2: Left picture shows “Wrong Positioning”: Radome has an uneven surface and is not positioned parallel to the antenna. Right picture shows “Correct Positioning”: Uniform distances as well as correct positioning and dimensioning of a radome. (Image source: InnoSenT)

In order to determine the correct, uniform distance, the following applies:

  • The spread of the waves is only slightly perturbed if they hit a radome at precisely half a wavelength (or a multiple thereof).
  • This means that the antenna surface (wave centre) must be positioned parallel to the cover, at a distance of λ/2 (or a multiple thereof).
  • With a center frequency of 24,125 GHz (with half a wavelength of about 6.2 millimeters (mm)), the optimum distance is approx. 6.2 mm.

The right material thickness

Here, the same principle applies as with determining the appropriate distance: in order to minimize disruption of the waves’ spread, they should hit the radome at half the wavelength. Similarly, the material thickness of the radome must also be selected appropriately for half the wavelength.

However, the manner in which the wave is altered by the substance of the radome (by penetrating the material) must also be taken into account. This adaptation corresponds to the conductivity of the material used (dielectric function ε). It shortens the wavelength by the factor √(εr).

For example, with plastics, this dielectric constant is between three and four, which, however, varies widely in practice. In order to obtain a ballpark figure, a calculation can be performed with the mean value of 1.5. The thickness of the material can then be calculated using the formula λ/2√(εr). This would equal 4 mm with these starting values.

Diagram of calculating the proper material thickness for a radome materialFigure 3: Example for calculating the proper material thickness for a radome material. (Image source:  InnoSenT)

In order to build the radome, extensive knowledge about the composition of the material used and the spread of electromagnetic waves is necessary. The information provided is intended only as guidance and to emphasize which aspects must absolutely be taken into account when constructing an antenna cover.

Disclaimer: The opinions, beliefs, and viewpoints expressed by the various authors and/or forum participants on this website do not necessarily reflect the opinions, beliefs, and viewpoints of Digi-Key Electronics or official policies of Digi-Key Electronics.

About this author

Marion Henneberger, Content Manager, InnoSenT

Marion Henneberger is responsible for Content Management at InnoSenT, the German Radar Technology company. Her job is to write on InnoSenT's innovative product solutions as well as radar technology in general. This also includes providing content on the many different applications that can be solved with Radar. The information is obtained directly from the InnoSenT Radar experts and prepared for public use. Therefore she regularly collaborates and talks with the company’s technicians, leadership and product managers.

In her role she transforms the technical data from the engineering into comprehensible content with the target to make the technology and the benefits understandable for ordinary people like you and me (assuming you are not a radar engineer). She enjoys to work for a company which pushes technical development to its limits and to experience the creation of significant inventions.