Protecting an antenna from the environment requires EM analysis plus materials expertise.
As an RF component, radome design is highly dependent on materials, dimensions, and geometry, as well as other factors. We’ll look at some of those in this part.
What about geometry?
Three geometric factors affect radome electrical performance: radome wall thickness, distance from antenna to radome, and radome angle. We have already looked at thickness and impact on reflections and transmission. Now we’ll look at the other two:
Antenna distance
The distance between the antenna and the radome affects performance. Depending on the radome thickness and distance, the initial reflection from the radome will induce multiple reflections between the antenna and radome. This leads to a standing-wave pattern where the amplitude varies as a function of radome distance. The amplitude variation will be minimized if the reflected wave from the radome is in phase with the transmitted wave. The optimum radome distance is
𝑑1 = 𝑚 × (𝜆0/2) where 𝑚=1, 2, 3,…
Since the goal is to minimize reflection of the incident wave, it is best to introduce a surface at the lowest-wave amplitude, which occurs at the half-wavelength point. Therefore, the radome thickness and the distance to the antenna should be in multiples of half-wavelengths, as seen in Figure 1.
As a radome can never be perfect, relative movements (vibrations) between antenna and radome will lead to large signal levels at the radar transceiver. These signals mostly look like normal Doppler signals caused by moving targets and can lead to the malfunction of the sensor system. Mechanical construction must prevent or at least damp relative movement between antenna and radome.
Angle
It might seem that since all the needed electrical properties of the material are known, designing the radome should be relatively straightforward. But there is also the incident direction of the RF wave onto the radome material. If the wave enters perpendicular to the radome (called boresight), reflections of the wave are minimal if the thickness is correct.
However, if the angle of arrival is different, the wave will incur some degree of increased reflection as the amount of radome material through which the EM waves must pass increases, shown in Figure 2. The magnitude of reflection increases as the incident angle of arrival increases.
To work around this, it is often necessary to “shape” the radome and so lower this angle. But there’s a tradeoff: this may increase the distance between the antenna and radome surface, and so a “compromise” value must be accepted. This angle problem is especially challenging for larger antennas, while, in contrast, a small antenna such as used for a cell tower with a flat radome does not face as difficult a challenge.
Go for the layered look
Not surprisingly, another tradeoff is bandwidth. Although many antennas operate within a narrow frequency range, many others do not. The bandwidth of an antenna affects the calculations on thickness, distance, and other variables. Wider bandwidths require compromise among the various performance parameters to achieve acceptable results across the band, rather than superior results at one narrow frequency.
To address these issues, radome designs can build on the single-layer (monolithic) radome and advance to sandwich and even multi-layer designs, as shown in Figure 3.
Obviously, the monolithic version is the simplest. It is a single-wall structure with a homogeneous half-wavelength-thick dielectric material. Electromagnetic waves undergo less reflection when they contact the dielectric substrate in the straight-on boresight direction, but reflection quickly worsens as the incident angle increases.
One way to improve performance is to use a basic “sandwich” radome, which has dielectric skins separated by a core material. The reflection can be reduced by the mutual cancellation of the reflections between the skins. As an added structural benefit, this type of radome has a high strength-to-weight ratio compared to a single-wall structure. The bandwidth of this type of radome can be increased by increasing the core thickness or by adding more sandwich layers of the proper dimensions.
Finally, the multi-layer radome structure is an extension of the sandwich. It can have any number of layers as needed to achieve desired properties such as low transmission loss over a wide frequency band, or to incorporate desired environmental and structural features — but at a high fabrication cost. Often this is fabricated as multiples of simpler A-sandwich radomes that are attached.
Materials
Radomes can be built of many materials; even wood was used in the early days. Wood, however, has many weaknesses, as it has relatively medium-to-high absorption along with low wave penetration.
Better options include foams such as polystyrene, which has low absorption and reflection as well as very good wave penetration. They can even be placed directly on the antenna. However, one challenge is their low stability and sensitivity to chemicals, which can reduce the protective effect of the housing.
Radar waves penetrate plastics, which makes them among the best choices for the radome designs. The specifics of plastics vary, and different plastics offer combinations of absorption, reflection, and penetration.
Even with plastics, there are less-obvious considerations. If the plastic also contains other substances, such as carbon in the case of black plastics, this can also cause measurement losses. Coatings that are often applied to plastics can also negatively impact penetration. A material with resistance to UV rays will prevent erosion in material performance.
Accumulated water due to rough or uneven surfaces can affect the ability of a radar system. Selecting a hydrophobic material to repel water or other liquids from accumulation helps prevent that problem.
Failures of mechanical properties can also affect the antenna, with the prevention of material degradation key to ensuring the mechanical and electrical properties remain consistent over time. High impact strength can prevent cracking from the impact of foreign objects, high flex strength can prevent deformation from loads, and sufficient hardness will prevent surface damage. All of these are all necessary characteristics for acceptable long-term performance in the “exposed” world of radomes.
Real-world radomes
Two examples show the breadth of radome use.
Figure 4 is the nose of a commercial aircraft and shows the multiple antennas (one for radar, two antennas-only) protected by a single radome. These are the weather radar antenna for detecting weather conditions such as storms and turbulence ahead of the aircraft; the localizer antenna to help the aircraft align with the centerline of the runway during an instrument landing; and the glide slope antenna to provide vertical guidance to ensure the aircraft descends at the correct angle for landing.
A very different kind of radome assembly is shown in Figure 5. This small proximity sensor assembly is in a rectangular box and integrates the active microwave electronics of the RF front end with its antenna structure; a similar but larger box is often used for cell-tower units.
The position of the radome in front of the antenna affects how the radar waves spread. In practice, this means there must be a uniform distance between the radome and antenna at any given point (parallel arrangement).
Even slight deviations, such as a small notch on the underside of the radome, alter radar wave propagation, as seen in Figure 6. Slanted radomes also adversely affect correct reflection. This also applies to rounded ends, protrusions, reinforcements, or grooves in the material.
The radome is an essential but easily underappreciated part of an RF system where the antenna must be protected against the “elements” as well as other stresses. The design and implementation of the radome requires the use of the implications of Maxwell’s equations as well as a detailed understanding of, and data on, the electromagnetic properties of the radome material.
References
Aerospace Radomes, Saint-Gobain Aerospace
Design of a radome, Innosent GmbH
The radome: More than an “aerodynamic housing”, www.engineeringpilot.com
Boost Your Radar’s Performance with Optimized Enclosures, RFbeam Microwave GmbH
Radome and mechanical design, Acconeer AB
Three Tips to Improve Radome Design, The Gund Company
mmWave Radar Radome Design Guide, Texas Instruments
A Fundamental and Technical Review of Radomes, MP Digest
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