In earlier blog issues, we reviewed the main features of MMIC transceivers used in automotive radar. There is still one aspect to cover: How to integrate the MMIC onto the PCB of the radar module, and how can we optimize the interface between the chip and antennas? To do this, we must first consider the type of antenna used in the system. We will look at two of the most popular solutions: microstrip antenna arrays and slots waveguides.
Considerations for Design
The antennas of FMCW radars are used for transmitting a continuous electromagnetic signal modulated over time. The antennas also receive the call reflected from objects in the surrounding environment and feed it back to the system for processing.
There are a few important factors to consider when designing antennas for FMCW automotive modules. The antenna bandwidth must be large enough to cover the desired frequency band (76-77GHz in the case of long-range radar and 77-81GHz in the case of corner and imaging radar depending on region and standards) while maintaining good radiation and matching properties. To guarantee the desired field of view and range, the antenna gain and beamwidth should be maintained over the entire frequency sweep range.
In order to maximize the field-of-view, it is common practice to use antenna arrays that have a half wavelength separation between rows. This can cause a strong coupling between antenna elements and performance degradation. The antenna should be designed to minimize any interference or reflections caused by other components, such as the module casing and vehicle parts like fascias or emblems.
The antenna must also be compact and small enough to fit into the space available for the radar module without compromising its efficiency. At the same time, manufacturing and assembly costs should be kept as low as they can. It would be easiest to print planar antennas on the PCB, but technological advancements in manufacturing have sparked interest in 3D designs, including waveguide-based geometry and lenses.
Patch Arrays Antennas
Microstrip patch radiators have been popular for many years due to their small size, low profile, and directional radiation patterns. The microstrip patch antennas could also cover the radar frequency band with low-to-moderate bandwidth. They are also easy to integrate as they are printed directly on the PCB and don’t require additional components or manufacturing processes.
Patch antennas are metallic structures printed over the ground plane on a PCB. The most common are linear series-fed arrays of patch antennas. However, other systems, such as comb-line and corporate-fed arrays, may also be used.
Figure 1: A simulation model of a 16-element patch antenna array
In a linear array of patches, changing each patch’s width will change that element’s contribution to the total radiation. The spacing between components and the lengths of the lines that connect them determine the phase distribution. Adjusting the spacing and widths makes it possible to shape the radiation patterns to meet the desired properties. For example, low sidelobes reduce interference.
Figure 2. Simulated field distribution of a series-fed linear patch array
The high-frequency operation of mmWave Radar creates challenges in the design of antenna patch arrays. Losses in the feed lines can be significant. This results in a loss of gain and efficiency of the exhibitions. Losses are due to the dielectric of the PCB and the high resistance of metallic lines. The lines should be as short as possible while using laminates of high quality as the substrate on which the antennas will be printed.
The bandwidth of each patch element on the PCB also limits the bandwidth of patch arrays fed in series. The feeding lines may also limit the bandwidth. It is also necessary to precisely control the phase of each signal provided to the component to achieve a constructive add-on of radiation in the desired directions. The distance and length of the lines between the patches determine this. The effect depends on frequency, which leads to a variation in the gain and direction of pointing along the operating frequency band.
In previous articles, we have shown that the optimal distance between antennas to achieve a maximum field of vision is half a wavelength. The adjacent patches of a serial-fed patch array are close together, resulting in a high mutual coupling that can reduce the performance. In some cases, high cross-polarization levels can be seen at specific frequencies, which reduces the array’s performance.
These factors make designing and manufacturing automotive radar patch antennas a complex task requiring advanced electromagnetic simulation software and extensive experience.
The manufacturing tolerances also significantly impact the performance of the patch array antennas used in automotive radar bands. The performance will be affected by etching tolerances, defects in surface finishes, and variations of material parameters. For the required consistency, high-quality materials and highly precise manufacturing fabrication techniques are needed, increasing the cost of boards and the entire system.
Slotted Waveguide Antennas
Slotted waveguides are widely used for automotive radar systems because of their high reliability and easy integration. They can also operate at higher frequency with less loss since the waveguide propagation medium is air. This virtually eliminates dielectric losses. Waveguides are typically made from conductive materials. In the past, copper or aluminum was used with high-precision machining to create the slots. Metalized plastic injection molding and 3D printers are more popular due to the advancements in manufacturing technology.
Image
Figure 4. 16-element slotted waveguide array simulation model
Slotted waveguide antennas are rectangular waveguides with narrow slots cut into the walls. These slots are fed by the signal that is transmitted through the waveguide. The waveguide’s places will determine the frequency and coupling of waveguide modes. The distance between spaces will determine overall radiation characteristics, including sidelobe levels.