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3.2Antenna

The Transmit pattern acts as an illuminator without angular discrimination capability. It has a strong intensity in the central 12 degrees coverage and it has intentionally increased sidelobes to broaden the illumination to wider angles but at reduced ranges.

The receive patterns shown in Fig. 2 implement the azimuth discrimination on receive. The three beams: Left, Center and Right (L,C and R) are dedicated to the detection of targets at long distances in a narrow angular coverage of about 12 degrees. The broader beams: Left-Left and Right-Right (LL and RR) are used for the detection of nearer targets at broader angles. The last two beams: Guard-Left and Guard-Right (GL and GR) are used as sidelobe cancellers.

The resulting two way patterns are shown in Fig. 3. and Fig. 4:

The angular coverage limited by the Guard antennas is of about 25 deg, but energy considerations limit the actual coverage to 20 deg depending on the target radar cross section. In Fig. 4 we show the realization of a rectangular coverage utilizing variable beamwidth beams.

T
his wide azimuth field of view pattern allows the FLR to detect and track targets both at long distances in straight segments of the road and at shorter distances in sharp curves. It is also this pattern that allows the early detection of “cut in” situations, which addresses to one of the most important customer complaints.

T
he patterns shown in Fig. 1 to 4 have been implemented in an antenna assembly described in Fig.5. The oval aperture at the center corresponds to the Tx beam. The circular aperture to the right includes the R, C and L beams. The oval aperture to the left includes the LL and RR beams. The two small horns at the extreme left and right are the GL and GR beams.

Fig. 6 shows a photograph of one of the five machined prototypes delivered by ERA. These very expensive prototypes were used for the validation of the antenna design and the integration of the first prototypes of the FLR.

The next step was the development of a mass production technology for the antenna. Our first choice for the antenna fabrication was metallized plastic (Ultem) as it was stated in the original proposal, but it was soon obvious that thermal considerations (the need of a heat sink for the heat developed in the RF module and conduct it to the ambient) required a good thermal conductor: i.e. a metal. Unfortunately, aluminum casting could not deliver the high tolerances required for a 77 GHz antenna without additional tooling. This solution was too expensive for a mass produced FLR.

A
fter a thorough search and analysis, the thyxomolded magnesium technology was chosen. This moulding technology is very similar to plastic molding, in which the magnesium is at a low temperature where it is partly solid and partly liquid. The mechanical tolerances of the resulting piece are similar to the obtained with plastic without the need of additional tooling.

The design made by ERA was transformed by the RoadEye team of mechanical engineers into a moldable design (draft angles, rounded corners…). The moulds were machined and after two rounds of corrections, the final magnesium parts were fabricated. In Fig. 7 the magnesium parts that compose the antenna assembly are shown. In the top row at the left, the lower half of the manifold and the front housing is shown. In the center the upper half of the manifold with the feeds and to the left the reflectors are shown. At the bottom of the picture we see the radome with the printed horizontal conductive lines that function as a polarization filter.

Several tenths of molded magnesium antennas were produced. Part of them were tested and compared with the original aluminum to validate the new design. Others were used to develop a coating that protects the antennas from corrosion and subjected to a battery of environmental test (salt spray, temperature cycling…) until they met a basic set of tests.

The performance of the antenna is shown in Fig. 8. The measurements were made in an anechoic chamber fully equipped for 77 GHz measurements. The graph shows a high accuracy measurement region of about 10 deg. Beyond this region, the target is only detected in a single beam (LL or RR) and the angular measurement is therefore coarser but sufficient for continuous tracking of close range targets. The shown accuracy has been obtained with an algorithm locally optimized. Tests over wide temperature range and waveforms with a global optimization of the algorithm show some degradation of the angular accuracy but still in the 0.25 deg bracket.