# Car Bumper Effects in ADAS Sensors at Automotive Radar Frequencies
## 5 Results and Discussion
The free-space transmission method was also used by these researchers, but the equipment used was a vector network analyzer (VNA). In [45; 46], uncertainties for attenuation measurements with a spectrum analyzer in building materials are estimated as 0.35 dB and 0.43 dB, respectively. Our uncertainty values are also acceptable compared with other radio-electric measurements in free space. In [47], the uncertainty budget reveals errors of around 40% when the field strength was measured with a spectrum analyzer. Ref. [48] reports the uncertainty values of an antenna gain when measured by different labs. Such uncertainty results are around 0.14 dB in best case scenarios. In summary, the results are in good agreement with those previously published in the literature for comparable experiments. In addition to the measurements, there has been an attempt to model the attenuation provided by the car bumpers using a multilayer transmission model, including various layers related to the composition of the plastic support and the different superposed layers of painting and varnishing with a variety of chemical compositions [41]. Although the theoretical effort is indeed valuable, the accuracy of the results does not seem to be too confident, and the measured attenuation figures recommend the consideration of something like a guard value in the radio link budget when the transmitter is covered by the plastic bumper. After discussing the main results obtained, the contents of this manuscript are compared with those of other published works, which are summarized in Figure 5:4  
As the study in [38] is more focused on a methodology to determine the permittivity than on the analysis of real samples, the measurement systems are focused on measuring small parts of the sample for homogeneity. For this reason, the substrate used in this study was not the same material as that used for the bumpers. Polycarbonate (PC) was chosen as the substrate material instead of polypropylene (PP) for homogeneous reasons. Still on the samples issue, the main difference of our study is that measurements were done in curved and flattened bumper samples. In [41], the measurements were performed in a lab environment with absorbers placed only on the wall closest to the antennas to avoid reflections from metallic components. The setup in [42] is also in a lab environment with no absorbers. We decided to use the controlled environment of an anechoic chamber and to cover any element inside it with absorbers. When testing the setup, we experimentally verified that, at the analyzed frequency range, any surface becomes an important source of reflection, even the frequency multipliers. This caused interferences in the received signal, thereby compromising the \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Reference**} & **Automotive Radar** & \multirow{2}{*}{**Sample Type**} & **Antennas** & **Measurement** & **Measurement** \\  & & & **Polarization** & **Scenario** & **Uncertainty** \\ \hline \multirow{4}{*}{[38]} & \multirow{2}{*}{76–81 GHz} & Painted & \multicolumn{2}{c}{Material} \\  & & polycarbonate samples & Not specified & Characterization Kit & \\  & & & & Optical free-space & \\  & & & & measurement & \\ \cline{3-6}  & & Samples with three & & & Error bars in \\  & & kinds of paints & & & \\  & & (shape and origin & & Lab environment & representing the \\  & & not specified) & & Absorber in one wall & standard \\  & & & & deviations \\ \hline \multirow{4}{*}{[42]} & \multirow{2}{*}{2017} & \multirow{2}{*}{76–81 GHz} & Actual bumper & \multirow{2}{*}{Not specified} & Free space method & \\  & & samples & & Lab environment & Not specified \\ \cline{3-6}  & & & No absorbers & \\ \cline{3-6}  & & Curved and flat & & Free-space method & \\ \cline{3-6}  & & actual bumper & Vertical & Anechoic chamber & \\ \cline{3-6}  & & samples & & Absorbers around the & \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Comparison with previously published works. accuracy of the results.