## 3 Antenna Characterization
### Radiation Pattern and Beam Steering  
The simulated and measured H-plane radiation patterns of the SIW LWA between beam angles from \(-45^{\circ}\) to \(+45^{\circ}\) are shown in Figure 7. In general, a good agreement between the simulated and measured spectral gain is observed. There is only a small and almost constant shift between the simulated and measured beam angle of about \(1^{\circ}\). This is traced back to minor inaccuracies in mounting the SIW LWA assembly to the measurement system. From Figure 7, one can furthermore observe that the maximum gain for 57 GHz and 66 GHz is at beam angles of \(-9.8^{\circ}\) and \(+8.4^{\circ}\), respectively. Thus, 1D beam steering of \(18.2^{\circ}\) was achieved when the RF was tuned within the frequency range of interest, from 57 to 66 GHz. The side lobe suppression was 14 dB at 50 GHz RF and changed towards lower values for increased radio frequencies, always staying above 11 dB. The 3-dB beam width of the LWA in the H-plane lay between \(\theta_{\text{3 dB,H}}=6.8^{\circ}\) at 50 GHz and \(\theta_{\text{3 dB,H}}=5.2^{\circ}\) at 66 GHz. This means that, when the gain of the antenna is flat, the directivity slightly increases with frequency. This increase in directivity is offset by higher losses at higher frequencies, which can be deduced from the decreasing \(S_{21}\).  
Figure 5: Scattering parameters of the SIW LWA in dB over the frequency range from 50 to 70 GHz. The simulated values of \(S_{11}\) (red, diamonds) and \(S_{21}\) (green, diamonds) are shown together with the de-embedded measured values of \(S_{11}\) (blue, solid line) and \(S_{21}\) (black, solid line).  
Figure 6: Simulated (solid line, blue) and de-embedded measured gain (crosses, red) of the presented LWA in dBi plotted over the radio frequency.  
Figure 7 also gives a first impression on the multibeam capabilities of the SIW LWA. Thereby, multibeam operation can be achieved by simply using multiple RF carrier frequencies, which are radiated at different beam angles according to their radiation patterns. Experimental results for multibeam data transmission are presented in the next section.  
Next, the beam steering behavior was analyzed. In Figure 8, the beam angle is plotted versus the frequency of operation. Again, a very good agreement between Eigenmode simulations, far-field simulations, and antenna measurements was observed. Overall, 40\({}^{\circ}\) beam steering was obtained for the entire 50-70 GHz frequency range, confirming the simulations of the design [16]. Furthermore, almost linear beam steering with a coefficient of approximately \(\delta\theta/\delta f=2.25^{\circ}/\mathrm{GHz}\) was achieved thanks to the almost linear dispersion slope shown in Figure 2. Together with the measured 3-dB beam widths, the resulting coherence bandwidths can easily be estimated based on the following equation and lie in the range from \(\delta f_{\mathrm{coh,50}}=3.0\) GHz at 50 GHz to \(\delta f_{\mathrm{coh,66}}=2.3\) GHz at 66 GHz.  
\[\delta f_{\mathrm{coh}}=\theta_{\mathrm{3 dB}}/(\delta\theta/\delta f). \tag{2}\]  
Figure 8: H Beam steering behavior of the SIW LWA as beam angle over frequency from far field simulations (triangles, blue), Eigenmode simulations (circles, red), measurement results (crosses, black), and their linear fit (green).  
Figure 7: H-plane radiation patterns as the measured (dots) and simulated (solid lines) gain of the SIW LWA over the beam angle theta in degrees at frequencies of 50 GHz (red), 57 GHz (green), 61 GHz (black), and 66 GHz (blue).