# Design of Triple-Band (DSRC, 5G, 6G) Antenna for Autonomous Vehicle Telematics
## 3 Results
### 6G CPW to MS Transition  
As the antenna frequency reaches the THz range, the SMA connector is no longer viable due to the small size and high operational frequency of the antenna. To circumvent this, a Waveguide Infinity Probe, selected from Microtech, fed the eight-by-eight 6G array antenna using a ground-signal-ground (GSG) topology. A coplanar waveguide to microstrip (CPW  
Figure 8: Fabricated two-by-two series-fed patch array with one-by-two Wilkinson power divider.  
Figure 7: S-parameter performance of the developed one-by-eight Wilkinson power divider.  
Figure 9: Simulated and measured S-parameter of the two-by-two series-fed patch array with one-by-two Wilkin power divider.  
to MS) transition was designed to integrate the power divider with the GSG probe. Three different CPW to MS transitions were developed based on a Klopfenstein function, an exponential function, and a linear function, as shown in Figure 10, using the principles outlined in [19; 20], for a coplanar waveguide with the ground (CPWG). Using a CPWG makes fabrication easier since the copper cladding on the bottom side does not need to be altered.  
Each transition was divided into N segments, and each segment is designed to achieve a 50 \(\Omega\) impedance. The separation between the signal and ground strips was recalculated for each segment to maintain the impedance. These dimensions were used in calculating the separation using MATLAB. The GSG probe connection was modelled using a perfect conductor boundary that connects the two ground strips with the excitation applied as a lumped port integrated across the ideal conductor to the signal strip, as shown in Figure 11.  
The Klopfenstein taper-based CPW to MS transition showed the most profound resonance and the widest bandwidth of 28.07 GHz. The exponential-based transition had the lowest bandwidth of 27.5 GHz, although the resonance was slightly deeper than the linear taper. The linear taper has a mid-size bandwidth of 27.78 GHz but the shallowest resonance. Figure 12 compares the S-parameter performance of the three CPW to MS transitions. Based on these results, the Klopfenstein taper was chosen.  
Figure 11: Ground–signal–ground probe modelling in HFSS.  
Figure 10: Comparison of three power divider designs using various functions to define the transition.  
The CPW to MS transition using a Klopfenstein function was then optimized by decreasing the length of the transition, increasing the width of the ground strips, and changing the lengths of the attached CPWG and microstrips. The final design achieves a 30 GHz bandwidth from 130-160 GHz, covering the entire band of the 6G antenna array with the power divider. This performance is shown in Figure 13, where port 1 is the microstrip side and port 2 is the CPWG. The geometry of this design can be fed by a GSG probe with a 150 \(\upmu\)m pitch, which is concurrent with the dimensional requirements of Microtech's Waveguide Infinity Probe. Following Microtech's recommendations, 5 \(\upmu\)m thick aluminum contact pads were placed on the CPWG strips. The contact pad effects on the antenna performance were negligible.  
Figure 14 shows the S-parameter and realized gain at the boresight of the developed eight-by-eight antenna array with the designed power divider and Klopfenstein taper-based CPW to MS transition. Although the power divider and CPW to MS transition reduce the overall realized gain of the antenna by 2 dBi, they increase the \(-10\) dB bandwidth by 7.71 GHz (109% increase). In summary, the developed 6G eight-by-eight antenna array with the power divider and CPW to MS transition has a bandwidth of 14.81 GHz (131.91-137.56 GHz, 140.53-146.87 GHz, and 153.06-155.88 GHz) and realized gain at the boresight of 15.1-19.8 dBi between the frequencies of interest.  
Figure 12: Comparison of performance of three different coplanar waveguide to microstrip transitions.  
Figure 13: S-parameters of the optimized Klopfenstein taper-based coplanar waveguide to microstrip transition.