## 4 Prototype Development and Measurement  
The proposed olmpeak MIMO antenna system is fabricated, and using in-house facilities, the measurements have been taken from the developed prototype. The fabricated prototype is shown in Figure 9. Figure 9a,b shows the proposed fabricated prototype model without connectors while Figure 9c,d shows the prototype with south-east connectors. In order to verify the design dimensions, a scale has been placed near the fabricated prototype.  
The measured and simulated s-parameters are superimposed on the plots in Figure 10. The solid-black lines are from the finite difference time domain-based electromagnetic simulations, whereas the dashed red lines are from the measured data. The slight ripples in the measurements are due to drifted calibrations and bent cables.  
Figure 8: Olmpeak MIMO antenna simulated s-parameters (**a**) reflection co-efficient (**b**) ports isolation.  
Figure 7: Olmpeak MIMO antenna (**a**) front view (**b**) back view.  
Figure 10: _Cont._  
Figure 9: OlympicA MIMO antenna prototype **(a)** without connector front **(b)** without connector back **(c)** with connector back **(d)** with connector front.  
The antenna is so thin and flexible that it requires a delicate mechanical balance to keep steady during the measurement. It is seen from the plots in Figure 10a-d that the measurement and simulated reflection coefficients are in great agreement. The slight shifts in the pole locations are due to slight bends of the substrate and also they are well within measurement uncertainty. Figure 10e shows the simulated and measured ports isolation performance between the antenna elements on the array. The slight difference between simulations and measurements can be attributed to the aforementioned calibration and bend issues. However, an isolation performance of at least 20 dB is achieved as seen in the figure. The measured isolation value is obtained as 24 dB at 28 GHz and 37 dB at 38 GHz.  
The array is symmetric from the center. The radiation patterns of the left two elements are identical to the right two elements in the array. Therefore, analyzing only half of the array would be adequate to obtain an idea for the overall array. We will present the results of Antenna 1 and Antenna 2 when the antennas are sequentially numbered from left to right. The simulated and measured radiation patterns of Antenna 1 and Antenna 2 at two principle planes of Phi = 90 and Theta = 90, at two resonating bands are shown in Figure 11. It is shown from Figure 10a-d that at 28 GHz both Antenna 1 and Antenna 2 have beams on the directions of Theta = 0 and Theta = 180 for Phi = 90, whereas they have main lobes on the directions of Phi = 45 and Phi = 135 for Theta = 90. Likewise, it is shown from Figure 11e-h that at 38 GHz, both Antenna 1 and Antenna 2 have main beams on the directions of Theta = +45 and Theta = 135 for Phi = 90 whereas they have main lobes on the directions of Phi = 60 and Phi = 120 for Theta = 90. The radiation patterns both simulated and measured are in a good agreement with slight disruptions due to the difficulty of thin substrate handling. The co- and cross-polarization of the MIMO system is shown in Figure 12. From the figure, it can be seen that X-pol levels among MIMO elements are well below \(-\)10 dB.  
Figure 10: OlympicA MIMO antenna measured s-parameters (**a**) antenna 1 (**b**) antenna 2 (**c**) antenna 3 (**d**) antenna 4 (**e**) ports isolation.  
Figure 11: Olympicak MIMO antenna radiation pattern (**a**) Antenna 1 28 GHz Phi 90 (**b**) Antenna 1 28 GHz Theta 90 (**c**) Antenna 2 28 GHz Phi 90 (**d**) Antenna 2 28 GHz Theta 90 (**e**) Antenna 1 38 GHz Phi 90 (**f**) Antenna 1 38 GHz Theta 90 (**g**) Antenna 2 38 GHz Phi 90 (**h**) Antenna 2 38 GHz Theta 90.