## 4 Antenna Prototype Fabrication and Measurement  
A prototype of QMSIW antenna was fabricated based on the optimized design. Figure 9 shows the fabricated prototype with insets showing the feed, vias, and shortening pins connecting the bottom and top copper plates. We used aluminum cylinders to form the via fence, otherwise copper pins were used for the shorting pins. Styrofoam spacers were inserted to maintain the height between the copper plates. Styrofoam is known to have similar electromagnetic material properties as air [19].  
The reflection coefficient (S11) of the fabricated prototype was measured using a vector network analyzer (Anritsu MS2038C) and its result was compared to the simulated S11 as illustrated in Figure 10. The test cable of vector network analyzer was connected to the antenna feed via an N-type coaxial connector. They had a good agreement except a slight difference in the resonant frequencies and minimum S11 values, 86.1 and 88 MHz, \(-12.9\) and \(-15.94\) dB for the measurement and simulation, respectively.  
Figure 8: Simulated radiation patterns of the optimized antenna: (**a**) 3D and (**b**) 2D azimuth patterns.  
Figure 10: Comparison of measured and simulated reflection coefficients of the fabricated prototype.  
Figure 9: A picture of the fabricated QMSIW antenna prototype.  
After verifying the fabricated antenna's resonant frequency, we proceeded to measure the radiation performance of the antenna. We noted that the measurement of the antenna gain of the fabricated prototype was difficult because of the large antenna size, which was hard to mount in an antenna measurement chamber. Alternatively, an in-house experiment setup as depicted in Figure 11 was used to measure the antenna's received power instead of the antenna gain. A 1.5 m long whip antenna (RH-10M) was used as a transmitting (Tx) antenna. It was connected to a signal generator (R&S SGT100A) whose waveform was controlled by a notebook. The fabricated QMSIW antenna was connected to a spectrum analyzer (Anritsu MS2038C) and the receiving power was recorded by sweeping the frequency from 60 to 100 MHz and by rotating the antenna 360 degrees along with the azimuth plane.  
The measured result is shown in Figure 12. The black line is the measured received power from 60 to 100 MHz with a 5 MHz interval. In this experiment, the Tx power from the signal generator was set to 0 dBm and the distance between the Tx and Rx antennas were 10 m. As can be seen, the received power increases as the frequency increases and then peaks at 86 MHz, which corresponds to the resonant frequency of the QMSIW antenna. After 86 MHz, the received power drops dramatically. We compare it with the simulated antenna gain which is the red line in Figure 12. More specifically, the red line is the simulated peak gain at \(\theta=90^{\circ}\) (i.e., azimuth plane). It also increases as the frequency increases and peaks at the simulated resonant frequency, 88 MHz. However, it gradually decreases after 88 MHz, which is different from the steep decrease in the measured received power. The latter may be caused by fabrication errors making the prototype antenna's very inductive after the resonant frequency, thus the EM energy was trapped inside the antenna cavity. The received power at 86 MHz by rotating the fabricated antenna is plotted in Figure 13 (black line). It is compared with the simulated radiation pattern at \(\theta=90^{\circ}\) (red line). Both are normalized to the maximum azimuth angle \(\phi=270^{\circ}\). It can be observed that the measured received power pattern shrinks about 0.5 dB from the simulated antenna gain pattern, but the overall patterns are similar to each other.  
Figure 11: Measurement setup for fabricated antenna’s received power.