## 4 Antenna Measurement Results  
The prototype of the CP EBG MPA was manufactured to validate the simulation results. The return loss S\({}_{\mathbf{II}}\) was measured using a Vector Network Analyzer (VNA). Figure 13 presents the fabricated MPA and our 3U CubeSat mockup model. As can be seen in Figure 13a, we had to use a total of 16 soldering nodes to fix the connecting vias to the EBG elements and the ground. In terms of structural robustness, this detail can be considered as a drawback of this fabricated antenna sample. However, we can improve this in the next fabricated samples by using plating technology to drill and firmly hold the vias between the top and the ground layers without the need for soldering. Figure 11: The gains over frequencies of the experimental EBG MPAs from Case (**a**) to Case (**h**). Figure 12: The gains over frequencies of the EBG MPA when varying the dimensions of the EBG elements. In Figure 13b, the mockup model consists of the aluminum frame structure and mounting surfaces that closely replicate the mechanical interface of a 3U CubeSat. The dimensions of the model are 30 cm \(\times\) 10 cm \(\times\) 10 cm, and this structure can be considered as three 1U cubes stacked on top of each other. The CP EBG MPA prototype was mounted on one of the end surfaces of the mockup and tested in an anechoic chamber to evaluate its radiation characteristics under realistic boundary conditions. This test can provide an accurate evaluation of the antenna gain, polarization, and far-field radiation pattern while accounting for the potential influence of the satellite structure. The anechoic chamber test setup is illustrated in Figure 14, in which we use a pair of two X-band, left-hand and right-hand circular polarization (LHCP and RHCP) helical antennas to receive the electromagnetic wave radiated from our antenna under test (AUT). The received signals are processed by our software to plot the far-field radiation patterns and to evaluate the quality of the CP wave. The process of qualifying the circular polarization of an antenna using two helical antennas is as follows: Firstly, we use the RHCP helical antenna to receive the radiated electromagnetic wave of our AUT (which is also RHCP) at 8.483 GHz, and record the  
Figure 14: The anechoic chamber setup for the antenna radiation pattern measurement. Figure 13: (**a**) The fabricated CP EBG MPA; (**b**) the structure of the 3U CubeSat model; (**c**) the antenna after being integrated into the CubeSat model. spatial received power values (in dB) over the azimuthal plane. Then, we repeat this step but replace the RHCP helical antenna with the LHCP one to record the spatial received power values in this case. Secondly, we determine the cross polarization discrimination (XPD) values (in dB) using the following formula:  
\[\textbf{XPD}=\text{Received Power (RHCP)}-\text{Received Power (LHCP)} \tag{2}\]  
After that, we calculate the spatial axial ratio (AR) values over the azimuthal plane at the target frequency of 8.483 GHz using the following formula:  
\[\textbf{AR}=log\left(\frac{10^{XPD/20}+1}{10^{XPD/20}-1}\right) \tag{3}\]  
To determine the 3 dB axial ratio bandwidth (3 dB ARBW), we repeated the above steps in a frequency span that surrounds our target frequency. Figure 15 displays the measured S\({}_{\textbf{II}}\) values of the CP EBG MPA. As observable in this figure, the measured return loss values at 8.483 GHz are approximately \(-15.4\) dB, which is slightly higher than the simulated results. The reasons for this decrease are probably due to the discrepancies in the antenna manufacturing process. However, the measured \(-10\) dB operational bandwidth is around 900 MHz, which is wider than the simulated result. The measured far-field radiation patterns in the YZ and XZ planes are shown in Figure 16. It is evident in these figures that the shapes of the measured patterns have good agreement with the simulation results.