# Low-Profile Antenna System for Cognitive Radio in IoST CubeSat Applications
## 3 Antenna Fabrication and Discussion of Results
### Radiation Characteristics  
The far-field radiation pattern characteristics were also performed for the proposed MIMO antenna. To calculate the antenna's performance, the peak gain and radiation efficiency values (%n) are determined for each element of the antenna. To conduct the measurement for any port, the rest of the MIMO antenna ports were terminated with a 50 \(\Omega\) load to avoid any reflected power. For sensing and frequency reconfigurable antennas, the peak gain values were 1.079 dBi and 0.86 dBi, respectively, while %n varied from 80-85% to 73-78%, respectively. Figures 12 and 13 show the simulated and measured peak gain and %n curves for both sensing and frequency reconfigurable NB antennas.  
Figure 11: Measured isolation coefficients: (**a**) between NB-P3 and WB-P1, (**b**) between NB-P4 and WB-P1, and (**c**) between NB-P3 and WB-P1.  
Figure 10: Measured reflection coefficients of the NB antennas: (**a**) NB-P3, (**b**) NB-P4.  
Figure 14 shows the plots for the antenna gain of port-1 and port-2 in terms of left-hand circular polarization (LHCP) and right-hand circular polarization (RHCP). In port-1, the antenna's RHCP gain is higher than its LHCP gain, and it can reach a maximum value of 1.079 decibels relative to an isotropic radiator (dBi) within the antenna's operational frequency band. Similarly, for port-2, the LHCP gain is greater than the RHCP gain, with a maximum value of 1.078 dBi achieved.  
The measured and simulated radiation patterns for the UWB antenna and narrowband reconfigurable antenna are shown in Figures 15 and 16, respectively. The far-field measurement setup is shown in Figure 16c. The proposed antenna was characterized by its  
Figure 14: Peak gain curves: (**a**) RHCP, (**b**) LHCP.  
Figure 12: Peak gain curves: (**a**) Sensing antenna, (**b**) frequency reconfigurable antenna.  
Figure 13: Radiation efficiency: (**a**) Sensing antenna, (**b**) frequency reconfigurable antenna.  
far-field measurements in an anechoic chamber. An anechoic far-field measurement setup typically includes the following components:  
1. Anechoic chamber: The chamber is large enough to accommodate the antenna under test and other required testing equipment. The walls, ceiling, and floor of the chamber are covered with radiation-absorbing material to prevent reflections and create an environment free of external interference.
2. Antenna positioning system: The antenna positioning system is used to precisely control the position and orientation of the antenna under test. This system typically includes a rotation stage and one or more translation stages, which allow the antenna to be positioned at various distances and angles from the measurement equipment.
3. Signal generator: The signal generator is used to generate the RF signal that excites the antenna under test. The frequency and power level of the signal can be adjusted as needed.
4. RF receiver: The RF receiver is used to measure the signal received by the antenna under test. The receiver is typically connected to an antenna or probe that is positioned at a fixed distance and angle from the antenna under test.
5. Data acquisition system: The data acquisition system is used to collect and store the measurement data. This system includes a computer, software, and any necessary interfaces for controlling the measurement equipment and recording the data.
6. Calibration equipment: The calibration equipment is used to calibrate the measurement system and ensure accurate results.  
The patterns show omnidirectional radiation patterns in both planes for wideband antenna, indicating they are useful for UHF band communication applications. The NB antennas have omnidirectional radiation and wide-beam directive patterns, alternating in both planes which makes them useful for sensing behavior. The simulated and measured results show good agreement for both antennas in both XZ and YZ planes.  
Figure 15: Simulated and measured radiation patterns of the wideband antenna in xz- and yz-plane. (**a**) P-1 and (**b**) P-2. (**c**) Measurement setup.