Results/Findings

7. Prototype Design

The final design of the working prototype that meets all objectives is described in this section. While two separate design concepts were fully developed, only one was built due to time, cost, and workload constraints. The tape measure dipole antenna prototype was built as it was the most feasible of the two designs. The description of this prototype has been broken down into the five major parts of the prototype. These parts are the TunaCan enclosure, coax cable to antenna interface, burn-wire circuit, balun, and coax cable.

7.1. TunaCan Enclosure

The top of the CubeSat features a securely attached enclosure designed to house a TunaCan. Wrapped around this enclosure are tape measure antennas as seen in Fig. 7.1. Before deployment, the antennas are neatly wrapped around the enclosure. To ensure controlled deployment, a burn wire circuit is used to burn the fishing line that holds the antennas in place. Additionally, there is a balun situated within the enclosure to manage signal transmission. This enclosure was 3D-printed with an infill of 40% as previous models with lower infill cracked and broke apart during construction. The enclosure successfully contained the antennas and fit the size requirements set in the objectives. It also contributed to the low weight of the design meeting another objective.

Figure 7.1 Tape measure enclosure

7.2. Coax to Antenna Arms Interface

To interface the coax cable to the antenna arms, ring terminals are needed. The ring terminals and antenna are securely fastened between 3D-printed blocks, ensuring a tight and stable connection. The M2.5 ring terminals are soldered to both the coax center connector and shield, creating a reliable electrical connection. To improve electrical contact between the ring terminals and antenna arms, the tape measure is sanded, enhancing the efficiency and performance of the antenna system. The completed RF interface can be seen in Fig. 7.2. This interface provided a reliable connection that met the requirements set out in the objectives. Later prototypes should use stronger and more reliable connections such as replacing 3D-printed components with delrin and aluminum blocks and replacing terminals with copper sheets.

Figure 7.2 Tape measure bolted inside the enclosure

7.3. Burn Wire Circuit

The fishing line holding the antennas is released by heating resistors by forcing more power through the resistor than it was rated. A MOSFET is employed to precisely control the deployment process. The circuit was built on a prototyping PCB with only one MOSFET and no backup pair of burn resistors. The schematic for the burn wire circuit is shown in Fig. 7.3. The two resistors of 8.2 ohms in series will both burn under the 731mA current. The resistors are rated for ¼ Watts of power and will receive a total of 8.7 Watts.

 

The parts used for the burn circuit are as follows:

 

  • MOSFET – 1 x 30N06
  • Resistors – 2 x 8.2Ohms, 1/4W
  • +5V,  +12V DC barrel connector

Figure 7.3 Circuit diagram for the burn circuit

The burn wire circuit is positioned at the top of the enclosure as seen in Figure. The fishing line is tied to four points on the enclosure to ensure stability during deployment, as seen in Fig. 7.4. The fishing lines are tied over the resistors to allow enough heat to cut the line. 

Figure 7.4 Burn circuit placed on the lid of the enclosure

7.4. Balun

In order for the bead shape balun to fit within the enclosure, it was necessary to ensure that its length was less than 2.75 cm, its diameters were less than 2.5 cm, and the overall height remained below 4 cm. 

 

Ferrite mixes # 31, 43, 61, 77 were evaluated to select the most suitable one that would achieve the desired impedance of 3000-4000 ohms. Appendix C contains the graphs illustrating the relationship between impedance and frequency for the four mixes [34]. It should be noted that the impedance values shown in the graphs were measured with a presence of a toroid-shaped balun with an inner diameter of 3.56 cm.

 

Considering the small size of the enclosure, a bead-shaped balun is used for the project. The choice of using a bead shape balun for the project was driven mostly by limited space inside the enclosure. The toroid shape, being larger in diameter, would have been difficult to fit. Additionally, bead shape baluns are commonly used to suppress electromagnetic interference (EMI) in cables, which aligns with the project’s requirements.

 

As observed from the graphs in Appendix C.2 and C.4, it is evident that ferrite mixes 43 and 77 are not suitable for this project, as their impedance remains below the 1200 ohm range even with multiple turns of coax. Both Mix #31 (Appendix C.1) and Mix #66 (Appendix C.3) meet the desired impedance range and be explored further.

 

The selected balun option for Mix #31 is the Fair-Rite bead with part number 2631102002. According to the datasheet, an impedance of 1700 ohms can be achieved with 3 turns as seen in Fig. 7.5[35]. By using 4 turns, the impedance can be estimated to be between 3000-4000 ohms as desired. However, for more than 4 turns, the choke impedance needs to be measured directly as it cannot be accurately estimated from the graph.

Figure 7.5: Impedance VS Frequency for Mix #31. [35]

Mix #61, as shown in Appendix C.3, can achieve an impedance of 4000 ohms. However, when searching for a Mix #61 balun, the available datasheets did not provide graphs with more than one turn and their corresponding impedance values. Figure 7.6 displays the impedance versus frequency graph for a single turn of the balun.

Figure 7.6: Impedance VS Frequency for Mix #61 for a single turn. [36]

When comparing the single turn impedances of Mix #31 and Mix #61, it is evident that Mix #61 has a higher impedance. This suggests that higher impedance can be achieved with Mix #61. However, due to the lack of accurate data on how the impedance increases with the number of turns, it is difficult to precisely estimate the impedance for multiple turns of Mix #61. 

 
Mix #31 was chosen for the project with dimensions which can be seen in Figure 7.7.

Figure 7.7: Dimensions of a Mix #31 bead shape balun [35].

Figure 7.8 shows the selected balun.

Figure 7.8: Selected Mix #31 balun.

7.5. Coax Cable

After selecting the balun mix and its size (12.8 mm inner diameter), a coaxial cable was chosen for the project. To achieve an impedance of 3000-4000 ohms, the coaxial cable must be wound around the balun four times. The coaxial cable should not exceed 2.8 mm in width to fit smoothly inside the balun’s inner diameter. The cable should also have pre-installed SMA connectors for testing.

 

The selected cable is Cinch Connectivity Solutions Johnson with part number 415-0025-MM500. The coax cable is RG178 with a diameter of 1.8mm, and can accommodate 4 turns around the balun with a bending radius of 5 mm [37].  In Figure 7.9 the selected coax cable can be seen and Figure 7.10 shows the coax cable wrapped around the balun 4 times.

 

Figure 7.9: Coax split into the center conductor and braided shield, ready to solder on ring terminals.

Figure 7.10: Balun is shown in the enclosure wrapped with 4 turns.

8. Testing & Validation

The testing and validation procedures, results, and conclusions are included in this section.

8.1. Testing Overview

The testing of the antennas and deployment method was split up into two separate days of testing with different testing procedures. However, the general goal of the testing for both days was identical. The prototype testing ensured that the following sections of the design met requirements:

 

  • Antenna deploys to full length
  • Burn wire circuit burns fishing line and deploys antennas
  • Antennas can be tuned to a specific center frequency
  • Balun isolates coax transmission line from dipole antennas.

8.2. RF Capabilities and Antenna Tuning

The antenna was tested and tuned to the desired frequency. During the process, multiple measurements were taken using an antenna analyzer. The antenna length was trimmed until it achieved resonance at the desired frequency. Ultimately, the length of each antenna was trimmed to 240 cm. At a frequency of 29.4 MHz, the Standing Wave Ratio (SWR) was measured to be 1.4, indicating good impedance matching. The input impedance of the antenna system was found to be 74.4 Ohms. This indicates that the antenna is well-tuned for efficient performance at the target frequency.

8.2.1. SWR Chart during Tuning

Before tuning the center frequency of the lowest SWR is around 26 MHz as seen in Fig.8.1. After tuning to 250 cm, the center frequency for the lowest SWR is 27 MHz, as seen in Fig.8.2.

Figure 8.1: 260 cm length initial SWR chart.

Figure 8.2: 250 cm length SWR chart.

8.2.1. SWR Chart during Tuning

Additional tuning later the SWR was getting closer to our desired frequency. Tuning the antennas to from 244cm to 240cm seen in Fig 8.3 and 8.4 respectively optimized our SWR to desired center frequency of 29.4 MHz. The SWR for the entire 29.3 to 29.52 MHz is below SWR=2.

Figure 8.3: 244 cm length SWR chart.

Figure 8.4: 240 cm length final SWR chart.

8.3. Deployment Testing

The deployment testing was split up into two days with two different testing procedures. The first deployment testing used a wooden platform to support the enclosure while the antennas deployed. The second deployment testing procedure used a custom 3D-printed base to support the enclosure while the antennas were deployed.

8.3.1. Deployment Testing I

The enclosure was securely mounted on a wooden platform placed atop a pole as seen in Fig. 8.5, preventing any surface drag or interference. To initiate the antenna deployment, a MOSFET is used to switch on the burn circuit. The burn circuit is powered through a variable power supply, allowing for direct power and no need for a battery.

 

The antenna system was then released but only extended to a total length of 80%. This was due to the antenna dragging on the wooden platform. Our test conditions prevented us from reaching full deployment, as the gravity on earth will pull the measuring tape down and make contact with the platform. The fully deployed antenna system can be seen in Fig 8.6. High winds were also challenging as the test was conducted on top of the ELW building at UVic. The conditions for the test are shown below in Table 8.1.

Table 8.1: Deployment Testing Conditions.

The remaining tests for burn circuit and antenna functionality were conducted, and detailed below in Table 8.2. The burn circuit was malfunctioning during the test and direct power from the PSU was used, bypassing the MOSFET to provide the needed current to the resistors. The following tests 3 and 4 were also successful and tunability and transmission line isolation were proven to be operating as expected. The antenna tunability was shown in the previous section. As for the transmission line isolation, this was tested by grabbing the cable to observe any effect on the SWR. The antenna proved to be isolated from the transmission line as no change in the SWR was seen. The balun selection was also correct in providing the necessary impedance.

Table 8.2:  Testing Sequence for First Test. *Test Date: July 25, 2023

Figure 8.5: Wrapped & tied down antenna elements.

Figure 8.6: Fully deployed antenna elements.

8.3.2. Deployment Testing II

The enclosure is mounted on a specially designed 3D printed base to prevent dragging as seen in Fig 8.7 and 8.8. The custom base ensures obstacle-free deployment and no dragging. During this deployment the burn circuit worked successfully and the antenna released as expected. However, the antenna began to wrap around the pole of the 3D printed base. The antenna however did fully deploy to full length but due to the test conditions gravity on earth will drag the antenna elements. This test was only for deployability and burn circuit. Testing for the tunability and coax isolation were not required for this test plan seen in Table 8.3. This was due to successful completion of tests in the previous testing phase.

Table 8.3 Test Sequence for Second Test. *Test Date: July 26, 2023 

Figure 8.7: Custom base

Figure 8.8: Deployed antennas on custom base

9. Cost Analysis

Table 9.1 shows the materials ordered for the prototype with their corresponding prices.

Table 9.1 Prices of Materials.

Note: An additional $16 was spent on shipping.

 

In Table 9.2 the number of hours spent on the project is shown.

Table 9.2 Number of hours spent on the project.

The tape measure antenna design already represents a highly cost-effective solution due to its simplicity and the affordability of materials used. The tape measure was the most expensive component of the antenna. It was selected because of its ability to reach 2.5 meters on each side without folding. Attempting to reduce cost could potentially compromise the antenna’s performance capabilities in this case.

10. Conclusion & Recommendations

A tape measure dipole antenna was designed for the 10-meter amateur radio band corresponding to the frequency range of 29.3 to 29.52 MHz while adhering to the requirements and constraints imposed by the CfaR team. This design was chosen for prototyping due to its simple design, affordable materials and feasibility for testing. 

 

The dipole antenna system has three main elements: the balun, burn circuit and enclosure. A lossy balun using a ferrite and coax cable was selected to suppress the common-mode current. The burn circuit used a MOSFET to switch the current flow through resistors for releasing the antenna. The enclosure was designed to accommodate the lossy choke balun, burn circuit and antenna arms.

Testing of the design showed that the antenna arms were deployable upon burning of the resistors. The carbon steel tape measure behaved as expected and released like a spring further unraveling the full length of the antenna. Testing conditions were a challenge and gravity played a major role in preventing clean deployments. All tests of the antenna deployment resulted in the dragging and collapse of the antenna arms. 

 

The RF capabilities were tested by ensuring a good electrical connection between the antenna and the input transmission line. The balun chosen for the design provided an impedance of ~4 kOhms, isolating the transmission line from the antenna. Antenna tuning was achieved and the antenna was proven to be tunable. The desired frequency for the antenna was tuned by trimming the material of the antenna ends. The final length was 240cm at SWR=1.46 at the center frequency and the entire frequency range had a SWR of below 2. 

 

It will be further recommended a better testing platform or rig is developed to test the deployment on Earth. The rig or platform must mitigate the effect of gravity and friction to prevent any forces acting on the antenna arms. This can be accomplished by suspending the enclosure and power sources from the roof. The performance of the antennas can also be further tested beyond measuring the SWR along the required frequency range to ensure the antenna will meet later decided power requirements. 

 

Furthermore, various sections of the design could be improved in further prototypes, including adding aluminum and Delrin blocks as well as a copper plate to replace current 3D-printed antenna holders and ring terminals respectively. 

 

This project was incredibly interesting from start to finish and provided valuable insight into antenna systems, spacecraft development, testing procedures, and design development. Once again, we would like to thank Levente Buzas and the entire CfAR team for their thorough support and detailed feedback throughout the entirety of the semester.